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Article

Assessment of Non-Sessile Invertebrates Associated with Mats of the Red Alga Phyllophora crispa at Giglio Island, Mediterranean Sea

by
Alexander Töpfel
1,*,
Melissa Steinhoff
2 and
Christian Wild
1
1
Marine Ecology Department, Faculty of Biology and Chemistry, University of Bremen, 28350 Bremen, Germany
2
Marine Botany Department, Faculty of Biology and Chemistry, University of Bremen, 28350 Bremen, Germany
*
Author to whom correspondence should be addressed.
Diversity 2025, 17(10), 728; https://doi.org/10.3390/d17100728
Submission received: 29 August 2025 / Revised: 25 September 2025 / Accepted: 14 October 2025 / Published: 17 October 2025
(This article belongs to the Section Marine Diversity)
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Abstract

The Mediterranean Sea hosts highly diverse habitats such as Posidonia oceanica meadows, coralligenous communities, and gorgonian forests. Stressors including warming, eutrophication, pollution, and overfishing are driving shifts towards algae-dominated systems, often with reduced biodiversity. Among these, recent research surprisingly revealed that the mat-forming red alga Phyllophora crispa, which overgrows seagrass and gorgonian habitats, supports high sessile invertebrate diversity. However, little is known about its associated non-sessile fauna. This study thus investigated non-sessile invertebrates in P. crispa using a newly designed appropriate sampling technique at two study sites around Giglio Island (Italy), Fenaio, and Secca II (distance ca. 600 m from each other). Across all samples, 5464 organisms were identified, mostly to family level. We recorded 169 non-sessile taxa, including 96 families, 41 copepod morphotypes, 21 ostracod morphotypes, and 11 unclassified taxa. The dominant phyla were Arthropoda (67%), Mollusca (14%), Annelida (9%), and Nematoda (5%). The most abundant families were Calliopiidae (Amphipoda), Leptognathiidae (Malacostraca), and Mytilidae (Bivalvia). Of the 169 taxa, 128 occurred at both sites, while 20 were unique to Fenaio and 21 to Secca II, suggesting high connectivity likely linked to mobility. Organism abundances ranged from 1315 to 5759 individuals per m2 seafloor. Diversity indices were as follows: Shannon 1.5–3.4, Simpson 0.6–1.0, and Pielou 0.6–0.9. These values are similar or even exceed previously reported values for sessile invertebrates (Shannon 2.2–2.5). Notably, P. crispa supported diversity levels higher than those reported for seagrass meadows (Shannon 2.0–2.1) and even tropical coral reefs (2.0). Our study thus confirms P. crispa as a biodiversity hotspot and suggests that these algae mats should be considered in biodiversity conservation strategies.

