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Article

A New Holoplanktonic Nudibranch (Nudibranchia: Phylliroidae) from the Central Mexican Pacific †

by
Jeimy D. Santiago-Valentín
1,
Eric Bautista-Guerrero
2,*,
Eva R. Kozak
1,
Gloria Pelayo-Martínez
1 and
Carmen Franco-Gordo
1,*
1
Departamento de Estudios para el Desarrollo Sustentable de Zonas Costeras, Universidad de Guadalajara, Gómez Farías 82, Cihuatlan, San Patricio Melaque 48980, Jalisco, Mexico
2
Laboratorio de Ecología Marina, Centro de Investigaciones Costeras, Centro Universitario de la Costa, Universidad de Guadalajara, Av. Universidad No. 203, Puerto Vallarta 48280, Jalisco, Mexico
*
Authors to whom correspondence should be addressed.
†
urn:lsid:zoobank.org:pub:50CF9D98-860D-47A9-B567-6B273E2D5DED.
Diversity 2025, 17(7), 479; https://doi.org/10.3390/d17070479
Submission received: 21 May 2025 / Revised: 7 July 2025 / Accepted: 7 July 2025 / Published: 11 July 2025
(This article belongs to the Section Marine Diversity)
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Abstract

Pelagic nudibranchs exemplify evolutionary convergences towards streamlined, transparent body forms adapted for life in the planktonic environment. Here, we describe a new genera and species, designated as Pleuropyge melaquensis gen. et sp. nov. This species belongs to the family Phylliroidae and is distinguished by key diagnostic characters, including a laterally positioned anus approximately one-third of the body length from the head, the absence of a cephalic disc, and an anterior hepatic caecum that is longer than the intestine. The description of P. melaquensis contributes to the classification of a third genus and a fourth species within the Phylliroidae family. This study offers novel insights into the functional and structural traits that have enabled nudibranchs to transition from benthic to pelagic environments.

1. Introduction

Nudibranchs (order Nudibranchia Cuvier, 1817) are marine gastropod mollusks belonging to the subclass Heterobranchia. Following a planktotrophic or lecithotrophic larval stage, these organisms typically lose their protective shell and develop external gills [1,2]. Approximately 3000 species have been described to date [3], the vast majority of which are benthic. However, a few evolutionary lineages have independently adopted pelagic lifestyles. Among these, the neustonic genus Glaucus Forster, 1777 floats inverted on the ocean surface [4], while members of the family Phylliroidae Menke, 1830, including Phylliroe spp. and Cephalopyge trematoides Chun, 1889, are true holoplanktonic organisms that inhabit the water column throughout their life cycle [5].
Pelagic adaptations are rare within Nudibranchia and represent examples of convergent evolution. These include streamlined, laterally compressed, and transparent bodies that enhance swimming efficiency and reduce visibility to predators in the open ocean. Members of Phylliroidae also possess a suite of specialized traits such as reduced or absent feet, locomotion through lateral undulations, and the presence of three or four hepatic caeca [5,6,7]. These features facilitate active predation on gelatinous zooplankton, including medusae, siphonophores, and salps [5]. Because their prey consume ecologically and commercially important taxa such as crustaceans and fish eggs [8], phylliroid nudibranchs may influence pelagic food webs more broadly than previously assumed.
The family Phylliroidae currently comprises two recognized genera: Phylliroe, which includes two species (P. lichtensteinii (Eschscholtz, 1825) and P. bucephala Lamarck, 1816), and Cephalopyge, represented by a single species, C. trematoides [5]. The phylogenetic placement of these holoplanktonic nudibranchs has undergone several revisions. Historically, Phylliroidae was classified as Cladobranchia not assigned [6]. However, recent studies have placed the family within the superfamily Dendronotoidea, grouping them with lineages that share morphological characteristics such as the absence of an oral hood and an undivided velum [7]. Despite this assignment, members of Phylliroidae are morphologically distinct from other dendronotoids due to their extensive adaptations to a pelagic lifestyle.
The geographic distribution of Phylliroidae is generally circumglobal, with occurrences concentrated in tropical and subtropical oceanic waters, particularly in the epipelagic zone [5]. However, confirmed records remain scattered and relatively rare, largely due to the fragility of these organisms and the challenges of capturing intact specimens during plankton tows [5]. Within the Eastern Tropical Pacific (ETP), members of Phylliroidae have been reported from several coastal regions, including Phylliroe bucephala along the Pacific coast of Colombia [9] and both P. bucephala and Cephalopyge trematoides off the coast of Peru [10]. In the nearby Gulf of California, both species have also been documented [11].
The Central Mexican Pacific (CMP), where the present study was conducted, spans approximately 17–23° N, 108–102° W and includes the coasts of Michoacán, Colima, Jalisco, and Nayarit. This region forms part of the ETP and is characterized by dynamic oceanographic conditions driven by seasonal upwelling, mesoscale eddies, and interannual variability associated with the El Niño–Southern Oscillation (ENSO), which promote elevated planktonic productivity and support diverse pelagic assemblages [12,13]. Despite this ecological richness, the holoplanktonic nudibranch fauna of the CMP remains poorly studied. Most existing records are incidental or based on photographs, and no formal descriptions of Phylliroidae species have been published from this region.
In this study, we describe a new genus and species of holoplanktonic nudibranch within the family Phylliroidae, based on specimens collected in the CMP. We provide detailed morphological and histological descriptions, supported by molecular evidence derived from three independent loci: mitochondrial cytochrome c oxidase subunit I (COI), mitochondrial 16S rDNA, and nuclear histone H3. The integration of molecular and morphological data contributes to a better understanding of the taxonomy, systematics, and evolutionary history of this elusive pelagic lineage.

