Differences in species delimitation models
By combining morphological diagnostics, the output of the molecular species delimitation methods and the application of proteome fingerprinting, we conclude to have collected and identified a total of 24 deep-sea ophiuroid species belonging to ten families. Of these, 11 belonged to species for which there was no molecular data in public repositories before. The specimens belonging to Ophiocten cf. umbracticum could not be morphologically verified exactly like O. umbraticum and might be a new species.
The only differences in delimitations by GMYC and ABGD were splits within O. abyssicola and O. aculeata. However, neither did morphological identifications reflect this, nor were these splits supported by clustering based on proteome fingerprinting. Also, the specimens from the two clades did not exhibit any geographic or bathymetric distribution patterns that the clades could refer to. All O. aculeata specimens from the NA were sampled at the Lóndsjúp and shared the typical characteristic features of O. aculeata such as smaller plates surrounding and separating the dorsal arm plates [30].
The splits within the O. aculeata, O. sarsii and O. abyssicola morphotypes might indicate cryptic lineages or ongoing speciation [46] However, it is known that GMYC tends to overestimate species diversity [33,47]. O. sarsii revealed a similar pattern as identified by [48] where two main mtDNA lineages across the Atlantic were identified. For O. abyssicola [49] it was observed that sequences from the NA and Western Indian Oceanform a clade while sequences from the Southwest Pacific form a distinct clade apart from this. So, whereas there seem to be (geographical) lineages within the species, this cannot be applied to our clades as delimited by GMYC since none of the resulting clusters shows a special affinity to the specimens from the Southwest Pacific.
Generally, the three methods reveal similar results, even though GMYC tends to result in more clades than the ABGD model and the BIN system. While the GMYC and the ABGD model are based on the relative distances to the sequences in the alignment, the BIN system compares the uploaded sequences with the available sequences in the database [38, 40].
It is noticeable that the BIN system was the only delimitation method that differentiated between the reference sequence of O. cuspidata and Ophiosabine anomala. This special case reveals potential pitfalls of non-integrative identifications. Closely related species may quickly be misidentified or falsely assigned to another species highlighting the importance of a sufficiently large dataset and degree of sampling per species for reliable species discrimination, especially for closely related species [50].
Thus, molecular species delimitations were aligned with results from morphological identification and supported by proteome fingerprinting. This was done because previous studies confirmed that a complementary interpretation of morphological identification and species delimitation tools leads to a better resolution of biodiversity [51,52]. In general, the mean intraspecific genetic distance in our data set was 0.99% which is comparable to what was observed in previous studies, e.g. 2,2% [47] and 0.88% [18].
Habitat heterogeneity and species richness
Various hypotheses attempt to explain the patterns of biodiversity and the “paradox” of high diversity in the deep sea [53]. One proposes that habitat heterogeneity, leading to a greater number of microhabitats, promotes higher biodiversity [9]. Variation of distinct geological structures, the biotic and abiotic factors create unique niches that require species to adapt and thus drive species separation. In this study, different habitats were classified by their habitat structure and substrate availability and then used to derive the biodiversity patterns of brittle stars. Although different habitats occur across the North Atlantic, no significant differences in relative species diversity can be recognized between the different habitats. However, there are clear differences in the species composition of the individual sampling regions.
The habitat with the highest recorded species richness is the cold-water coral reef of Londsjup located at the Aegir Ridge, which is the shallowest sampling station and closest to the shore habitat. The biotic habitat is primarily composed of two coral species, Desmophyllum pertusum (Linnaeus, 1758) and Madrepora oculata Linnaeus, 1758 [54,55]. In this study, Ophiosabine anomala (n=1) and Ophiura robusta (n=2) were the only species exclusively found at the Lóndsjúp, although neither one of them is known to be specifically associated with CWC as habitat [30,56]. The species richness at Londsjúp might be due to the 3D-structure provided through the corals enhancing the availability of microhabitats and function for the brittle stars depending on their species-specific biology [9]. Cavities between the corals can function as refuge and shelter, especially for juvenile and small brittle stars as documented for O. aculeata [57]. While climbing on the coral, their elevated position may enhance foraging opportunities by providing access to additional food sources. O. abyssicola, found in high abundance at the Londsjúp, was observed perched on corals with their arms reaching into the water column, potentially reflecting an advantageous elevated position for food foraging [16].
Also, ocean ridges enhance the abundance of benthic organisms by extending and elevating the seafloor and providing hard substratum [20]. These provide habitat for species typically found on continental margins [21] that may not survive on abyssal plains [5,58]. Additionally, hard substratum can be used by other organisms to create biotic structures and the ridge structures themselves can also provide cracks and caves that are used as a shelter by brittle stars to be protected against predators [16].Additionally, ridges can influence the environmental conditions [21], for example, the Greenland-Iceland-Faroe Ridge creates different habitats in the northern Atlantic [59]. Resulting variations of abiotic factors can lead to variability in the species distribution [21]. The steep slopes also affect the ocean currents redistributing the food availability which also influences the richness and abundance of the deep-sea floor on a smaller regional scale as well as on a larger geographical scale [60]. Even if the structure and sediment composition of the ocean ridges potentially offer various microhabitats, they can also act as functional distribution barriers inhibiting the cosmopolitan distribution of species [7] The significance of potential barriers can vary from species to species depending on their life biology.