1. Introduction

The Mediterranean Sea is the largest inland sea on earth, with only one natural connection to the Atlantic, the Strait of Gibraltar, and one man-made connection to the Red Sea, the Suez Canal [1]. Despite its geological isolation from the surrounding oceans, it harbors a great biodiversity with an estimated 17,000 eukaryotic species, which in comparison to world estimates make up between 4% and 18% of the world’s marine biodiversity [2], while only representing 0.82% of the world’s surface area and 0.32% of the world’s oceans volume [3,4]. Due to its geological isolation from neighboring oceans, the Mediterranean Sea is home to a remarkable biodiversity and a high degree of endemism estimated at around 20% [4,5,6]. It thereby represents a special location where driving factors of biodiversity loss have much more drastic effects compared to other habitats or locations [6]. A natural driver of biodiversity change in the Mediterranean is the immigration of Atlantic species, whereas Lessepsian immigrants are generally considered as anthropogenically induced. Global warming constitutes another pivotal factor, as rising temperatures may force species to shift poleward in search of cooler waters. Such movements can create critical conditions in semi-enclosed basins like the Mediterranean Sea, where natural barriers restrict the northward migration of southern species that would otherwise disperse continental margins. Additionally, pollution, overexploitation, and habitat degradation further exacerbate biodiversity change in the region [6,7]. Its role as a biodiversity hotspot is achieved through habitat heterogeneity and structural complexity of its habitats.
Next to gorgonian forests characterizing the seascapes of coralligenous communities [8], coralligenous concretions—unique calcareous formations of biogenic origin produced by the accumulation of encrusting algae growing under dim light conditions—represent another key structural component of Mediterranean benthic environments [9]. One of the best-studied ecosystem engineers in the Mediterranean Sea is the endemic seagrass Posidonia oceanica (L.) Delile, 1813, which forms extensive meadows from 0 to 45 m water depth [10]. P. oceanica meadows are biodiversity hotspots through providing permanent habitat structure, spawning ground, and nursery for many associated species [11,12,13]. Seagrass meadows also contribute to nutrient cycles and the production of detrital material [14].
Some of these biodiversity hotspots are declining at an accelerating pace in recent decades, e.g., P. oceanica meadows have lost between 13 and 38% of their areal extent, while the overall cover and shoot density have been thinning out in the remaining areas [10]. The receding of this ecosystem happens due to the combined effects of multiple stressors [15]. Temperatures above ~28 °C reduce the photosynthetic rate of P. oceanica, resulting in an increase in respiration in relation to photosynthesis, which can lead to a negative carbon balance [16,17]. Sea surface temperatures (SSTs) are usually highest in the eastern Mediterranean Sea, with SSTs of up to 30 °C in August 2023, compared to 28 °C in the central Mediterranean Sea and 24 °C near the Strait of Gibraltar [18]. Above all, rising SSTs oppose thermal stress, damage the seagrass, and trigger P. oceanica shoot mortality [19,20]. The Anthropocene [21] has caused negative impacts on most marine ecosystems, with threats like habitat loss and degradation being the main impact drivers in the Mediterranean Sea [6,9,22]. These stressors can change the dynamics of a community, causing the system to transition into an alternative state characterized by a prevalence of more tolerant species [23,24]. These so-called phase shifts are typically associated with consequences, such as the loss of ecosystem functioning, structural complexity [25], and ultimately a change in community composition [26] and loss of biodiversity [23,27].
However, this does surprisingly not necessarily apply to all new algal habitats, as recent studies show high habitat complexity [28] and high biodiversity [29] associated with the fleshy, mat-building red alga Phyllophora crispa (P.S.) Dixon 1964. P. crispa shows opportunistic traits, as it can survive and proliferate even when detached and drifting in the water column [30]. Following the Costa Concordia shipwreck in 2012, dense P. crispa mats were reported more around Giglio Island, suggesting that large-scale disturbance events can create favorable conditions for its expansion [29]. These mats subsequently established P. crispa as a mat-forming ecosystem engineer, capable of reshaping benthic community structure and influencing local biodiversity patterns [31]. Existing studies thus indicate that P. crispa mats are expanding not only around Giglio, even as environmental stressors are rising. This algal assemblage has gained increased attention in recent years and was observed overgrowing already damaged P. oceanica meadows and hard-bottom communities, thus being the new dominant state during/after a phase shift [32].
P. crispa acts as an engineering species, forming continuous mats or turfs in sciaphilic habitats and reaching a thickness of up to 15 cm, while growing on hard substrates [29,31]. It occurs across the Mediterranean [33,34], the Black Sea [35,36], and the Atlantic [34,37] and is, together with P. oceanica meadows and coralligenous communities, one of the most abundant habitats in this area [38]. Not only do P. crispa mats have a profound impact on environmental parameters such as water movement, light intensity, and temperature [31], but also harbor a great diversity of sessile invertebrates [28]. With only a few assessments of Mollusca [39] and Annelida, Mollusca, and Arthropoda [29], a comprehensive study of non-sessile invertebrates remains missing. With this work, we aim to answer the following research questions:
  • What are the abundances and diversity of non-sessile invertebrates associated with P. crispa?
  • Which are the most abundant families in P. crispa mats?
  • Are there spatial differences in associations with P. crispa?
To answer these research questions, a field campaign in 2023 was conducted at Giglio Island in the Italian Mediterranean Sea in order to use a newly developed sampling device to assess the diversity of non-sessile invertebrates associated with P. crispa mats.

2. Materials and Methods

2.1. Study Area and Sampling Activities

The study was conducted from 4 June to 25 June 2023 on the island of Giglio, in the Tuscan Archipelago National Park (42°21′57″ N, 10°54′7″ E, Tyrrhenian Sea), Italy. While the coast is made up of rocks and smooth cliffs, the underwater seabeds around the island are characterized by granite slopes, next to sandy bottoms, P. oceanica meadows, coralligenous habitats, and different algal mats. For this study, the two study sites Secca II and Fenaio (Figure 1) were chosen as sampling sites because of similar current conditions as well as good accessibility throughout the whole season. Currents in the study area were qualitatively observed during SCUBA diving, generally flowing out of the Bay of Campese. Their occurrence is further supported by the presence of different gorgonians (Eunicella cavoliniid, Paramuricea clavata), which require sustained water movement for suspension feeding [40]. Temperature and salinity in the area are typically 18 °C daily mean and 38 PSU [28]. Furthermore, P. crispa occurs on both study sites at similar depths. SCUBA divers collected all samples at a depth of 31.7 ± 4.2 m with eight replicates taken at each site, resulting in a total of n = 16 samples.
Samples were taken using a PAM collector (surface area 201.06 cm2), which was designed and tested in a preliminary study (see Figure A1 in Appendix A). It is closable at the top and the bottom and was designed to minimize the loss of organisms during the sampling and transportation process. At the sampling location, the sampling spot was chosen randomly by the sampling diver, and P. crispa was sampled when the mat displayed a thickness of at least 5 cm, following the threshold applied in previous assessments of P. crispa [38,41,42]. The sampling diver put his hand through the net, carefully removed the algae from the substrate, closed the collector at the top and the bottom, and transferred the whole collector containing the sample into a transport barrel. All samples were carefully transported to a designated room at the Institute for Marine Biology (IfMB, located within Campese) immediately after sampling. The room was kept at a constant 20 °C, and a day–night rhythm was ensured by activating the lights from 7:30 to 8:30 am and deactivating them between 7:30 and 9:00 pm. Samples were kept in an aerated tub, and the analysis was performed within 48 h, starting from the time of sample collection.