2. Materials and Methods

2.1. Study Area

Specimens were collected in March–April 2018 and May 2019 from three sites off the Central Mexican Pacific (CMP) coast. The first two sites were close to Navidad Bay: ‘NAVI’, situated at a distance of 5 km from the coast with an estimated depth of approximately 90 m, directly in front of the coast with a depth of approximately 10 m. The third site was in Manzanillo Bay, designated as ‘MANZ’, with an estimated depth of approximately 16 m (see Figure 1). The CMP exhibits a seasonal hydroclimatic regime, characterized by two distinct periods. The initial period, spanning from January to June, is characterized by relatively cool temperatures (less than 25 °C), high primary and secondary productivity, and dry conditions (minimal or no rainfall). This period is also marked by the presence of shallow mixed and dissolved oxygen (DO) minimum layers. The subsequent period, from July to December, is distinguished by warm temperatures (greater than 25 °C), low primary and secondary productivity, and frequent rainfall, including seasonal rains and hurricanes. During this period, mixed and DO minimum layers deepen [12,13].

2.2. Sampling and Morphological Description

Samples were collected during surface trawls using plankton nets with (250 µm mesh) equipped with a General Oceanics flowmeter at the mouth of the net. The NAVI and LAB samples were gently transferred to a cooler and taken to the laboratory where the nudibranchs were separated alive from the sample, allowing for observations of their movements before preservation. The MANZ sample was fixed on board for subsequent separation. The specimens were examined under a stereoscope (Stemi 2000-Zeiss®, Oberkochen, Germany) and identified based on external and internal morphological characteristics following the taxonomic criteria of Steinberg [14], Tokioka [15], and Goodheart [16]. Observations included body size, color, and specific anatomical features. Descriptions of swimming movements of the live organisms were also included. Specimens from all sites were fixed with 96% ethanol for molecular analysis, while seven organisms collected in LAB were preserved in 4% formaldehyde for subsequent histological analysis. The holotypes and paratypes were deposited in the “Colección de Zooplancton de El Colegio de la Frontera Sur” (ECO-CHZ).

2.3. Histology

A total of seven organisms were examined by means of histological analysis. Each organism was subjected to a process of dehydration using an eight-stage ethanol series (70–100%), followed by clearing in two-stage xylene and embedding in paraplast using a Leica® EG1160 tissue embedding machine. The duration of each stage was 1 h. Tissues were sectioned into 7 μm-thick slices using a Leica® RM2125RT semi-automatic rotary microtome. Subsequently, the sections were stained with hematoxylin and eosin, and mounted onto glass slides using EntellanTM. A thorough examination and photographic documentation of all samples was conducted using a Carl Zeiss® AxioScope optical microscope.

2.4. Molecular Analysis

The genomic DNA was extracted from three specimens using an SV Promega kit (Promega®, Singapore) following the manufacturer’s instructions. Three genes were amplified separately from individual genomic DNA samples by a polymerase chain reaction technique (PCR). One partial sequence of the mitochondrial cytochrome c oxidase subunit 1 gene (COI; 680 pb) was amplified using degenerate primers: dgLCOI490 (5′-GGTCAACAAATCATAAAGAYATYGG-3′) and dgHCOI21908 (5′-TAAACTTCAGGGTGACCAAARAAYCA-3′) [17]; the 16S mtDNA (~480 bp) was amplified with the primers 16LRN1398 (5′-CGCCTGTTTAACAAAAACAT-3′) and 16SRHTB (5′-ACGCCGGTTTGAACTCAGATC-3′) [18]; and nuclear gene histone (H3,~400 bp) was amplified with the following set of primers H3af (5′-TGGCTCGTACCAAGCAGACVGC-3′) and H3ar 5′-ATATCCTTRGGCATRATRGTGAC-3′) [19].
Templated DNA (20–100 ng) was used in a 11.5 μL final volume reaction (7.1 μL H2O, 0.8 μL MgCl2 (25 mM), 0.7 μL dNTP (2.5 mM), 0.13 μL forward primers (25 mM), 0.13 μL reverse primers (25 mM), 2.5 μLTaq buffer 5X (Promega®), 1 μLTaqflexi DNA polymerase (Promega®). The standard thermocycling protocol PCR conditions for all genes were as follows: one cycle at 94 °C for 5 min of denaturing; 35 cycles at 94 °C for 1 min; 48 °C (COI), or 55 °C (16S), or 62 °C (H3) for 1 min, annealing, 72 °C 1 min, and a final extending of one cycle at 72 °C 5 min. The PCR products were visualized on a 2% agarose gel through electrophoresis TAE (Tris-acetate-EDTA) buffer. Subsequently, the PCR fragments were purified using a Wizard SV Gel and PCR Clean-Up System (Promega®) in accordance with the manufacturer’s instructions. Finally, sequencing was performed at Macrogen Inc. (Seoul, Republic of Korea).
The sequences were manually edited using Geneious Basic (ver. 4.8.5) to ensure quality, trim the read ends, assemble forward and reverse sequences, and obtain a consensus sequence for each specimen, using MUSCLE under default parameters. To confirm the identities, all sequences were trimmed and checked for frameshift errors in alignment using the Basic Local Alignment Search Tool (BLAST; blast.ncbi.nlm.nih.gov, accessed on 1 May 2025). The gene sequences were deposited in GenBank (https://www.ncbi.nlm.nih.gov, accessed on 30 May 2025) of the National Center for Biotechnology Information (NCBI) with accession numbers: COI (PV648734, PV648735, PV819806), 16S (PV668483, PV6684834, PV828418), and H3 (PV832065, PV832065, PV832065).
To determine the systematic position of the examined nudibranch, the COI, 16S, and H3 were concatenated and aligned with sequences from other Dendronotoidea species downloaded from the NCBI GenBank. Alignments were performed using Mafft version 6, online [20]. Prior to phylogenetic reconstruction, alignments were created without inserts and hypervariable base positions. Bayesian inference methods (BI) and the maximum likelihood (ML) were used for phylogenetic analysis. The Bayesian inference (BI) analysis was conducted using Mr. Bayes 3.2.1 [21] with Markov Chain Monte Carlo simulations of over 200,000 generations (frequencies < 0.001). The Hasegawa–Kishono–Yano (HKY) substitution model was selected for its superior performance in generating the Bayesian information criterion score, as determined by jModelTest v2.1.10 [22]. The appropriate burn-in value was determined by examining the standard deviation of split frequencies. A 50% majority rule consensus tree was constructed from all generations sampled after the burn-in period. The ML analyses were performed using MEGA v. 11 [23] with 1000 bootstrap replicates [24]. The best-fitting evolutionary substitution model was General Time Reversible + G model determined using JModelTest 2.0 software. Furthermore, genetic distances among species of the superfamily Dendronotoidea were obtained using the variance estimation bootstrap method (1000 replications) with the Kimura 2-parameter model and Gamma distribution, including transitions and transversions. The analysis was carried out using MEGA v. 11 [23].

