Benthic Infauna in the Shallow-Water Hydrothermal System of Banderas Bay, Mexico: A Two-Period Comparison

Abstract

At a depth of approximately 9 m off the coast of Banderas Bay, hydrothermal activity occurs through various seabed vents, discharging liquids and gases that reach temperatures of up to 89 °C and pH values lower than the surrounding seawater. This study examines the composition of the benthic infauna inhabiting the sediments of this hydrothermal system in two time periods: November 2017 (previously reported) and September 2023 (recorded for this study). In total, for both samplings, we identified 17 benthic infaunal groups—amphipods, isopods, cumaceans, tanaidaceans, crabs, shrimps, copepods, snails, limpets, caecids, chitons, bivalves, scaphopods, polychaetes, amphioxus, ophiuroids, and bryozoans—belonging to these ten taxonomic classes: Malacostraca, Maxillopoda, Gastropoda, Polyplacophora, Bivalvia, Scaphopoda, Polychaeta, Leptocardii, Ophiuroidea, and Stenolaemata. Additionally, we identified galleries of polychaetes, vermetids, and peracarids. Despite the stressful hydrothermal conditions, statistical analyses of both sampling campaigns revealed no significant differences in abundance, highlighting the potential persistence and adaptability of benthic communities in hydrothermally influenced habitats.

1. Introduction

Submarine hydrothermal systems occur from abyssal depths to the intertidal zone [1]. This broad distribution has led to their classification into two distinct phenomena: deep-sea hydrothermal systems (DSHSs), located at depths greater than 200 m, and shallow-water hydrothermal systems (SWHSs), located at depths shallower than 200 m [1,2].

Beyond depth, there are specific differences between these two types of hydrothermal systems. For example, DSHSs host a high proportion of obligate taxa (species uniquely adapted to these environments) [1], such as the giant tubeworm (Riftia pachyptila), the giant mussel (Bathymodiolus thermophilus), and the blind shrimp (Rimicaris exoculata). Energy production in these systems relies almost exclusively on chemosynthesis, resulting in a predominance of symbiotic relationships and comparatively lower species richness compared to SWHSs. Additionally, diatoms are absent, and hydrothermal vent temperatures can exceed 400 °C.

In contrast, SWHSs are dynamic environments where both chemosynthesis and photosynthesis can occur, as sunlight enables photosynthesis while geothermally reduced compounds support chemosynthesis [1,3,4,5]. These systems exhibit more species diversity and richness than DSHSs [1] and lack obligate taxa [6]. Diatoms and algae are present, and hydrothermal vent temperatures range from 10 °C to 119 °C [1,7,8,9,10,11].

Most SWHSs on Earth owe their origin to oceanic volcanoes. However, those reported in the Mexican Pacific are associated with continental margins affected by active tectonic extension processes [2]. Of the five reported in this region, one is located in Banderas Bay, Nayarit [12], and the remaining four along the coasts of the Baja California Peninsula: Punta Banda [13], Bahía Concepción [14], Los Cabos [15], and Puertecitos [16].

This study focuses on the Shallow-Water Hydrothermal System in Banderas Bay (SWHSBB), Nayarit, Mexico. To date, biological studies in this hydrothermal system remain scarce, as shown by the fact that only 77 benthic diatoms species have been reported [17]—seven phyla of bacteria (Proteobacteria, Firmicutes, Thermotogae, Chloroflexi, Aquificae, Bacteroidetes, and Cyanobacteria) [18], the amphioxus Branchiostoma californiense [19], eight classes of benthic infauna (Malacostraca, Maxillopoda, Gastropoda, Bivalvia, Scaphopoda, Polychaeta, Leptocardii, and Stenolaemata) [11], and six morphospecies of cumaceans (Cyclaspis sp. 1, Cyclaspis sp. 2, Diastylis sp. 1, Oxyurostylis sp. 1, Pseudoleptocuma sp. 1, and Pseudocuma sp. 1), from the families Bodotriidae, Diastylidae, and Pseudocumatidae [20]. These studies have contributed to knowledge on the biological richness of the SWHSBB; however, many researchers have based them on one-off and non-systematic observations. To achieve a more precise characterization of the composition and dynamics of benthic communities in this hydrothermal system, it is essential to intensify research efforts. In this context, we conducted a new survey on the benthic infauna associated with the SWHSBB in 2023. The aim was to analyze its composition and abundance and compare the results with previously reported data from Rodríguez-Uribe et al. [11] at the same study sites and areas to assess the community structure and its relationship with the dynamics of the hydrothermal system.