2.2. Species Identification

The sample was divided into thirds for further analysis, and each third was analyzed by one person. The non-sessile invertebrate macrofauna was removed and roughly categorized using a binocular to reduce the stress of anesthesia on the invertebrates as much as possible. Subsequently, the subsamples were pooled and sieved with a 55 µm meiofauna net. The residue was transferred along with the algal material into a smaller 1.8 L plastic bowl. Six tablespoons of magnesium chloride (MgCl2), each 25 g, were added to the bowl. Ca. 15 min later, the content of the bowl was stirred, and after an additional 15 min, it was sieved by using a 500 µm net. The residue was transferred into another bowl and again divided into thirds. Organisms were presorted and afterwards identified to at least family level, except for Copepoda, Ostracoda, and Nematoda, which were only identified to superorder, order, and phylum level, respectively. For the groups of Copepoda and Ostracoda, morphotypes were created based on pictures taken via digital photomicrography. For the rest of the organisms, pictures of common specimens were taken as well. Mytilid bivalves found in P. crispa samples were included in the analyses, as many species are sedentary and are considered not strictly sessile [39], as they are able to detach and reattach themselves, and thus do not fully conform to the definition of sessility. Moreover, some of the mollusk families may have been underestimated in previous studies due to limitations of earlier sampling techniques. Our approach, by contrast, allows a more accurate representation of the assemblage structure. For identification of families, relevant scientific papers and literature were used (see Table A1 in Appendix A). After identification, the wet weight of the sample was measured by shaking the thalli to get rid of excess water and then weighing it.
In order to calculate the surface area of P. crispa samples and correlate the results to the data, the algal thalli were freed from epiphytes, laid out flat on graph paper, and flattened with a transparent plastic pane. A picture of the flattened thalli was taken using a camera (Canon Powershot G7X Mark II, Canon Inc., Tokyo, Japan), which was mounted to a fixed tripod. The following fixed settings were used to take the photos: F4.0, ISO 200, 1/4. The surface area was then calculated using ImageJ (version 1.53a), and the resulting surface area was multiplied by two to take both sides of the thalli into account. The calculations of organism densities were carried out incorporating the calculated surface areas and the counting of organisms.

2.3. Diversity Descriptors

Non-sessile invertebrate diversity was assessed using three descriptors: Shannon diversity index, Simpson diversity index, and Pielou evenness index. The indices were calculated as means per site (Appendix A, Formulas (A1)–(A3)). Values for Copepoda and Ostracoda were calculated separately due to identification at different taxonomic levels compared to the rest of the data. Statistical differences in families between sites, as well as differences in diversity descriptors between sites, were assessed using Wilcoxon signed-rank tests. Data were organized using Excel (version 16.07) while plots and analyses were made with RStudio (version 2025.05.0+496).

