Benthic Peracarids (Crustacea) from an unexplored area of Patagonian channels and Fjords

Abstract Background The intricate geomorphology of the coastline in the Chilean Channels and Fjords region, together with the freshwater inputs from the ice fields provide the area with very unique ecological characteristics and a variety of habitats that favour great marine biodiversity. However, although Chilean Patagonia has been the focus of several expeditions and ecological surveys, the greatest emphasis has been either on the populated coasts of the Beagle Channel and the Straits of Magellan to the south or the area to the north of Golfo de Penas, leaving vast areas that remain largely unexplored. This leads to a latitudinal gap in the faunistic information and hinders zoogeographic studies to assess biogeographical connections along the eastern coasts of the Pacific. Peracarida is a taxonomic group that provides an excellent model for such studies because of their high abundance and biodiversity, benthic habits, small size and limited dispersal capacity. New information A dataset providing the first and only records of the benthic Peracarida between the latitudes 48–51.5°S of the Pacific coast of Chile is presented here, hence closing a geospatial gap for the study of the biogeographical connections of the Peracarida along the Eastern Pacific coast. The dataset comprises a total of 141 georeferenced records of 60 sublittoral species of Tanaidacea, Isopoda and Amphipoda. This and other studies reveal that the coastal fauna of the region follow a latitudinal distribution pattern at a larger scale and nested assemblages inside the channels and fjords that can be regarded as a consequence of the more restrictive conditions in the inner parts. In the present scenario of global warming that is expected to affect particularly polar and subpolar regions, the present dataset serves as a reference for the distribution patterns of benthic organisms with low dispersal capacity.


Introduction
The Patagonian Channels and Fjords constitute one of the largest estuarine systems of the world, extending along around 84,000 km of coastline . They receive important freshwater inputs from glaciers and pluviosity, that, together with seasonal variations in temperature, cause hydrographic gradients in salinity and temperature from the inner parts to the open ocean (Chuecas and Ahumada 1980). It is an area of highly differentiated, fragmented ecosystems that offers a unique opportunity for studying the distribution patterns of the fauna in habitats with environmental gradients.
While the southern channels and fjords of the Magellan Region have been the focus of several expeditions and ecological surveys, the greatest emphasis has been on the populated coasts of the Beagle Channel and the Straits of Magellan (De Broyer et al. 2007): for example, the scientific expedition of the HMS Challenger in 1873-1876 and the Lund expedition (Brattström and Johanssen 2011), the more recent joint Chilean-German-Italian campaign Victor Hensen (Arntz and Gorny 1996) and periodic sampling efforts carried out by the Universidad de Magallanes (Ríos et al. 2007). By contrast, the coastal ecosystems of the less accessible area between the Golfo de Penas and the Smith Channel (48-52°S) remain largely unexplored. The sedimentary bottoms were surveyed by the scientific expedition CIIMAR Fiordo 2  from which general results for the macrofauna were presented by Mutschke (2008), who stressed the scarcity of knowledge of the amphipod fauna in the region; species lists and biogeographic remarks have been carried out for the Polychaeta (Montiel et al. 2001) andCrustacea Decapoda (Retamal andArias 2000). On the other hand, the hard substrates of this central zone had never been surveyed until the survey carried out in Bernardo O'Higgins National Park (henceforward BONP) on-board the R/V Nueva Galicia (Aldea et al. 2011, Aravena et al. 2018, Esquete and Aldea 2015, Esquete et al. 2012, Palacios 2018 and whose data pertaining to peracarid crustaceans are presented here. The superorder Peracarida is one of the most abundant and diverse taxa in the marine benthos (Cartes et al. 2001, González et al. 2008, Spears et al. 2005. Perhaps the most significant synapomorphy that characterises the group is the presence of a marsupium where the larval stages develop, with the consequent absence of a dispersive phase (Johnson et al. 2001). The displacement capacity of the adults varies across orders, families and species, with proven consequences on species geographical distribution (e.g. Bober et al. 2018). This makes the Peracarida an excellent model taxon for hypothesis testing in biogeographical research; however, they are rarely the focus of such studies: biogeographic studies of the Peracarida of the coasts of Southeast Pacific do not generally include the Magellan Region. The distribution of algae-associated Peracarida along the Chilean coasts from the northern limit down to 42°S was studied by Thiel (2002). His work revealed a latitudinal pattern, with two well-differentiated main distribution areas separated by an extensive transitional zone. Later, Thiel et al. (2003) assessed the diversity and distribution of the peracarids of Chile including the Magellan Region and Antarctic territory, highlighting the contrast between the high species richness and scarcity of data for many taxa. More recently, González et al. (2008) compiled a list of the Chilean Peracarida and their latitudinal distribution, pointing out the scarcity of records in certain latitudes and a high level of endemism, with few species with broad latitudinal ranges. Every new survey comes with the discovery of undescribed species (see, for example, Esquete et al. 2012, Esquete andAldea 2015) showing that, despite the efforts of classic expeditions and recent surveys,knowledge of the Peracarid fauna in the area is still in its infancy.
The present dataset includes records of Peracarida along four degrees of latitude where the underwater biodiversity remained unexplored. It provides data that allow us to link the biogeography of the southern Patagonian coast with the rest of the Eastern Pacific. Additionally, the present work contributes to the knowledge of the biodiversity of the Magellan Region. All in all, the relevance of the present dataset lies in three main factors: 1.
the importance of the Peracarida for biogeographic studies, due to their diversity and direct development; 2.
the geomorphologic and ecological particularities of the area covered; and 3. the fact that it closes a latitudinal gap in the data available for this taxonomic group along the coasts of the eastern Pacific Ocean.
The potential of the usage of this dataset is exemplified here with nestedness analysis. It describes the species composition patterns within a continental biota and in isolated areas such as fragmented habitats and islands (Ulrich et al. 2009), which would represent a particular case of beta diversity (Ulrich and Almeida-Neto 2012). As such, nestedness constitutes an excellent means for assessing spatial patterns in terms of composition and distribution patterns in isolated ecosystems (Atmar and Patterson 1993, Escalante Espinosa and Morrone 2001).
Nestedness analysis was performed following Marin and Delgado (2001), based on the matrix of presence and absence of species by sampling site. Using the Nestcalc software (Atmar and Patterson 1993), the temperature of the observed matrix was calculated, which could vary in a range of 0-100°. A value T = 0° would correspond to a perfectly nested pattern, while T = 100° would correspond to a pattern where the set of species would be totally random (Atmar and Patterson 1993). In addition, the simulated temperature was A total of 23 sites were sampled by SCUBA divers (Fig. 1). At each site, five replicate squares of 25 × 25 cm (0.063 m2) were scraped off all the organisms, including fauna and smaller algae. Kelps were excluded from the sample and left intact on the substrate. Two samples were taken at both 5 and 15 m depth at each site (totalling 10 squares per site). Samples were fixed in 5% buffered formalin and subsequently sorted, preserved in 70% alcohol and finally the organisms identified to the lowest possible taxonomic level.  Study area with the 23 sampling sites in the BONP, Chilean Fjords and Channels Ecoregion. Dark grey area corresponds to BONP.