2. Materials and Methods

The SWHSBB is located in Banderas Bay at a depth of approximately 9 m and 400 m from the coast (Figure 1). In September 2023, a sampling campaign was conducted at the same study sites as Rodríguez-Uribe et al. [11]: 20°44′54.7″ N, 105°28′40.6″ W (Site 1, S1); 20°44′54.8″ N, 105°28′40.4″ W (Site 2, S2); and 20°44′54.9″ N, 105°28′38.4″ W (Site 3, S3), ensuring the comparability of the results. The location and georeferencing of these three study sites were determined using a Garmin^® eTrex^® Touch 35 GPS, Olathe, KS, USA, ensuring the precise identification of the same sampling sites.

For the September 2023 sampling campaign, we followed the same methodology employed by Rodríguez-Uribe et al. [11]. They subdivided each study site (n = 3) into three areas (n = 9) based on bottom temperature and proximity to hydrothermal vents and designated the vents as the center of each site. Three sediment samples were collected in each area (n = 27). The area of direct hydrothermal influence was designated as Area 1 (A1), as it was closest to the vent. Area 2 (A2) represented the intermediate area with moderate hydrothermal influence, whereas Area 3 (A3) was the farthest from the vents and was considered an area without hydrothermal influence.

Temperature and pH were recorded in each area of the three study sites using a YSI™ Professional 1030 multiparameter probe (Pro1030), Yellow Springs, OH, USA, equipped with a submersible cable extending up to 20 m. For the sediment sampling and physicochemical measurements, we sectioned each area using PVC quadrants. Sediment samples were collected using 10 cm $\times$ 10 cm PVC plastic cores. In the laboratory, we separated the organisms from the sediment samples. First, we sieved each sediment sample through two ALCON™ 8″, Mumbai, India, diameter brass sieves, one No. 20 with an 850 µm mesh size and the other No. 50 with a 300 µm mesh size, stacked on top of each other and placed inside a glass tray. To better visualize the organisms, the procedure was performed under a 6× magnification lens and with the aid of a flashlight. Subsequently, we stored them in sterile flasks with 96% undenatured ethyl alcohol. We performed the identification under an Optika™ 50$\times$ stereoscopic microscope, Italy, and classified the organisms into invertebrate groups according to the guidelines of De León-González et al. [21] and Brusca et al. [22].

In this study, we assessed invertebrate group richness, defined as the count of unique invertebrate groups at each site and within each study area. We used the term taxonomic richness within the text. Additionally, we recorded the abundance of organisms within each invertebrate group, expressed as the total number of individuals and their relative percentage within each group per site and area.

For statistical analysis, the Kruskal–Wallis H test [23], a non-parametric test, was used since the data did not follow a normal distribution. The test was performed with a 95% confidence level using SPSS^® Statistics 27.0.1 software. This analysis enabled us to evaluate whether there were significant differences in the abundance of invertebrate groups between sites and areas in the two sampling campaigns: November 2017 (reported by Rodríguez-Uribe et al. [11]) and September 2023 (conducted in this study).