3. Results

A total of 943 individuals were collected in five sampling events (NAVI = 1, LAB = 3, MANZ = 1), between March and April 2018. Abundances ranged from 0.07 to 6.22 ind/m3, with the highest abundance at LAB in April 2018 (916 individuals, 6.22 ind/m3). The individuals occurred at temperatures between 24 and 25 °C (Table 1).

3.1. Systematics

Class: Gastropoda Cuvier, 1795
Subclass: Heterobranchia Burmeiter, 1837
Order: Nudibranchia Cuvier, 1817
Suborder: Cladobranchia not assigned
Superfamily: Dendronotoidea Allman, 1845
Family: Phylliroidae Menke, 1830
Genus: Pleuropyge gen. nov.
Zoobank ID: urn:lsid:zoobank.org:act:007961CC-2973-4F0D-AD8C-481E7F142765
Species: Pleuropyge melaquensis sp. nov.
Zoobank ID: urn:lsid:zoobank.org:act:D16D8C61-3903-40AB-8400-F07A4221C694

3.2. Type Material

Holotype: ECO-CHZ 12568, March 2018; Navidad Bay site ‘NAVI’ (19°09′03″ N, 104°44′50″ W). Paratype: ECO-CHZ 12569, March–April 2018 and May 2019; Navidad Bay: ‘NAVI’, ‘LAB’ (19°14′00″ N, 104°41′30″ W), and Manzanillo Bay ‘MANZ’ (18°99′48″ N, 104°26′07″ W).

3.3. Etymology

The generic name Pleuropyge from the Greek plevrá (=side) and pygí (=anus); in reference to the position of the anus, the main characteristic that separates these organisms from the rest of the Phylliroidae family. The species name melaquensis refers to the locality San Patricio Melaque, near the three collection sites.

3.4. Diagnosis

Holoplanktonic nudibranch with an ellipsoid body, transparent and laterally compressed. Rhinophores are slightly larger than the head. Visible muscle fibers run along the central axis of the body. The foot is located below the posterior region of the pharynx. There are no mucous cells observed. The mandible is composed of two jaw plates with small, irregularly spaced spicules. The odontophore has a radular formula of 14–15 × 1.1.1. The anus has a pleuroproctic position. Three hepatic caeca extend from the stomach, two towards the posterior and one antero-dorsal, and the heart is located dorsally.

3.4.1. External Morphology

The holoplanktonic nudibranch is characterized by a transparent, elongate body that is laterally compressed throughout its life cycle. In the lateral view, the body has an ellipsoid shape (Figure 2A). The rhinophores are slightly longer than the head (Figure 2B), and a cephalic disc is absent. Pigmented dots are distributed throughout the body, with some specimens exhibiting concentrated bands along the dorsal and ventral margins (Figure 2A). The tail exhibits a rounded morphology, although some specimens exhibit a central convex curvature. Longitudinal muscle fibers are visible along the central axis of the body (Figure 3A). The foot is located in the posterior to the pharynx (see Figure 2A), and mucous cells are not observed (Figure 3B). The mean length of preserved organisms is 6.71 ± 1.93 mm (Table S1).