3. Results

3.1. Analyses of Non-Sessile Invertebrate Assemblages

A total of 5464 specimens were found across all samples, containing 169 non-sessile taxa, including 96 families, an additional 41 different morphotypes of Copepoda, 21 different morphotypes of Ostracoda, and 11 unclassified phyla. The most abundant phyla within P. crispa mats were Arthropoda (67% of total organisms found), Mollusca (14%), Annelida (9%), and Nematoda (5%), followed by Echinodermata, Chaetognatha, Nemertea, Plathelminthes, and Cnidaria (all < 2%) (Figure 2). Data for both study sites were pooled because there were only minor differences between the sites for all of the phyla. The largest share of Arthropoda was Copepoda (1114) and Ostracoda (816), together making up 35% of all organisms. These two groups were not identified to order level because the pictures that were taken were not precise enough to display the morphological traits needed for clear identification.
Of the 5464 specimens found, 3058 were identified at least to family level. Within these, the most abundant families across all samples were Calliopiidae (Amphipoda), Leptognathiidae (Malacostraca), and Mytilidae (Bivalvia). All of these families were significantly more abundant at Secca II compared to Fenaio (Figure 3). All families of Amphipoda were confirmed as benthic, often found associated with macroalgae or sediments. The phylum Arthropoda also displayed the largest number of identified families (42), followed by Mollusca (30), Annelida (13), and Echinodermata (6). Densities of non-sessile specimens across the two sites varied between 1315 and 5759 individuals per m2 seafloor, with the lowest number calculated at Fenaio and the highest at Secca II. The densities differed significantly, with 4316 ± 769 individuals per m2 seafloor at Secca II and 2726 ± 1099 individuals per m2 seafloor at Fenaio (p = 0.005). A multivariate analysis of the entire community showed no significant difference between the sites (PERMANOVA, F = 1.51, R2 = 0.008, p = 0.142). When examining the distribution of families between the sites, we found substantial overlap, with 128 out of a total 169 taxa present at both sites. Out of these 169, 20 taxa, including 10 families, 4 not further identified classes, and 6 morphotypes, were only found at Fenaio, while 21 taxa, including 15 families and 6 morphotypes, were exclusively found at Secca II (Figure 4). The families and morphotypes exclusive to Fenaio were Epialtidae, Processidae, Pasiphaeidae, Anthuridae, Ampharetidae, Chromodoridae, Polyceridae, Muricidae, Asterinidae, Toxopneustidae, non-identified (non-ident.) Bivalvia, non.-ident. Crinoidea, non-ident. Ophiuroidea, non-ident. Cnidaria, and Podoplea 13, 15, 16, 37, 38, 42. Exclusively found at Secca II were Palaemonidae, Mysidae, Phliantidae, Cyproideidae, Ampeliscidae, Corophiidae, Idoteidae, Tanaididae, Orbiniidae, Amphinomidae, Naticidae, Eulimidae, Turbinidae, Siphonariidae, Parechinidae, Podoplea 28, 35, 39, Gymnoplea 3, 4, and Podocopida 19.

3.2. Diversity Descriptors

The highest diversity indices were observed at Fenaio, with the highest values of Shannon (3.2) and Simpson (0.9). Values for the evenness (Pielou’s index) were similar across all groups and sites (average 0.8 ± 0.1), with the lowest value recorded for Ostracoda at Secca II (0.8) and the highest for Copepoda at Secca II (0.9). Shannon indices differed noticeably between Copepoda, Ostracoda, and the remaining groups investigated, with average values for Copepoda of 2.5, Ostracoda of 2.0, and for the remaining groups of 3.0 at Secca II and 3.2 at Fenaio (average 3.1) (Table 1).

4. Discussion

P. crispa mats are capturing increased attention in recent years as this habitat-building alga [29] is displacing several traditional habitats in the Mediterranean Sea, such as rocky hard-bottom communities and seagrass meadows, primarily due to anthropogenically induced environmental stressors. The loss of these traditional habitats potentially causes a loss of biodiversity [28]; however, P. crispa has been found to foster exceptional invertebrate biodiversity [28,29,38]. Our results show a high density and diversity of non-sessile invertebrates associated with P. crispa mats and give a first in-depth look into the non-sessile invertebrate diversity associated with the red alga.

4.1. Composition of Non-Sessile Invertebrate Assemblages in P. crispa Mats

Similarly to a previous study of Bonifazi et al. [29], we found Arthropoda, Mollusca, and Annelida among the most abundant phyla. However, in our study, Arthropoda were the most abundant phylum, while Bonifazi et al. [29] found Annelida (Polychaeta) as the most abundant. Differences occur likely due to the different sampling techniques. As Bonifazi et al. [29] used 20 cm × 20 cm quadrates without any nets attached, non-sessile organisms could escape, leading to inaccurate results. We used a collector equipped with a net to prevent organisms from escaping, suggesting that our results achieve higher accuracy in classifying organisms into their respective phyla. Arthropoda also appear in Cystoseira habitats as the most abundant phylum [55]. In our study, the most abundant families across all samples were Calliopiidae (Amphipoda), Leptognathiidae (Malacostraca), and Mytilidae (Bivalvia), which is different in some regards to previous assessments of mollusks associated with P. crispa, as they found Gastropoda instead of Bivalvia to be the most abundant group of mollusks [39]. Similarly to P. crispa, Cystoseira habitats in the Aegean Sea consisted of the order Amphipoda as the most dominant taxon, with 36 different species, followed by Isopoda (23) and Decapoda (19) [56]. Interestingly, more than a third of all specimens belonged to Copepoda and Ostracoda. Copepoda serve as major grazers of phytoplankton and therefore build the base of the food web for other organisms to prey upon [57]. Harpacticoid copepods are common inhabitants of phytal assemblages such as P. crispa, where several closely related species of the so-called phytal-dwelling families often co-occur [58]. Other phytal habitats, such as those created by the brown alga of the genus Cystoseira, have a similar structure compared to P. crispa mats and are much better investigated in terms of copepod assemblages. These habitats seem to shelter high diversity, 24 species alone of Harpacticoida belonging to 20 genera distributed within 11 families were identified in the eastern Mediterranean Sea of Turkey [59]. Seagrass meadows show a similar trend, with Harpacticoida dominating the associated orders of copepods [60,61,62]. As a big share of the Copepoda displayed characteristics such as cylindrical body shapes with a prosome having the same width as the urosome, it can be hypothesized that these belong to the order Harpacticoida. That is because these body features are well-known to be characteristic for copepods of this order [63]. Phytal or epibenthic species of Harpacticoida, in general, display a host of body shapes, including lateral and dorsal-ventral compression of the body [64]. The phylum of Nematoda was the fourth most abundant; however, no families were identified. Nematodes play an important role as energetic links between smaller (bacteria) and larger organisms (macrofauna), facilitating the mineralization of organic matter [65]. Echinoderms, being the fifth most abundant phylum, can play a key role in benthic ecosystems and can contribute to the complexity and resistance against perturbations. They do so by acting as keystone species that regulate dominant prey and algal populations. Through their trophic interactions and habitat-modifying activities, they enhance network connectivity and buffer the system against perturbations, thereby prolonging stability and recovery capacity [66].