To evaluate the effect on the composition of the benthic infauna community of the physicochemical and marine conditions of each area, into which each study site was subdivided, and to establish differences between the two sampling periods (2017 and 2023), non-metric multidimensional scaling (NMDS) was applied [24]. In addition, we used non-parametric analysis of similarity (ANOSIM) [25] to determine statistically significant differences between groups, taking into account the community structure [26]. These analyses were performed in PAST 4.03 software using the Bray–Curtis distance.

3. Results

3.1. Physicochemical Parameters

During the November 2017 and September 2023 sampling campaigns, we measured the pH and temperature at the time of hydrothermal discharge in the three designated areas (A1, A2, and A3) of each study site (S1, S2, and S3). During the September 2023 sampling campaign, pH and temperature were recorded in the three areas of each study site between 10:00 and 14:00 h, at a depth of approximately 9 m. The values recorded are in

Table 1. The temperature and pH in each area of each study site of the Shallow-Water Hydrothermal System in Banderas Bay (SWHSBB) at two time periods. Site 1 (S1), site 2 (S2), site 3 (S3), area 1 (A1), area 2 (A2), and area 3 (A3).

Parameter	S1			S2			S3
	A1	A2	A3	A1	A2	A3	A1	A2	A3
November 2017
pH	7.67	7.93	8.05	7.71	8.00	8.03	7.66	7.91	8.08
Temperature (°C)	87.0	31.0	27.4	85.0	30.0	27.3	88.0	30.5	27.8
September 2023
pH	7.72	7.90	8.06	7.76	7.95	8.08	7.79	7.98	8.07
Temperature (°C)	78.0	32.0	28.3	82.0	31.0	28.2	79.0	31.0	28.1

The data from the November 2017 sampling campaign were taken from Rodríguez-Uribe et al. [11]. The data from the September 2023 sampling campaign were collected for the present study.

. The temperature readings during hydrothermal discharge events ranged from 27.3 °C to 88.0 °C in 2017 and from 28.1 °C to 82.0 °C in 2023. The pH values ranged from 7.66 to 8.08 across both periods. These measurements reflect spatial and temporal variation in hydrothermal influence among the sites and across the two sampling periods.

3.2. Benthic Infauna

According to Rodríguez-Uribe et al. [11], during the November 2017 sampling campaign, they reported 371 benthic infaunal organisms and 226 galleries. They classified the 371 individuals into 15 groups (amphipods, isopods, cumaceans, tanaidaceans, crabs, shrimps, copepods, snails, limpets, caecids, bivalves, scaphopods, polychaetes, amphioxus, and bryozoans) belonging to eight taxonomic classes (Malacostraca, Maxillopoda, Gastropoda, Bivalvia, Scaphopoda, Polychaeta, Leptocardii, and Stenolaemata). Additionally, they reported galleries of polychaetes, vermetids, and peracarids. Amphipods were the most abundant group (49.60%), whereas crabs (0.27%) and limpets (0.27%) were the least abundant (Figure 2). At the site level, S2 exhibited the highest abundance (58.22%), followed by S3 (30.73%), while S1 had the lowest (11.05%). Regarding taxonomic richness, all three study sites presented the same number of groups (11) (

Table 2. The total abundance and taxonomic richness of benthic infaunal groups recorded at the three study sites of the Shallow-Water Hydrothermal System in Banderas Bay (SWHSBB) during two sampling campaigns.