3.4.2. Internal Morphology

Most internal organs are in the anterior half of the body, with a slight overlap when viewed laterally (Figure 2A). The mandible is red and composed of two jaw plates (Figure 2C); the cutting edge of the jaw plates is smooth, with small, irregularly spaced spicules. The odontophore (Figure 2D and Figure 3D) has a radular formula of 14–15 × 1.1.1. The median tooth is denticulate on both sides of the central spine with six denticles, while the lateral teeth have interior denticulation (Figure 2D). The mandible opens into the triangular-shaped pharynx, whose epithelium consists of cuboidal to elongate cells; muscular layers surrounding pharyngeal epithelium with fibers oriented transversely (Figure 3C). The pharynx leads to the esophagus which connects to the stomach (Figure 2A). In life, these structures vary in coloration from translucent to shades of orange and pink among specimens.
The intestine is dark brown, short, extending horizontally towards the posterior. The anus is located laterally (pleuroproctic), positioned at about one-third of the body length from the head (Figure 2A,E,F). Three hepatic caeca extend from the stomach, two posterior and one anterior. All are brown with darker pigments and transparent in unfed specimens. The posterior hepatic caeca extend almost to the end of the tail, while the anterior hepatic caecum reaches the head of the organism (Figure 2A). The epithelium of the hepatic caeca has cuboidal to columnar cells (Figure 3F). The heart is located dorsally, above the stomach (Figure 2A). Two hermaphrodite organs are located between the posterior hepatic caeca and beneath and slightly posterior to the stomach. Due to overlapping, they often appear as one organ (Figure 2A,G). Each lobule consists of a solid central mass which has numerous gonad follicles (Figure 2G). In life, the central mass has a pinkish coloration, and the spherical follicles vary between yellow and orange. Lobules with oogonia lie in the periphery and spermatogonia in the median part (Figure 3E). The hermaphroditic duct is extremely long and often tangles with the hermaphroditic glands (Figure 2G).

3.5. Description of Swimming and Behavior

Live specimens exhibit active swimming behavior, utilizing lateral undulations to navigate the water (Video S1). When alive, these organisms demonstrate notable body plasticity, capable of transitioning from a nearly spherical shape to an elongated elliptical form. This flexibility results in substantial variability in total body length, with height-to-length ratios ranging from 20.7 to 47.4% in preserved specimens (Table S1). Defecation was observed in several individuals, confirming the presence of a laterally positioned anus (Video S2). Several live specimens were observed with elongated, thread-like strands of egg tissue. However, attempts to capture photographic evidence of these strands were unsuccessful.

3.6. Molecular Phylogenetic Analyses and Pairwise Genetics Distance Inferences

The molecular phylogeny was constructed using 24 sequences, each consisting of 1283 base pairs. The phylogenetic reconstruction employing maximum likelihood (ML) and Bayesian methods (BI) with mitochondrial (COI and 16S) and nuclear (H3) concatenated genes demonstrates moderate to high posterior probability and bootstrap values for the Dendronotoidea superfamily, within which are sequences belonging to the families Bornellidae, Dotidae, Lomanotidae, Hancockiidae, Dendronotidae, Scyllaeidae and Phylliroidae. Phylogenetic analyses placed the new species P. melaquensis and Phylliroe bucephala in a well-supported monophyletic clade that corresponds to the family Phylliroidae (Figure 4).
The genetic distance (K2P) calculated from COI-16s-H3 concatened genes among a selected group of Dendronotoidea superfamily species is summarized in Table 2. Among all intergeneric comparisons, the lowest genetic distance was found between P. melaquensis and Phylliroe bucephala from central California (0.032 ± 0.005). In contrast, P. melaquensis sp. nov. exhibited major genetic distances (≤0.420 ± 0.005) from species within other families (Table 2).

4. Discussion

In this study, we describe Pleuropyge melaquensis gen. et sp. nov., a previously unknown holoplanktonic nudibranch from the CMP. Morphological characteristics and molecular analyses support its placement within the family Phylliroidae, and clearly differentiate it from the known genera Phylliroe and Cephalopyge. The discovery of P. melaquensis expands our understanding of the morphological variation and degree of pelagic specialization within this little-known family, and highlights the existence of an intermediate form that bridges traits observed in Cephalopyge and Phylliroe.

4.1. Morphological and Molecular Evidence for Placement in Phylliroidae

Members of the family Phylliroidae are holoplanktonic, meaning they spend their entire life cycle in the open ocean [25]. The only two genera previously described in this family, Cephalopyge and Phylliroe, exhibit key adaptations for pelagic life. These include a laterally compressed and transparent body for camouflage, gelatinous tissues for buoyancy [5], and a reduction in benthic appendages such as the foot [25]. Furthermore, they possess a branched, visible digestive gland system, likely an adaptation for maximizing nutrient uptake in food-scarce environments [26]. These morphological traits are likewise present in P. melaquensis. In addition to the morphological characteristics, the swimming behavior of P. melaquensis is consistent with other members of Phylliroidae. Specimens exhibit active swimming behavior, utilizing lateral undulations to move through the water column (Table 3). This mode of locomotion is shared with all other members of the family and has been proposed as a key exaptation that facilitated the transition of Phylliroidae into pelagic environments [7].
Complementing these morphological and behavioral traits, molecular evidence further supports the inclusion of P. melaquensis within Phylliroidae. The phylogenetic analyses conducted in this study align with previously published transcriptome-based phylogenies for Cladobranchia [16,27] and gene-based studies using the 18S ribosomal and 16S mitochondrial markers for the superorder Nudipleura [28]. These studies, consistent with our results, place Phylliroidae within the superfamily Dendronotoidea. Importantly, our results place P. melaquensis within a well-supported clade that also includes Phylliroe bucephala, confirming its phylogenetic affiliation with the family. Taken together, these lines of evidence provide a robust basis for recognizing P. melaquensis as a member of Phylliroidae.