4.2. Abundance and Diversity of Non-Sessile Invertebrates in P. crispa Mats

We identified 5464 specimens on a total algal surface of almost 14,000 cm2 (1.4 m2), equaling 3902 individuals per m2 algae. Compared to Bonifazi et al. [29], this presents a number almost twice as high (1990 individuals per m2 algae), underscoring the importance of an adequate sampling technique for non-sessile organisms. It also confirms P. crispa acting as a biodiversity hotspot and a diverse habitat-building red alga. The densities of organisms averaged at 4316 ± 769 individuals per m2 seafloor at Secca II and 2726 ± 1099 individuals per m2 seafloor at Fenaio, showing a noticeable, but not significant, difference between the sites. Comparing the abundance to sessile invertebrates, it becomes obvious that the abundances of previous studies focusing on sessile invertebrates were much higher, with roughly 64,000 individuals associated per m2 P. crispa [28]. The diversity and cover of a sessile animal assemblage, however, do not predict its associated non-sessile fauna [67]. When looking at the families present at both sites, we observed a high similarity with 128 taxa, including 68 families, 9 higher taxa not further identified, and 51 morphotypes, out of a total 169 taxa present at Secca II and Fenaio. Only a small number was exclusive to either one of the sites. Most of the unique families at the study sites belonged to the same phyla. Only at Secca II, four families of Amphipoda were exclusive. Most of the families contribute to nutrient cycling by feeding on algal matter, detritus, and other invertebrates, thereby influencing the flow of energy/matter within these habitats. This similar assemblage could reflect similar environmental conditions leading to similar family distribution, with both sites being exposed to nutrient-carrying currents as well as similar water depths at which P. crispa occurs.
Diversity descriptors differed between Copepoda, Ostracoda, and the other groups investigated. While values for Shannon diversity for Copepoda (2.6 at Secca II and 2.4 at Fenaio) and Ostracoda (1.9 at Secca II and 2.0 at Fenaio) were similar to previously reported values for sessile invertebrates with 2.2, 1.1, 2.5, and 2.2 [28,38,41,42], it should be noted that our calculations could be more precise if identification was improved. Due to the lack of high-resolution images, a more accurate identification of Copepoda and Ostracoda was not possible. Therefore, these results should be treated with caution. The Shannon index for the rest of the organisms, however, was even higher with 3.0 at Secca II and 3.2 at Fenaio, which places our results very well in line with values calculated for sessile invertebrates (2.2, 2.2, 1.1, 2.5) and even exceeds them [28]. A similar value compared to our results was recorded for mollusks with a Shannon index of 3.4 [39]. The only substantially higher values were reported for bryozoan assemblages in coralligenous reefs off Sicily, with Shannon values of 6 and 13.5. Such exceptionally high values are likely attributable to the increased availability of habitable space for bryozoans provided by the numerous pores and crevices characteristic of the investigated coralligenous reef [45]. The significance of our results becomes apparent when compared to values calculated for other habitats in the Mediterranean Sea, like P. oceanica (2.2, 2.0), coralligenous reefs (1.4, 1.8, 2.1, 2.6), and Cystoseira zosteroides (2.0) [9,28,43,44,45,46]. Our results stand out even more when compared to traditional ecosystems like coral reefs (2.0), mangroves (1.9), and kelp forests (2.4) [47,51,52]. These comparisons highlight P. crispas’ importance as a biodiversity hotspot and solidify even further that our understanding of this mat-building algae is not yet complete, as our study presents the first accurate in-depth investigation of non-sessile invertebrates.
The high number of individuals, as well as the high values for diversity descriptors, may be explained by the high surface area of P. crispas thalli, which is also reflected in the 2D to 3D conversion factor. This factor shows how large the enlargement of the surface area is for organisms to live on, compared to the surface area of the substrate they grow on. We calculated a conversion factor of 5.3 ± 1.9, which is similar to previous assessments of P. crispa with 4.9 ± 0.2 [28]. Structural complexity often correlates with high associated diversity [68,69,70,71,72,73] and may lead to reduced water movement and, in turn, increases sedimentation [28,31]. This could improve food source accessibility, leading to higher richness.