Taxonomic Class	Invertebrate Group	November 2017					September 2023
		Sites			Number of Individuals	Abundance (%)	Sites			Number of Individuals	Abundance (%)
		S1	S2	S3			S1	S2	S3
Malacostraca	Amphipods	17	107	60	184	49.60	36	96	95	227	51.13
	Isopods	5	3	2	10	2.70	4	3	2	9	2.03
	Cumaceans	2	21	6	29	7.82	5	3	11	19	4.28
	Tanaidaceans	0	4	0	4	1.08	0	1	2	3	0.68
	Crabs	1	0	0	1	0.27	2	0	4	6	1.35
	Shrimps	2	4	6	12	3.23	4	7	7	18	4.05
Maxillopoda	Copepods	2	6	2	10	2.70	1	8	4	13	2.93
Gastropoda	Snails	2	4	0	6	1.62	4	4	6	14	3.15
	Limpets	0	1	0	1	0.27	0	2	1	3	0.68
	Caecids	4	13	1	18	4.85	0	0	1	1	0.23
Polyplacophora	Chitons	0	0	0	0	0	1	4	3	8	1.80
Bivalvia	Bivalves	1	4	3	8	2.16	1	4	6	11	2.48
Scaphopoda	Scaphopods	0	0	3	3	0.81	1	0	0	1	0.23
Polychaeta	Polychaetes	4	49	18	71	19.14	17	30	43	90	20.27
Leptocardii	Amphioxus	0	0	8	8	2.16	4	6	9	19	4.28
Ophiuroidea	Ophiuroids	0	0	0	0	0	0	0	2	2	0.45
Stenolaemata	Bryozoans	1	0	5	6	1.62	0	0	0	0	0
Abundance		41	216	114	371	100	80	168	196	444	100
Taxonomic richness		11	11	11			12	12	15

The data from the November 2017 sampling campaign were taken from Rodríguez-Uribe et al. [11]. The data from the September 2023 sampling campaign were collected for the present study.

). At the area level, A1, which is directly influenced by hydrothermal activity, recorded the lowest abundance across all three sites, whereas A2 and A3 recorded the highest abundance values (

Table 3. The total abundance and taxonomic richness of benthic infaunal groups recorded at the three areas of the Shallow-Water Hydrothermal System in Banderas Bay (SWHSBB) during two sampling campaigns.

Taxonomic Class	Invertebrate Group	November 2017					September 2023
		Area			Number of Individuals	Abundance (%)	Area			Number of Individuals	Abundance (%)
		A1	A2	A3			A1	A2	A3
Malacostraca	Amphipods	13	87	84	184	49.60	39	92	96	227	51.13
	Isopods	3	5	2	10	2.70	2	3	4	9	2.03
	Cumaceans	6	14	9	29	7.82	2	7	10	19	4.28
	Tanaidaceans	3	1	0	4	1.08	0	1	2	3	0.68
	Crabs	1	0	0	1	0.27	0	5	1	6	1.35
	Shrimps	0	5	7	12	3.23	2	8	8	18	4.05
Maxillopoda	Copepods	1	8	1	10	2.70	0	5	8	13	2.93
Gastropoda	Snails	1	4	1	6	1.62	1	5	8	14	3.15
	Limpets	1	0	0	1	0.27	0	2	1	3	0.68
	Caecids	6	10	2	18	4.85	0	1	0	1	0.23
Polyplacophora	Chitons	0	0	0	0	0	5	3	0	8	1.80
Bivalvia	Bivalves	1	3	4	8	2.16	7	2	2	11	2.48
Scaphopoda	Scaphopods	2	1	0	3	0.81	0	0	1	1	0.23
Polychaeta	Polychaetes	9	23	39	71	19.14	9	29	52	90	20.27
Leptocardii	Amphioxus	0	3	5	8	2.16	3	10	6	19	4.28
Ophiuroidea	Ophiuroids	0	0	0	0	0	0	0	2	2	0.45
Stenolaemata	Bryozoans	4	1	1	6	1.62	0	0	0	0	0
Abundance		51	165	155	371	100	70	173	201	444	100
Taxonomic richness		13	13	11			9	14	14

The data from the November 2017 sampling campaign were taken from Rodríguez-Uribe et al. [11]. The data from the September 2023 sampling campaign were collected for the present study.

).