4.2. Diagnostic Features Supporting a New Genus and Species

In addition to the traits which justify its placement within Phylliroidae, P. melaquensis exhibits key diagnostic characters that distinguish it from the previously described genera in the family, supporting the establishment of a new genus. In comparison with Phylliroe spp., P. melaquensis differs notably in rhinophore morphology and digestive anatomy. Phylliroe possesses some of the largest rhinophores in the subclass Heterobranchia, extending toward the posterior half of the body [7], whereas in Pleuropyge they are relatively short, extending only slightly beyond the head. The structure of the digestive system also differs: Phylliroe has four hepatic caeca, with two located anteriorly and two posteriorly [7], while Pleuropyge possesses only three, comprising one anterior and two posterior caeca (Table 3).
A comparison of P. melaquensis with the monospecific species of Cephalopyge (C. trematoides) reveals several additional differences. One of the most prominent is the position of the anus. Cephalopyge retains the ancestral condition, with the anus positioned dorsally, just behind the rhinophores [14,15]. In contrast, Pleuropyge exhibits a laterally positioned anus (pleuroproctic), approximately one-third of the body length from the head. Another important distinction is the absence of a cephalic disc in Pleuropyge, a structure that is consistently observed in C. trematoides. Although its function in Cephalopyge remains unknown, in other nudibranchs it has been associated with sensory and feeding roles [29,30,31].
Further morphological differences between Pleuropyge and Cephalopyge include the shape of the tail, proportions of the hepatic ducts, and the number of hermaphrodite lobules. While previous authors have noted that such traits may vary due to contractility, specimen size, starvation, or fixation [14,15], one feature that appears stable and taxonomically informative is the relative length of the anterior hepatic caecum and the intestine. In all examined specimens of P. melaquensis, the anterior hepatic caecum was longer than the intestine. By contrast, in C. trematoides, the intestine is typically longer, and a 1:1 ratio is the closest reported. The consistency of this trait across specimens suggests its diagnostic value.
Another trait that may hold species-level relevance is the number of hermaphrodite lobules. While C. trematoides has been reported to exhibit between two and seven lobules [14,15], all examined specimens of P. melaquensis consistently displayed exactly two. Although this falls within the reported range for Cephalopyge and thus cannot be considered diagnostic, its uniformity in Pleuropyge suggests a potentially informative difference that merits further investigation.
The molecular analyses provide further support for the recognition of Pleuropyge as a distinct genus. Our phylogenetic results indicate a close relationship between Pleuropyge and Phylliroe but resolve them as separate lineages. Although molecular data for Cephalopyge were not available for inclusion in our analyses, the clear morphological distinctions outlined above support the exclusion of P. melaquensis from that genus. Given the pronounced morphological similarities among the three genera, we anticipate that future molecular data will place Cephalopyge within the same clade as Pleuropyge and Phylliroe. However, based on the current combination of molecular and morphological evidence, Pleuropyge is recognized as a distinct lineage within Phylliroidae.
The observed low genetic diversity (3%) between Pleuropyge and Phylliroe can be explained by the fact that nudibranchs with planktonic life stages have the capacity to disperse over large distances in open water. This phenomenon results in extensive gene flow and minimal genetic divergence within lineages, as evidenced in cosmopolitan species such as Fiona pinnata and Glaucus atlanticus, where populations maintain genetic homogeneity due to their capacity to traverse oceans via floating substrates [32,33].
With regard to the phenomenon of divergence, recent molecular and phylogenetic studies have demonstrated that some families of nudibranch have undergone evolutionary divergence and incomplete lineage classification, resulting in low genetic distances between genera. This has resulted in low levels of genetic difference between closely related species and genera, particularly when utilizing mitochondrial markers such as the COI and 16s gene. For instance, Eubranchus malakhovi and its closely related species exhibit minimal mitochondrial DNA divergence, which is indicative of a recent divergence approximately 1.5 million years ago [34]. Moreover, research conducted on Dorid nudibranchs has demonstrated that their mitochondrial genomes exhibit a high degree of conservation, characterized by minimal variation in gene content and structure across different species [35,36]. This phenomenon contributes further to the observed low level of genetic divergence. In groups exhibiting high morphological plasticity (i.e., Phylliroidae), the use of different molecular markers (mitochondrial and nuclear) can yield varying levels of resolution, with certain markers potentially failing to capture the complete extent of divergence at the genus level. Consequently, a genetic distance of 3% is consistent with the observed patterns in nudibranch systematics, thereby underscoring the necessity for integrative approaches that combine molecular, morphological, and ecological data for robust genera delimitation.