5. Conclusions

We conclude that Phyllophora crispa harbors a great diversity of associated non-sessile invertebrates. This study presents a first in-depth insight into the non-sessile diversity, as previous studies had only looked at the sessile fauna associated with P. crispa. Diversity descriptors showed values exceeding those of sessile invertebrates and even those of traditional ecosystems such as seagrass meadows and coral reefs. Therefore, we can confirm and extend P. crispas role as a biodiversity hotspot and biodiversity refuge along with previous studies on epiphytic epifauna [28,38,39,41,42]. We suggest further investigations of P. crispa to confirm its role on a larger scale across the whole Mediterranean Sea and to further deepen our understanding of this habitat. Further assessments on a broader scale could also give insights into the connection between certain areas or patches of P. crispa in terms of exchange of biodiversity. This information could be implemented in future biodiversity conservation strategies, particularly as traditional habitats continue to decline in the face of accelerating environmental change.

Author Contributions

Conceptualization, A.T., M.S. and C.W.; methodology, A.T., M.S. and C.W.; validation, A.T., M.S. and C.W.; formal analysis, A.T. and M.S.; investigation, A.T. and M.S.; resources, C.W.; data curation, A.T.; writing—original draft preparation, A.T.; writing—review and editing, A.T., M.S. and C.W.; visualization, A.T.; supervision, C.W.; project administration, C.W.; funding acquisition, C.W. All authors have read and agreed to the published version of the manuscript.

Funding

This study was supported by baseline funding of the Marine Ecology department, University of Bremen and by the Institute for Marine Biology (Campese, Italy). A.T. and M.S. received funding via the ERASMUS+ program.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

The corresponding dataset will be openly available in the PANGAEA data repository (www.pangaea.de) upon publication.

Acknowledgments

The authors would like to thank Jenny Tuček and Mischa Schwarzmeier (Institute for Marine Biology), as well as Reiner and Regina Krumbach (Campese Diving Center) for letting us use their premises and their assistance in logistical support and sampling efforts. We would also like to thank Paul Weber for his assistance in data collection.

Conflicts of Interest

The authors declare no conflicts of interest.

Appendix A

Diversity 17 00728 g0a1
Figure A1. Sampling device for non-sessile invertebrates associated with Phyllophora crispa, which was designed to minimize the escaping of non-sessile organisms. The net at the top and at the bottom (metal lid) is closable.
Figure A1. Sampling device for non-sessile invertebrates associated with Phyllophora crispa, which was designed to minimize the escaping of non-sessile organisms. The net at the top and at the bottom (metal lid) is closable.
Diversity 17 00728 g0a1
Table A1. Literature used for identification.
Table A1. Literature used for identification.
Author(s)YearTitle
Riedl, R.2011Fauna und Flora des Mittelmeers
Stresemann, E.1992Wirbellose
Hayward, P.J. & Ryland, J.S.1999Handbook of the Marine Fauna of North-West Europe
Larink, O. & Westheide, W.2006Coastal Plankton. Photo guide for European Seas
Sabelli, B.1982Conchiglie. Caratteristiche e ambienti di vita dei Molluschi
Doneddu, M. & Trainito, E.2005Conchiglie del Mediterraneo
Debelius, H. & Kuiter, R.H.2007Nacktschnecken der Weltmeere
Suarez-Morales, E. et al.2020Class Copepoda
Rodriguez-Lazaro, J. & Ruiz-Munoz, F.2012A General Introduction to Ostracods: Morphology, Distribution, Fossil Record and Applications
Debelius, H.2000Krebsführer
Bellan-Santini et al.1980The Amphipoda of the Mediterranean
Holdich, D.M. & Jones, J.A.1983Tanaids
Smaldon, G.1993Coastal Shrimps and Prawns
Ballesteros, E. & Llobet, T.2015Illustrierter Naturführer Mittelmeer
Wirtz, P. & Debelius, H.2003Niedere Tiere Mittelmeer und Atlantik
Geiß, Günter1990Weichtiere, Krebse, Stachelhäuter des Mittelmeeres
Green, J. & Macquitty1987Halacarid Mites
Formula (A1): The Shannon-Wiener Index H of a population consisting of N individuals from different species, where each n represents the count of individuals belonging to species i, is denoted as follows:
H =   − ∑ i = 1 s p i × l n p i
Formula (A2): The Pielou’s Evenness Index J is calculated using the Shannon-Wiener Index H, divided by Hmax as follows:
J = H H m a x
Formula (A3): The Simpson’s Diversity Index D uses S as the total number of different species in a population, ni as the number of individuals of species, and i and n as the total number of individuals of the population:
D = 1 − ∑ i = 1 s n i ( n i − 1 ) n ( n − 1 )