During the September 2023 sampling campaign conducted for this study, we collected 444 benthic infaunal organisms and 263 galleries. We classified the 444 individuals into 16 groups (amphipods, isopods, cumaceans, tanaidaceans, crabs, shrimps, copepods, snails, limpets, caecids, chitons, bivalves, scaphopods, polychaetes, amphioxus, and ophiuroids) belonging to nine taxonomic classes (Malacostraca, Maxillopoda, Gastropoda, Polyplacophora, Bivalvia, Scaphopoda, Polychaeta, Leptocardii, and Ophiuroidea). Amphipods and polychaetes were the most abundant groups, accounting for 51.13% and 20.27% of the total abundance, respectively (Table 2; Figure 3). The least abundant groups were caecids (0.23%) and scaphopods (0.23%) (Figure 2). At the site level, site S3 exhibited the highest abundance (44.14%), followed by S2 (37.84%), while S1 had the lowest (18.02%). Taxonomic richness was also highest at S3, with 15 groups, while S1 and S2 had the same number, 12 (Table 2). At the area level, A1, the area under direct hydrothermal influence, had the lowest abundance values across all three sites, whereas the highest abundance was recorded in A2 and A3 (Table 3; Figure 4). Specifically, A1 accounted for 15.54% of the total abundance, A2 for 41.22%, and A3 for 43.24%. Regarding taxonomic richness, A2 and A3 recorded the highest values, with 14 groups each, while A1 had the lowest, with nine. Regarding the number of galleries, 153 were recorded in A1, 80 in A2, and 30 in A3. Of these, 14.07% were associated with peracarids, 17.87% with vermetids, and 68.06% with polychaetes.

During the two sampling campaigns at the SWHSBB, we recorded 815 benthic infaunal organisms and 489 galleries. Across both campaigns, 17 invertebrate groups were identified (amphipods, isopods, cumaceans, tanaidaceans, crabs, shrimps, copepods, snails, limpets, caecids, chitons, bivalves, scaphopods, polychaetes, amphioxus, ophiuroids, and bryozoans) corresponding to ten taxonomic classes (Malacostraca, Maxillopoda, Gastropoda, Polyplacophora, Bivalvia, Scaphopoda, Polychaeta, Leptocardii, Ophiuroidea, and Stenolaemata). To view photographs of representative specimens from each invertebrate group, you can refer to the Supplementary Materials section.

3.3. Statistical Analysis

The results of the Kruskal–Wallis H test indicated no significant differences in the total abundances of organisms between the study sites analyzed in the November 2017 and September 2023 campaigns. Specifically, when comparing the abundances at S1, S2, and S3 from November 2017 with their respective values from September 2023 (S1 2017 vs. S1 2023, S2 2017 vs. S2 2023, and S3 2017 vs. S3 2023), we found no statistically significant differences ($p=0.388$) (Figure 5). Similarly, the Kruskal–Wallis test was applied to compare total abundances between the areas within each study site for both sampling campaigns. The results showed no significant differences in organism abundance across the study site areas between November 2017 and September 2023. Specifically, we compared the abundances at A1, A2, and A3 from 2017 to their respective values in 2023 (A1 2017 vs. A1 2023, A2 2017 vs. A2 2023, and A3 2017 vs. A3 2023), and we did not detect significant differences ($p=0.342$) (Figure 6).

To explore differences in benthic infaunal composition among the study areas and between the two sampling periods (2017 and 2023), we carried out an NMDS analysis using Bray–Curtis similarity. The resulting ordination (Figure 7) showed no clear separation between sampling years, suggesting a high degree of similarity in community composition over time. Consistently, the ANOSIM test yielded a non-significant result $\left(R=0.09342, p=0.1077\right)$, indicating that the benthic infaunal communities did not differ significantly between the 2017 and 2023 sampling periods.