4.3. Morphological Gradient and Pelagic Adaptations

Beyond discrete diagnostic traits, a broader comparison of body plan features across Cephalopyge, Pleuropyge, and Phylliroe reveals a gradient of morphological specialization for pelagic life. One particularly informative feature is the position of the anus, a trait often used to infer phylogenetic relationships in gastropods [37]. During development, gastropods undergo a torsion process that rotates the body 180°, typically placing the anus above the head [38]. In nudibranchs, this torsion is frequently reversed, a change thought to enhance mobility and improve sensory integration in benthic or pelagic environments [39,40,41,42]. Cephalopyge retains the ancestral dorsally located anus, positioned just behind the rhinophores, whereas both Pleuropyge and Phylliroe exhibit a laterally positioned anus, which may reflect a condition associated with adaptation to pelagic life [43,44].
Another trait that varies among the three genera is the configuration of the hepatic caeca. Cephalopyge and Pleuropyge both possess one anterior and two posterior caeca, while Phylliroe has two caeca located anteriorly and two posteriorly [14,15] (This study). The presence of an additional anterior caecum in Phylliroe may offer enhanced digestive capacity, an advantage in nutrient-poor pelagic environments where efficient nutrient absorption is critical [45].
Rhinophore morphology also differs and may have implications for swimming and sensory function. In Phylliroe, the rhinophores are notably elongated, extending toward the posterior half of the body, and may have been proposed to aid in swimming stability [7] or sensory detection in the water column [29]. In contrast, the rhinophores of Cephalopyge and Pleuropyge are smaller and remain confined to the anterior portion of the body.
Taken together, these morphological traits suggest that P. melaquensis represents an intermediate form within Phylliroidae. It retains several ancestral characteristics observed in Cephalopyge, while also expressing features more like the highly specialized Phylliroe. Identifying such intermediate forms is essential for resolving the functional and ecological diversification of pelagic nudibranchs and highlights the importance of continued morphological and molecular exploration within this group.

5. Conclusions

The description of Pleuropyge melaquensis gen. et sp. nov. adds a third genus and fourth species to the family Phylliroidae and highlights the hidden diversity that exists among holoplanktonic nudibranchs, which is often overlooked due to the loss of diagnostic characteristics following fixation. By integrating morphological, histological, molecular, and behavioral data, this study provides new insight into the functional and structural traits that have enabled nudibranchs to transition from benthic to pelagic environments. The distinct combination of ancestral and derived traits observed in P. melaquensis gen. et sp. nov. also underscores the potential existence of undescribed or misclassified species within this family. Future research, including the inclusion of molecular data for Cephalopyge, additional specimens of Phylliroe, and comparative material from across the species ranges, will be essential to resolve phylogenetic relationships and fully understand the diversity within Phylliroidae.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/d17070479/s1.

Author Contributions

The authors confirm their contribution to the paper as follows. Study conception and design: C.F.-G., E.R.K., E.B.-G., J.D.S.-V. and G.P.-M. Data collection and methodology: C.F.-G., E.R.K., E.B.-G., J.D.S.-V. and G.P.-M. Analysis and interpretation of results: E.B.-G., J.D.S.-V. and E.R.K. Draft manuscript preparation: C.F.-G., E.R.K., E.B.-G. and J.D.S.-V. Funding acquisition: C.F.-G. All authors have read and agreed to the published version of the manuscript.

Funding

The present study was funded by the Programa de Apoyo a la Mejora en las Condiciones de Producción de los Miembros del SNI y SNCA–PROSNI, the Programa de Fortalecimiento de Institutos, Centro y Laboratorios de Investigación 2022 of the Universidad de Guadalajara, México and research support was also provided by the National Geographic Society (NGS-100354C-23). The University of Guadalajara provided research funds for field sampling on board the “León Marino”.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

Data are contained within the article and Supplementary Materials.