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Figure 1. Locations of the sampling sites within the study area of Giglio Island, Italy. Red dots mark the sampling sites Fenaio (F) and Secca II (S), where Phyllophora crispa was taken. Black arrow indicates the main current in the area.
Figure 1. Locations of the sampling sites within the study area of Giglio Island, Italy. Red dots mark the sampling sites Fenaio (F) and Secca II (S), where Phyllophora crispa was taken. Black arrow indicates the main current in the area.
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Figure 2. Relative abundance of non-sessile invertebrate phyla associated with Phyllophora crispa. The bar plot displays the proportional composition (% of total individuals) of non-sessile invertebrate phyla found within P. crispa mats across all collected samples (n = 16).
Figure 2. Relative abundance of non-sessile invertebrate phyla associated with Phyllophora crispa. The bar plot displays the proportional composition (% of total individuals) of non-sessile invertebrate phyla found within P. crispa mats across all collected samples (n = 16).
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Figure 3. Average number ± SE of specimens found within families of the four most abundant phyla (A) Arthropoda, (B) Mollusca, (C) Annelida, and (D) Echinodermata. The fourth most abundant phylum (Nematoda) was not included due to inaccurate taxonomic identification. Arthropoda do not include Copepoda and Ostracoda due to inaccurate taxonomic identification. Asterisks indicate significant differences between sites based on Wilcoxon-signed rank test with p < 0.05 (*), p < 0.01 (**).
Figure 3. Average number ± SE of specimens found within families of the four most abundant phyla (A) Arthropoda, (B) Mollusca, (C) Annelida, and (D) Echinodermata. The fourth most abundant phylum (Nematoda) was not included due to inaccurate taxonomic identification. Arthropoda do not include Copepoda and Ostracoda due to inaccurate taxonomic identification. Asterisks indicate significant differences between sites based on Wilcoxon-signed rank test with p < 0.05 (*), p < 0.01 (**).
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Figure 4. Area-proportional Venn diagram displaying numbers of total, shared, and unique (in brackets) taxa found in investigated Phyllophora crispa mats at the study sites Secca II (green) and Fenaio (purple).
Figure 4. Area-proportional Venn diagram displaying numbers of total, shared, and unique (in brackets) taxa found in investigated Phyllophora crispa mats at the study sites Secca II (green) and Fenaio (purple).
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Table 1. Diversity indices (richness = number of non-sessile families or sessile phenotypes, H′ = Shannon, D = Simpson) and evenness for investigated Phyllophora crispa mats as well as other biodiversity hotspots based on literature data. Indices and evenness presented are based on classical formulas. a Ascidiacea, A Arthropoda, AN Annelida b Bryozoa, c Cnidaria, C Chaetognatha, CO Copepoda, e Entoprocta, E Echinodermata, f Foraminifera, m Mollusca (Bivalvia), M Mollusca, N Nematoda, NENemertea, OS Ostracoda, p Polychaeta (Sedentaria), P Plathelminthes, r Rotifera, s Porifera, ST Polychaeta (Serpulidae). t Data collated from multiple other publications. u Excluded Cnidaria, Bryozoa, and Ascidiacea from the analysis. v Respective study included barnacles and phoronids that were not included in the current analysis. x Excluded Polychaeta from the analysis. y Excluded Copepoda from the analysis. NW North West, E East, N North, SW South West. Modified after El-Khaled et al. [28].