4. Discussion

During the first sampling campaign in November 2017, they reported 371 infaunal organisms and 226 galleries [11]. In the second campaign, conducted in September 2023 as part of the present study, we collected 444 organisms and 263 galleries. The latter represents a 19.68% increase in the total abundance of benthic infaunal organisms in the SWHSBB. This increase could be related to the rainy season characteristic of September in the region [27], which contributes a greater influx of nutrients to the marine sediments [28,29], thereby favoring a higher abundance of benthic species. However, statistical analyses indicated no significant differences in the abundance between the two sampling campaigns. No significant differences were found between sites or areas when compared to the study by Rodríguez-Uribe et al. [11], suggesting that the organisms found in the A1 areas of each study site belong to the adjacent community, although in smaller quantities. These findings align with those reported by Cardigos et al. [30], Marques-Mendes [31], Melwani & Kim [32], and Couto et al. [33], who reported similar patterns. The lower abundances recorded in the A1 areas of each site could be attributed to the stressful environmental conditions of the SWHSBB, which are primarily characterized by high temperatures and acidity from the hydrothermal discharges. Although the elevated temperature of these discharges dissipates rapidly in the water column, it still impacts the nearby sediments and promotes mineral precipitation in the vent mounds. This process may hinder the colonization of other groups in the study sites and areas.

Hydrothermal discharges at the SWHSBB are composed of liquids and gases, reaching temperatures of up to 89 °C and exhibiting lower pH levels than the surrounding seawater. The gas phase is primarily composed of N₂ (88%) and CH₄ (12%), with only trace amounts of H₂S, CO₂, H₂, Ar, and He [34]. The discharged liquid originates from meteoric water that mixes with seawater upon release. This fluid is significantly enriched in Si, Ca, Li, B, Ba, Rb, Fe, Mn, and As compared to seawater, and has low contents of Na, K, Cl, HCO₃⁻, SO₄²⁻, and Br, which are introduced through seawater mixing [34]. These hydrothermal conditions, although stressful for the benthic infauna of the SWHSBB, as described in the previous paragraph, favor other organisms, such as benthic diatoms as observed in the study by Estradas-Romero & Prol-Ledesma [17], who assessed the richness and abundance of benthic diatoms at two hydrothermal sites—the SWHSBB and the SWHS of Bahía Concepción (SWHSBC)—located on the Gulf coast of the Baja California Peninsula. Their results indicated 77 species of benthic diatoms at the SWHSBB compared to 66 species in the control areas (under ambient conditions without hydrothermal influence). At the same time, at the SWHSBC, they reported 101 species, compared to 75 in the control areas. Meanwhile, the predominance of nitrogen and methane [34], along with the abundance of sulfide minerals such as pyrite, cinnabar, and Tl sulfide, create a reducing environment in the SWHSBB [18]. This chemical diversity supports the presence and predominance of Thermotogae bacteria, which are thermophilic and chemolithoautotrophic sulfur-metabolizing [18].

In the 2023 sampling campaign, we identified 16 groups corresponding to nine taxonomic classes, whereas Rodríguez-Uribe et al. [11] reported 15 groups belonging to eight classes (

Table 4. A comparison of benthic invertebrate groups recorded in Shallow-Water Hydrothermal System in Banderas Bay (SWHSBB) in the 2017 and 2023 sampling campaigns. The ✓ indicates that the group is present, while the ✗ shows that it is absent.

Taxonomic Class	Invertebrate Group	Present in
		2017	2023
Malacostraca	Amphipods	✓	✓
	Isopods	✓	✓
	Cumaceans	✓	✓
	Tanaidaceans	✓	✓
	Crabs	✓	✓
	Shrimps	✓	✓
Maxillopoda	Copepods	✓	✓
Gastropoda	Snails	✓	✓
	Limpets	✓	✓
	Caecids	✓	✓
Polyplacophora	Chitons	✗	✓
Bivalvia	Bivalves	✓	✓
Scaphopoda	Scaphopods	✓	✓
Polychaeta	Polychaetes	✓	✓
Leptocardii	Amphioxus	✓	✓
Ophiuroidea	Ophiuroids	✗	✓
Stenolaemata	Bryozoans	✓	✗

The data from the 2017 sampling campaign were taken from Rodríguez-Uribe et al. [11]. The data from the 2023 sampling campaign were collected for the present study.