Acknowledgments

The authors would like to express their gratitude to Armando Alvizar-Martínez for his technical support during the zooplankton sampling and José Angel Cohuo Colli for his support in storing the samples in the “Zooplankton Collection of El Colegio de la Frontera Sur” (ECO-CHZ). J.D.S.-V. was supported by a postdoctoral fellowship (SECIHTI; ID: 619025). Also, the authors would like to express their gratitude to the Laboratorio de Posgrado of the Centro Universitario de la Costa Sur-UDG” and the Laboratorio de Ecologia Marina of the Centro Universitario de la Costa-UDG, for the use of facilities for molecular and histological processing, and the anonymous reviewers for their valuable contributions that helped to improve the manuscript.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Figure 1. Sites with records of organisms from the Phylliroidae family in the Eastern Tropical Pacific. Red points: Collection locations in this study: NAVI: Bahía Navidad; LAB: Laboratorio in Bahía Cuastecomates, MANZ: Bahía Manzanillo, CMP: Central Mexican Pacific. Blue points: Previously reported occurrences of Phylliroidae: CO: Colombia, CH: Chile, GC: Golfo de California; PE: Peru.
Figure 1. Sites with records of organisms from the Phylliroidae family in the Eastern Tropical Pacific. Red points: Collection locations in this study: NAVI: Bahía Navidad; LAB: Laboratorio in Bahía Cuastecomates, MANZ: Bahía Manzanillo, CMP: Central Mexican Pacific. Blue points: Previously reported occurrences of Phylliroidae: CO: Colombia, CH: Chile, GC: Golfo de California; PE: Peru.
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Figure 2. External and internal morphology of Pleuropyge melaquensis gen. et sp. nov. (A) lateral view. Scale = 1000 µm (B) Anterior part and rhinophores. Scale = 500 µm (C) Mandible. Scale = 200 µm (D) Odontophore. Scale = 100 µm (E,F) Anus. Scale = 500 and 200 µm (G) reproductive system. Scale = 500 µm. a: anus; ag: accessory glands; ahc: anterior hepatic caecum; e: oesophagus; f: foot; g: gonads; h: heart; hd: hermaphroditic duct; ko: kidney organ; m: mandible; phc: posterior hepatic caecum; r: rhinophores; s: stomach; ag: accessories gland, t, teeth.
Figure 2. External and internal morphology of Pleuropyge melaquensis gen. et sp. nov. (A) lateral view. Scale = 1000 µm (B) Anterior part and rhinophores. Scale = 500 µm (C) Mandible. Scale = 200 µm (D) Odontophore. Scale = 100 µm (E,F) Anus. Scale = 500 and 200 µm (G) reproductive system. Scale = 500 µm. a: anus; ag: accessory glands; ahc: anterior hepatic caecum; e: oesophagus; f: foot; g: gonads; h: heart; hd: hermaphroditic duct; ko: kidney organ; m: mandible; phc: posterior hepatic caecum; r: rhinophores; s: stomach; ag: accessories gland, t, teeth.
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Figure 3. Pleuropyge melaquensis gen. et sp. nov. histology. (A) Longitudinal section of muscle fibers (arrow). Scale = 20 µm (B) Longitudinal section of the foot. Scale = 200 µm (C) Longitudinal section of the pharynx. Scale = 50 µm (D) Longitudinal section of the mandible (arrows: jaw plates). Scale = 100 µm (E) Cros-section in posterior half of body. Scale = 100 µm (F) Longitudinal section of the hepatic caecum. Scale = 50 µm. dg: digestive gland; oc: oocyte s: spermatid.
Figure 3. Pleuropyge melaquensis gen. et sp. nov. histology. (A) Longitudinal section of muscle fibers (arrow). Scale = 20 µm (B) Longitudinal section of the foot. Scale = 200 µm (C) Longitudinal section of the pharynx. Scale = 50 µm (D) Longitudinal section of the mandible (arrows: jaw plates). Scale = 100 µm (E) Cros-section in posterior half of body. Scale = 100 µm (F) Longitudinal section of the hepatic caecum. Scale = 50 µm. dg: digestive gland; oc: oocyte s: spermatid.
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Figure 4. Systematic position of Pleuropyge melaquensis gen. et sp. nov. recovered from analysis of Bayesian inference (BI) and the maximum likelihood (ML), using concatenated mitochondrial (COI, 16S mtDNA) and nuclear (H3) gene sequences. The values of each node correspond to the posterior probability and bootstrap values of major clades that were reconstructed with BI and ML strict consensus tree, respectively (BI/ML). The green box delineates the Phylliroidae family. New species sequences are in bold. GenBank accession numbers are provided in Table S2.
Figure 4. Systematic position of Pleuropyge melaquensis gen. et sp. nov. recovered from analysis of Bayesian inference (BI) and the maximum likelihood (ML), using concatenated mitochondrial (COI, 16S mtDNA) and nuclear (H3) gene sequences. The values of each node correspond to the posterior probability and bootstrap values of major clades that were reconstructed with BI and ML strict consensus tree, respectively (BI/ML). The green box delineates the Phylliroidae family. New species sequences are in bold. GenBank accession numbers are provided in Table S2.
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Table 1. Abundance and environmental data for Phylliroidae from sampling sites in the Central Mexican Pacific. NAVI: Bahía Navidad; LAB: Laboratorio in Bahía Cuastecomates, MANZ: Bahía Manzanillo.
Table 1. Abundance and environmental data for Phylliroidae from sampling sites in the Central Mexican Pacific. NAVI: Bahía Navidad; LAB: Laboratorio in Bahía Cuastecomates, MANZ: Bahía Manzanillo.