Akin to ocean ridges, seamounts disrupt the abyssal and create a distinct habitat compared to the surrounding seafloor (8) with potentially altered food supply to the deeper ocean [21,61]. A special region in this dataset is the abyssal hill in the NACES MPA region, a component of the largest marine protected area in the North-East Atlantic [62], situated at the base of the recently discovered seamount "Mount Doom" [25]. The sediment in this area is significantly influenced by the adjacent seamount, characterized by volcanic glass, which provides a hard substratum for benthic species.
The ophiuroid fauna at the investigated seamount overlaps with both the abyssal plain and the deep seafloor, as indicated by the presence of species such as O. ljungmani and A. bullata and cf. Ophiuroglypha sp. A. This is consistent with previous studies [63,64] in which the ophiuroid fauna on seamounts reflects species found elsewhere at comparable depths and on the surrounding seabed. Despite the small sample size of ten specimens used in this study, the identification of five different species shows a remarkable level of biodiversity in the area studied. This observation is consistent with the findings of [63] and may reflect the species richness associated with the highly variable topography, substrate, and bathymetry of seamounts at smaller scales [65], which means that seamounts are not necessarily centers of endemism for brittle stars [64].
Chemosynthetic ecosystems have been found at hydrothermal vents and cold seeps that have a primary production due to chemoautolithotrophic bacteria [8,66]. The hot, chemical-rich fluid that exists at hydrothermal vents can support reducing microbes creating a specific and unique habitat patch, with an associated community around the vent [66]. Even though chemosynthetic environments are known to host a variety of endemic species in various taxa [67] amongst which are also brittle stars [68], none was discovered within this study. It is known that species from non-chemosynthetic habitats do visit vent sites and hydrothermal conditions [68]. All four species O. abyssicola, O. spectabilis, O. purpureus and O. clavigera found at the vent field often associated with cold-water corals and sponges [16,69,70]
Compared to the obvious variety of the habitats before the fine sediment of the abyssal plains consists mainly of biological particles produced by planktonic organisms with siliceous or calcareous features and rock weathering on land (8). This relatively homogeneous composition is interspersing habitats like abyssal hills or rocky habitats provide heterogeneity [71,72]. This could lead to an underestimation of biodiversity due to the presence of more hard substratum habitats and small-scale differences in the abyssal terrain [71,72]. In this study, stations classified as abyssal plains showed no inhibited biodiversity. Although species richness did not significantly differ from other habitats, the dominance of a few species like A. bullata in individual sampling regions is remarkable.
Comparing the appearance, the deep seafloor area of the Azores has a similar character to the abyssal plains, fine sediment, and limited hard substrate available. However, this sampled area is in the broader region around the MAR, which is characterized by faults, fracture zones and abyssal plains covered with volcanic rocks [73]. Fracture zones show high species diversity was observed on the steep slopes and rocks [74]. The geomorphological elements with steep vertical walls [25] of the close by fracture zone also influence seafloor diversity by potentially channeling ocean currents, increasing food availability and thus improving conditions for filter-feeders at the surrounding abyssal plains (21, 75,76].
In the deep sea, some species can exhibit a widespread distribution [77,78] even if a uniform species distribution is exceptional [10,67]. The potential distribution range of individuals is a significant factor in structuring populations and the role of dispersal strategies for connectivity is discussed [53,79]. As most benthic organisms have a bipartite life cycle with a planktonic larvae phase which plays a central role in the potential distribution in marine systems [53]. The limited larval swimming ability and their small size support the hypothesis that the dispersal of species with planktonic larval stages is determined by the current extrinsic and the pelagic larval duration (PLD) [79]. It is assumed that the maximum PLD is driven by type of larvae [13]. We found the broadcasting species A. bullata with associated leicotrophic larvae, hence implying a presumed longer PLD [80]. Amphiophiura bullata is known to be a widely distributed brittle star, with documented findings in the Indian Ocean and Atlantic [81]. Also, in our dataset, A. bullata was found at different locations, further supporting the theory of larval dispersal potential and the species' distributional potential. Moreover, it has been argued that brooding species are more restricted in their dispersal [82]. As an example of known brooding species, our dataset contains O. clavigera and O. anomala [15,83], and both species were only found at one site, highlighting the hypothesis that a brooding life cycle limits the dispersal potential of species. The variability in the reproductive strategies of brittle stars emphasizes the importance of considering the individual life cycle and lifestyle of the studied species when assessing biodiversity. Regionally distributed species are more vulnerable to threats than widespread species such as A. bullata.
Implications for the results of this paper
Assessing factors shaping species distribution remains challenging when we consider the large geographical distances between sampling locations and the relatively small size of the sampling area compared to the vast unsampled regions in the North Atlantic. Smaller scale variations, food availability and individual life traits of the species contribute to the composition of benthic communities, creating a mosaic of influences that vary across different spatial scales [57,60,71]. The interconnected complexity of biotic and abiotic factors contributing to biodiversity patterns cannot be attributed to one ultimate factor. The data presented herein offer an initial insight into species connectivity and distribution patterns in the North Atlantic Ocean while emphasizing the significance of expanding sampling points to facilitate a deeper understanding of connectivity in the deep sea. Despite the lack of a significant correlation between habitat heterogeneity and biodiversity, our observations suggest potential connections, particularly in structured environments such as cold-water coral reefs like Lóndsjúp, and it is evident that each sampled habitat exhibits a distinct species composition contributing to the overall biodiversity in the North Atlantic.