Echinoderidae

Anguas-Escalante, Abril, Jesús-Navarrete, Alberto De, Landers, Stephen C. & Sørensen, Martin V., 2024, Kinorhynch assemblages on the southern Gulf of Mexico continental shelf, Zoologischer Anzeiger 313, pp. 228-240 : 236-238

4.2. Echinoderidae assemblage and total Kinorhynch community

This study represents the first Echinoderidae diversity analysis in sGoM. The results indicated a rich undescribed fauna, which includes Fissuroderes sp. 1 and 18 Echinoderes species. The finding of Fissuroderes sp. 1 represents the first record of the genus from sGoM, and the Echinoderes species records were increased from five species ( E. unispinosus , E. zacharyi , E. augustae , E. bookhouti , E. spinifurca ) to nine, including

E. charlotteae , E. joyceae , E. juliae and E. romanoi as new records.

The presence of E. juliae in sGoM exhibits more evidence of its wide distribution, since it was described from USA Northeast Pacific deep-sea (Sørensen et al., 2018) and recently also found at the Hikurangi Margin, New Zealand as well as near the Atacama Trench, Chile ( Grzelak and Sørensen, 2022; Grzelak et al., 2021). Likewise, E. unispinosus has shown an enigmatic distribution that includes the sGoM (Alvarez-Castillo´et al., 2020; present study). This species has its type locality in the Northeast Atlantic, north of the Azores ( Yamasaki et al., 2018b), but specimens have also been found in the Pacific Ocean, at bathyal depths off California (Sørensen et al., 2018) and New Zealand ( Grzelak and Sørensen, 2022), as well as in the Indian Ocean, in the Mozambique Channel ( Cepeda et al., 2020). Although there are sufficient morphological arguments to justify the identifications of these species, it is still necessary to investigate whether they are cryptic species, because of their biological abilities alone would not be enough to disperse widely (Sørensen et al., 2018; Sørensen and Pardos, 2020). However, the hypothesis that there are mechanisms capable of dispersing kinorhynch species over wide distances either by marine currents, thermohaline circulation, stepping stones, or human vectored dispersal is increasingly accepted ( Herranz and Leander, 2016; Yamasaki et al., 2018a; 2019; Grzelak et al., 2021; Grzelak and Sørensen, 2022). Such an assumption has only once been supported by genetic information in specimens of Echinoderes ohtsukai Yamasaki & Kajihara, 2012 from Canada and Japan, which showed identical genetic sequences. The presence of E. ohtsukai in Canada appears to be due to the import of Japanese oysters for aquacultures ( Herranz and Leander, 2016).

Remarkably, E. juliae is the only abundant sGoM species not reported previously in the studies from nGoM. In general, the distribution of the most abundant Echinoderidae species in sGoM includes the most common species from nGoM. Among those, the highest abundance of E. spinifurca , E. bookhouti , and E. augustae in the group of shallow depth was determinant to the dissimilarity with the other two depth groups. This is because of the abundance of these species in the other groups did not exceed two specimens, E. spinifurca was found in all three depth groups while E. bookhouti , and E. augustae in shallow and medium depth sites. Similarly, in the nGoM studies these three species have been the most abundant at depths less than 200 m ( Hoffman et al., 2021; Kennedy et al., 2024; Landers et al., 2018; 2019; 2020). The dominance of E. spinifurca , E. bookhouti , and E. augustae in the group of shallow sites also influenced the results of its diversity. Even though the group of shallow stations obtained the highest abundance and sample coverage, Hill diversity was the lowest in comparison with the other two depth groups.