DateSampling Site# IndividualsInd/m3Temperature (°C)
14 March 2018NAVI140.0825.5
17 March 2018MANZ50.5424.7
26 April 2018LAB9166.2224.1
20 May 2019LAB40.0725
21 May 2019LAB40.1725
Table 2. Pairwise genetic distance (K2P) matrix among selected Dendronotoidea species using mitochondrial (COI, 16S mtDNA) and nuclear (H3) concatenate genes (1283 pb). The matrix displays values of genetic distances (lower left diagonal) and standard error estimated(s) (upper right diagonal).
Table 2. Pairwise genetic distance (K2P) matrix among selected Dendronotoidea species using mitochondrial (COI, 16S mtDNA) and nuclear (H3) concatenate genes (1283 pb). The matrix displays values of genetic distances (lower left diagonal) and standard error estimated(s) (upper right diagonal).
COI-16S-H3123456789101112131415161718
1Pleuropyge melaquensis 0.0050.0310.030.0320.030.0310.030.0290.0340.0330.0290.0330.030.0290.030.030.034
2Phylliroe bucephala0.032 0.030.030.0310.030.0310.0310.030.0360.0350.0290.0340.0290.0290.0290.030.035
3Bornella valdae0.4400.426 0.0130.0150.0150.0150.0150.0180.0180.0180.0160.0190.0160.0180.0160.0180.018
4Bornella hermanni0.4030.3980.144 0.0140.0170.0170.0160.0170.0180.0180.0180.0170.0170.0170.0170.0180.019
5Bornella johnsonorum0.4340.4280.170.175 0.0170.0170.0170.0170.0190.0170.0160.0190.0180.0170.0180.0170.018
6Doto millbayana0.4050.4090.1840.2060.203 0.0020.0050.0120.0130.0120.0120.0160.0160.0160.0160.0170.017
7Doto dunnei0.4050.4110.1840.2080.2080.005 0.0060.0120.0130.0120.0110.0160.0160.0160.0160.0170.017
8Doto coronata0.4080.4220.1720.2030.1960.0350.036 0.0120.0130.0130.0120.0180.0150.0170.0160.0170.018
9Doto antarctica0.3840.3940.2110.2140.2150.1190.1170.121 0.0120.0110.0090.0170.0170.0170.0170.0160.017
10Doto amyra0.4330.4480.2330.2370.2470.140.1380.1420.128 0.0090.0130.020.0170.0170.0180.0170.017
11Doto ussi0.4410.4540.2360.2370.2350.1430.1410.1460.1190.091 0.0110.0190.0180.0180.0180.0170.017
12Doto greenamyeri0.3950.3970.2010.2160.1990.120.1170.1210.0770.140.118 0.0150.0170.0170.0160.0160.016
13Lomanotus vermiformis0.4490.4590.2350.2260.2380.1980.1990.210.2070.2260.2140.18 0.0170.0170.0170.0170.02
14Hancockia californica0.3960.4020.2140.2190.2190.1790.1820.1710.1880.1920.2040.1910.191 0.0170.0160.0160.018
15Dendronotus regius0.390.4020.2090.2140.2110.1780.1790.1860.1970.2070.2220.1940.2140.207 0.0120.0130.018
16Dendronotus kalikal0.4130.4080.2030.2060.2140.1790.1830.1850.1890.2060.2040.1810.2190.1870.137 0.0080.015
17Dendronotus frondosus0.4140.4140.2170.2220.2220.1920.1950.2020.1830.1990.1950.1810.2160.1970.1460.069 0.014
18Notobryon thompsoni0.4440.4630.240.2360.2230.2150.2120.2270.2030.210.210.1910.2420.2130.2110.1840.18
19Notobryon wardi0.4230.420.230.240.2320.2090.2070.2110.2120.2180.2250.1960.2270.2170.1970.1860.1820.06
20Scyllaea fulva0.4590.4770.2210.240.2150.2060.2040.1990.2280.2570.2460.2230.2330.2010.1950.1870.1840.188
21Scyllaea pelagica0.4540.4720.2210.2380.2180.2040.2020.1990.2260.2570.2460.2230.2350.2040.1940.1860.1840.188
Table 3. Morphological comparison between Phylliroidae species. * refers to a data reported in 15.
Table 3. Morphological comparison between Phylliroidae species. * refers to a data reported in 15.
GenusPleuropygeCephalopygePhylliroe
SpeciesP. melaquensisC. trematoidesP. bucephalaP. lichtensteinii
Average body length6.71 ± 1.93 mm~25–10 mm~20–40 mm~12–15 mm
Body formLong, flattened ellipseLong, flattened ellipseFlattened, leaf or fish-likeFlattened, leaf or fish-like
Cephalic discAbsentLocated dorsally above mouthAbsentAbsent
Rinophores contractileContractile, slightly larger than the headContractile, slightly larger than the headNon-contractile, extend toward the posterior half of the bodyNon-contractile, extend toward the posterior half of the body
Radula1.1.11.1.16.1.66.1.6
Buccal massAbsentAbsentPresentPresent
Number of hepatic caecaThreeThreeFourFour
Location anusOn the left, laterally positioned at about one-third of the body length from the headDorsal, behind rhinophoresOn the right, mid-bodyOn the right, mid-body
Number of hermaphroditic organs22–7 (5 *)22–3
Bioluminescent cellsAbsentAbsentPresentPresent
Swimming BehaviorEntire bodyEntire bodyCaudal finCaudal fin
ReferenceThis study14, 157, 3636, 43
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Santiago-Valentín, J.D.; Bautista-Guerrero, E.; Kozak, E.R.; Pelayo-Martínez, G.; Franco-Gordo, C. A New Holoplanktonic Nudibranch (Nudibranchia: Phylliroidae) from the Central Mexican Pacific. Diversity 2025, 17, 479. https://doi.org/10.3390/d17070479

AMA Style

Santiago-Valentín JD, Bautista-Guerrero E, Kozak ER, Pelayo-Martínez G, Franco-Gordo C. A New Holoplanktonic Nudibranch (Nudibranchia: Phylliroidae) from the Central Mexican Pacific. Diversity. 2025; 17(7):479. https://doi.org/10.3390/d17070479

Chicago/Turabian Style

Santiago-Valentín, Jeimy D., Eric Bautista-Guerrero, Eva R. Kozak, Gloria Pelayo-Martínez, and Carmen Franco-Gordo. 2025. "A New Holoplanktonic Nudibranch (Nudibranchia: Phylliroidae) from the Central Mexican Pacific" Diversity 17, no. 7: 479. https://doi.org/10.3390/d17070479

APA Style

Santiago-Valentín, J. D., Bautista-Guerrero, E., Kozak, E. R., Pelayo-Martínez, G., & Franco-Gordo, C. (2025). A New Holoplanktonic Nudibranch (Nudibranchia: Phylliroidae) from the Central Mexican Pacific. Diversity, 17(7), 479. https://doi.org/10.3390/d17070479

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