Table 1. Diversity indices (richness = number of non-sessile families or sessile phenotypes, H′ = Shannon, D = Simpson) and evenness for investigated Phyllophora crispa mats as well as other biodiversity hotspots based on literature data. Indices and evenness presented are based on classical formulas. a Ascidiacea, A Arthropoda, AN Annelida b Bryozoa, c Cnidaria, C Chaetognatha, CO Copepoda, e Entoprocta, E Echinodermata, f Foraminifera, m Mollusca (Bivalvia), M Mollusca, N Nematoda, NENemertea, OS Ostracoda, p Polychaeta (Sedentaria), P Plathelminthes, r Rotifera, s Porifera, ST Polychaeta (Serpulidae). t Data collated from multiple other publications. u Excluded Cnidaria, Bryozoa, and Ascidiacea from the analysis. v Respective study included barnacles and phoronids that were not included in the current analysis. x Excluded Polychaeta from the analysis. y Excluded Copepoda from the analysis. NW North West, E East, N North, SW South West. Modified after El-Khaled et al. [28].
HabitatLocationRichnessTaxaEvennessH′DReference
Phyllophora crispaGiglio Island,
Secca II		92 x,y9 A,AN,C,c,E,M,N,NE,P,p0.83.00.9Present study
Phyllophora crispaGiglio Island, Fenaio92 x,y9 A,AN,C,c,E,M,N,NE,P,p0.93.20.9Present study
Phyllophora crispaGiglio Island,
Secca II		251 CO0.92.60.9Present study
Phyllophora crispaGiglio Island, Fenaio221 CO0.92.40.9Present study
Phyllophora crispaGiglio Island,
Secca II		171 OS0.81.90.8Present study
Phyllophora crispaGiglio Island, Fenaio181 OS0.82.00.8Present study
Phyllophora crispaNW Mediterranean2239 a,b,c,e,f,m,p,r,s0.72.20.3[28]
Posidonia oceanicaNW Mediterranean1797 a,b,c,f,m,p,s0.82.10.3[28]
Phyllophora crispaNW Mediterranean171 ST0.61.1/[38]
Posidonia oceanica leavesNW Mediterranean101 ST0.40.2/[38]
Posidonia oceanica shootsNW Mediterranean101 ST0.40.2/[38]
Phyllophora crispaNW Mediterranean331 b0.22.2/[41]
Posidonia oceanica leavesNW Mediterranean291 b0.21.3/[41]
Posidonia oceanica shootsNW Mediterranean291 b0.22/[41]
Phyllophora crispaNW Mediterranean811 f0.82.5/[42]
Posidonia oceanica leavesNW Mediterranean581 f0.91.3/[42]
Posidonia oceanica shootsNW Mediterranean581 f0.71.2/[42]
Phyllophora crispaNW Mediterranean261 M0.93.4/[39]
Posidonia oceanicaNW Mediterranean335 a,b,f,p,s0.92.00.3[43]
Coralligenous reefsNW Mediterranean556 a,b,c,f,p,s0.82.10.3[44]
Coralligenous reefsMediterranean786 t7 a,b,c,f,m,p,s0.92.60.2[9]
Coralligenous reefs canopyE Mediterranean2095 f,M,OS,ST0.31.8/[45]
Coralligenous reefs frameE Mediterranean2095 f,M,OS,ST0.51.4/[45]
Coralligenous reefs canopyE Mediterranean1361 b0.295.95/[45]
Coralligenous reefs frameE Mediterranean1361 b0.5113.5/[45]
Cystoseira zosteroidesNW Mediterranean786 a,b,c,f,p,s0.82.00.3[46]
Coral reefSW Indian Ocean4575 a,c,f,m,s0.92.00.3[47]
Coral reef turf algaeW Indian Ocean48 u2 p,m1.01.00.5[48]
Coldwater coral reefN Atlantic Ocean2137 a,b,c,f,m,p,s1.02.70.2[49]
Coldwater coral reefN Atlantic Ocean774 a,b,c,s0.81.60.4[50]
Mangrove forestCaribbean Sea546 a,b,c,m,p,s0.81.90.3[51]
Kelp forestNE Pacific Ocean79 v6 a,b,c,m,p,s1.02.40.2[52]
Antarctic hard bottomWeddell Sea608 x6 a,b,c,f,m,s0.92.20.3[53]
Halimeda biohermCoral Sea474 x5 a,b,c,m,s0.71.60.4[54]
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Töpfel, A.; Steinhoff, M.; Wild, C. Assessment of Non-Sessile Invertebrates Associated with Mats of the Red Alga Phyllophora crispa at Giglio Island, Mediterranean Sea. Diversity 2025, 17, 728. https://doi.org/10.3390/d17100728

AMA Style

Töpfel A, Steinhoff M, Wild C. Assessment of Non-Sessile Invertebrates Associated with Mats of the Red Alga Phyllophora crispa at Giglio Island, Mediterranean Sea. Diversity. 2025; 17(10):728. https://doi.org/10.3390/d17100728

Chicago/Turabian Style

Töpfel, Alexander, Melissa Steinhoff, and Christian Wild. 2025. "Assessment of Non-Sessile Invertebrates Associated with Mats of the Red Alga Phyllophora crispa at Giglio Island, Mediterranean Sea" Diversity 17, no. 10: 728. https://doi.org/10.3390/d17100728

APA Style

Töpfel, A., Steinhoff, M., & Wild, C. (2025). Assessment of Non-Sessile Invertebrates Associated with Mats of the Red Alga Phyllophora crispa at Giglio Island, Mediterranean Sea. Diversity, 17(10), 728. https://doi.org/10.3390/d17100728

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