Towards the intermediate depths, habitats can provide the optimum region with intermediated disturbance and productivity, and maximum diversity can be achieved ( Gray and Elliot 2009). This has been observed in the general patterns of meiofauna diversity (nematodes and copepods) that suggest a peak of diversity at intermediate depths ( Vanreusel et al., 2023). In the present study Echinoderidae were richest at shallow to medium depth sites, increasing from 15 species to 25 species, and the distribution of abundances was more uniform as Hill Shannon showed approximately twice as many species as Hill–Simpson (90 % SC). This peak of diversity is related to intermedium values of temperature (6–10 ◦ C), salinity (34–35 ppt) and (2 – 4 mg /L) DO, which reduces the capabilities competitive of dominant species in shallow sites, and this in turn optimal conditions for other species ( E. juliae and E. joyceae were the most abundant). Probably, the disturbance caused by freshwater efflux decreases as depth increases, and organic matter availability also decreases. Therefore, in the medium depth sites the intensity of disturbance is moderate, which suggests that the intermediate disturbance hypothesis is true in the sites studied here ( Connell, 1978).

Beyond the intermediate depths, disturbance and productivity are reduced, consequently the diversity also decreases in the deeper depths ( Gray and Elliot 2009). The group of deep sites of the present study had the lowest total abundance than the other groups, but similar richness as shallow sites, the distribution of the abundance between species was mostly uniform with Hill–Shannon higher than Hill–Simpson (90 % SC). However, the dominant species of shallow sites were not present ( E. augustae , E. bookhouti ) or present with only 2 specimens ( E. spinifurca ). Environmental characteristics in the group of deep stations are cold water (4–5 ◦ C), salinity mostly 34 ppt and higher rank of DO, where E. unispinosus was the most adapted species with the greatest abundance. Disturbance in this depth seem to be lowest or absent, which is reflected mostly in the lower abundance and in absence of opportunistic species, probably associated with natural reductions in food (mostly as organic matter) as well as biotic interactions as depredation and competition ( Giere, 2009b; Vanreusel et al., 2023).

Species composition of Echinoderidae along depth differed significantly between depth groups, the environmental characteristics of each group are regulating the presence and abundance of the species. Seven species are shared in the three groups and environmental variations influence the variation of their abundance. The remaining species were restricted to two depth groups or only one group. The most abundant species contributed to the major dissimilarity between groups. The studies of nematodes in sGoM have clearly found differences of the species composition across the depth, and there are three principal factors proposed as drivers of species composition in meiofauna along depth: pressure, oxygen, and availability of food ( Armenteros et al., 2022; Vargas-Espositos et al., 2023). Among them, only oxygen was measured in the present study, and it was the third variable that characterised the sample sites ( Landers et al., 2022), but the analysis showed no strong influence at the species. However, disturbance caused by efflux of freshwater and pollution of oil platforms could be affecting the Echinoderidae assemblage by promoting of development of opportunistic species probably E. spinifurca , E. bookhouti , and E. augustae ( Cardoso-Mohedano et al., 2022; Machain-Castillo et al., 2019).

Regarding total kinorhynch community records from nGoM has a total of 32 species belonging to 12 genera (Landers et al., 2018; 2019; 2020; Hoffman et al., 2021); the present study reports a greater richness in the sGoM with 46 species representing 14 genera. Surprisingly, sample effort in nGoM have been more extensive in space and time, in contrast with the single sampling campaign and 24 sites sampled along sGoM. In the continental shelf from Louisiana to Florida 37 sites were studied and 32 species were found (Landers et al., 2018). In Mobile Bay and along the Alabama continental shelf a total of 11 species were reported from 13 sites ( Hoffman et al., 2021), nine species from 10 locations ( Landers et al., 2020), and seven species from 13 sites ( Kennedy et al., 2024). Also, along the Louisiana continental shelf 18 species were reported from 16 sampling sites ( Landers et al., 2019). Differences in richness of sGoM and nGoM probably is related with differences in the disturbance caused mostly by freshwater efflux of rivers, which in nGoM is probably more intense because of the area is more uniformly shallow ( Cardoso-Mohedano et al., 2020).

In general, the same trend of the Echinoderidae assemblage was observed for the total kinorhynch community regarding the pattern of diversity and species composition. The depth gradient of the sites studied influenced the Kinorhyncha diversity and species composition by the variation of environmental conditions, and the disturbance (freshwater efflux, productivity, oil activities).