Taxonomic coverage
Description: The Peracarida identified to the lowest possible taxonomic level. Description: A total of 60 peracarid species were identified in the area, making up a total of 141 georeferenced records (Aldea and Esquete 2020). Amphipoda were the most diverse, representing 85% (52 species), followed by the Isopoda (10%, 6 species) and the Tanaidacea (8%, 5 species).

Additional information
The results showed that the Peracarida of the BONP presented a nested pattern, with a larger number of species in the outermost part of the channels and subsets of those in the innermost. The temperature of the matrix was 20.762°C; p < 0.01 (Fig. 2).
This study widens the known distribution limit of several species previously recorded in Patagonia and the Southern Ocean (compiled in De Broyer et al. 2007, Horton et al. 2020: for instance, the amphipod Aora maculata (Thomson, 1879) has been recorded in several locations throughout the Southern Ocean and this is the first record in the American continent. Likewise, the amphipods Jassa ingens (Pfeffer, 1888), Jassa thurstoni Conlan, 1990and Heterophoxus trichosus K.H. Barnard, 1932and the isopod Ischyromene eatoni (Miers, 1875, whose distributions were previously confined to the Scotia Arch Islands and the Southern Ocean, are registered for the first time on this latitude. Colomastix castellata K.H. Barnard, 1932 was described from Islas Malvinas (Falkland Islands) and is found for the first time since the original description. Uristes subchelatus (Schellenberg, 1931) is found for the first time out of the Magellan Strait. A total of 50% of the species occurred in one site only, which can be interpreted as a consequence of the heterogeneity and habitat fragmentation of the area and the niche specificity of Peracarid species.
Generally, species distribution patterns observed at the present time are the result of several ecological, evolutionary and biological processes. Given that nested patterns can be the result of extinction and/or colonisation processes (González and Poulin 2005), the results presented here have three possible interpretations: 1.
as a colonisation process from the open ocean towards the inner parts of the channel; 2.
as a consequence of an increased heterogeneity and hence the higher number of available niches towards the outer part of the channels; or 3.
as a consequence of a gradient of environmental conditions along the channels being less favourable to the development of the Peracarids towards the inner part.
Nested patterns have been found previously in the marine fauna of the region, generally consistent with the latitudinal gradient: fish parasite species (González and Poulin 2005) and polychaete assemblages (Moreno et al. 2006) adjust to this pattern, in both cases revealing that the majority of the species find their optimum at a certain range of latitude. Perhaps more interestingly, a biogeographical study of the calanoid copepods of the southern Chilean channels showed a nested pattern characterising various subsets of a larger assemblage corresponding to inner "microbasins" within the channels (Marin and Delgado 2001).

Conclusions
The combination of the results presented here and those of the mentioned previous studies seems to indicate that, whereas at a larger scale, there are clear latitudinal patterns in the distribution of the marine species along the southeast Pacific coasts, the distribution on a finer scale responds to more specific ecological preferences of the species and the more extreme environmental conditions in the inner part of channels and fjords. This dataset compiles the first and only records of the benthic Peracarida in the channels and fjords of the Pacific coast of Chile between the 48-51.5°S, hence closing a latitudinal gap for the study of the biogeographical connections of the group along the Eastern Pacific coast. In the present scenario of global warming that is expected to affect particularly polar and subpolar regions, the present dataset serves as a reference for the distribution patterns of benthic organisms with low dispersal capacity.