) in the 2017 campaign. We identified three specific differences that stand out: in 2023, no specimens of bryozoans (class Stenolaemata), while in 2017, no specimens representing the classes Polyplacophora and Ophiuroidea were reported (Table 4).

We attributed the absence of bryozoans in the 2023 sampling to their ecological characteristics. These colonial, sessile invertebrates possess a calcium-based skeleton, exhibit suspension-feeding habits, and are potentially affected by local acidification. Their presence in the first sampling could have been incidental, influenced by the dynamic conditions of the SWHSBB, which might explain their absence in the subsequent sampling.

Polyplacophoran mollusks, commonly found in intertidal and deep-sea environments, typically adhere to rigid substrates such as rocks and algae. The lack of firm surfaces in the study area likely accounts for their low abundance and exclusive record in the most recent sampling. A similar situation could apply to ophiuroids. Although their presence in shallow hydrothermal systems is feasible, these organisms are mobile and tend to burrow beneath the sediment surface. However, they are typically found in areas farther from hydrothermal vents [32]. Their absence during the November 2017 samplings is likely due to their ability to relocate actively away from the unfavorable conditions of the SWHSBB.

Knowledge of biodiversity requires assigning organisms to their respective species, which necessitates the expertise of specialists and a historical understanding of the fauna in the studied locality. This problem is known as a taxonomic impediment, and classification at higher taxa has constituted a methodological option for monitoring studies of benthic communities. Under this approach, we have advanced in the prospecting of the SWHSBB, but we hope to advance knowledge at the species level in the future.

Although the identification of benthic infauna was limited to the class level, these findings underscore the importance of continued monitoring of the SWHSBB to better understand temporal dynamics in community composition and abundance under persistent hydrothermal influence. Given the ecological sensitivity of shallow-water hydrothermal systems and their proximity to coastal development, long-term studies are crucial for assessing the resilience of benthic infaunal communities and detecting potential anthropogenic impacts. Furthermore, documenting the presence of tolerant or opportunistic species, such as polychaetes and microbial indicators of geochemical shifts, may offer valuable insight into the natural variability and adaptive processes occurring in these extreme marine environments.

5. Conclusions

This study evaluated and compared the benthic infauna associated with the SWHSBB across two sampling campaigns: one conducted in November 2017 and reported by Rodríguez-Uribe et al. [11] and the other carried out for the present study in September 2023. The comparison provides a broader perspective on the biodiversity of these communities in a shallow-water hydrothermal environment. Despite a 19.68% increase in total organism abundance between the two periods, statistical analyses (Kruskal–Wallis test, NMDS, and ANOSIM) revealed no significant differences in either abundance or taxonomic composition across sites, areas, or years. These results suggest a temporal stability in the community structure of the SWHSBB. Specifically, composition and abundance were the parameters assessed to describe community structure, showing that a relatively consistent group of benthic organisms can adapt to the stressful conditions of this environment, characterized by high temperatures, low pH levels, and rich geochemical composition. However, the absence of the class Stenolaemata in 2023 and the exclusive presence of Polyplacophora and Ophiuroidea in 2023 suggest potential fluctuations in community constituents, which may reflect ecological turnover or sampling limitations. The continued dominance of amphipods and polychaetes, along with the low representation of more sensitive taxonomic groups, supports the hypothesis that only a limited subset of benthic invertebrates is adapted to tolerate hydrothermal stressors.

The benthic infaunal community described here constitutes a first step toward a more detailed analysis of community structure in terms of composition and abundance in the SWHSBB. Nonetheless, further long-term and spatially extensive studies, as well as the identification of benthic infauna at the genus and species levels, are essential to distinguish resident hydrothermal organisms from occasional or transient ones and to understand better the ecological dynamics and resilience of these benthic communities in shallow hydrothermal environments.