Antarctic Pycnogonids Research Articles

Antarctica is less isolated with increasing depth - evidence from pycnogonids

No other group of animals typifies the uniqueness of Antarctic life more than Pycnogonida (sea spiders), with 20% of all known species found in the Southern Ocean, and 64% of these endemic to the Antarctic. Despite nearly 200 years of research into pycnogonids and other benthic phyla in Antarctica, the parameters which drive the distribution and diversity of benthic fauna are still poorly understood. This study aimed to investigate the diversity and connectivity of pycnogonid communities on either side of the Antarctic Polar Front, with an emphasis on the role of water depth, using an occurrence dataset containing 254 pycnogonid species from 2187 sampling locations. At depths shallower than 1000 m, communities to the north and south of the Antarctic Polar Front were distinct, while below this depth this geographic structure disintegrated. The Polar Front, or the expanse of deep ocean it bisects, seemingly acts as a semipermeable barrier to species exchange between well-sampled shallow communities. The less sampled and less understood deep sea appears to be better connected, with high levels of shared species following the northward flow of Antarctic Bottom Water. The exceptionally high diversity and endemism of Antarctic pycnogonids may reflect an apparent competitive advantage in cold waters which leaves them vulnerable to ongoing ocean warming, with increased competition and predation pressures.

Sea spiders (Arthropoda, Pycnogonida) from ten recent research expeditions to the Antarctic Peninsula, Scotia Arc and Weddell Sea - data.

This dataset contains information on specimens of Southern Ocean Pycnogonida (Arthropoda), that were collected from ten different research cruises, spanning 13 years. The individual aims and objectives of each cruise can be found in their cruise reports. The specimens have been collated into a single dataset, forming the basis of J. Maxwell's PhD. The dataset will be used to investigate the community structure of Antarctic pycnogonids and the factors which influence its composition. This dataset is published by SCAR-AntOBIS under the licence CC-BY 4.0. Please follow the guidelines from the SCAR and IPY Data Policies (https://www.scar.org/excom-meetings/xxxi-scar-delegates-2010-buenos-aires-argentina/4563-scar-xxxi-ip04b-scar-data-policy/file/) when using the data. If you have any questions regarding this dataset, please do not hesitate to contact us via the contact information provided in the metadata or via data-biodiversity-aq@naturalsciences.be. This dataset adds vital occurrence and abundance data for pycnogonids from 10 previously unexamined research cruises from the Weddell Sea, Antarctic Penisula and the islands of the Scotia Arc. It includes the first pycnogonid data from the Prince Gustav Channel. The 197 sampling stations within this dataset represent an 11% increase in the number of stations where pycnogonids have been recorded in the Southern Ocean, southern South America and New Zealand waters and an 18% increase for above 60 degrees latitude. Presence data for any observed epifauna are also included.

Body Size of Temperate Sea Spiders: No Evidence of Oxygen-Temperature Limitations.

AbstractOxygen limitation has been proposed as one of the key factors that limits body size at high temperatures (the oxygen-temperature hypothesis). Geographic patterns in body size are thought to be driven in part by the effects of temperature on oxygen supply and demand, particularly when the increased oxygen demand of tissues at higher temperatures outpaces the ability of large organisms to supply internal tissues with oxygen. We tested the effects of temperature on the rate of oxygen consumption of two temperate sea spider (Pycnogonida) species, Achelia chelata and Achelia gracilipes, across a range of body sizes. We measured oxygen consumption at 5 temperatures: 12, 16, 20, 24, and 28 °C. Oxygen consumption of both species increased significantly with temperature, but the effect did not depend on body size; thus, we found no evidence to support the oxygen-temperature hypothesis. While previous interspecific studies on Antarctic pycnogonids have found that larger-bodied animals have more porous cuticles, thus potentially offsetting their higher aerobic metabolic demand by increasing oxygen diffusivity, the pore area of the cuticle of the two temperate species did not change with body size. This suggests that the generally small size of warm-water sea spiders may be due to selective factors other than oxygen limitation.

Polar gigantism and the oxygen-temperature hypothesis: a test of upper thermal limits to body size in Antarctic pycnogonids.

The extreme and constant cold of the Southern Ocean has led to many unusual features of the Antarctic fauna. One of these, polar gigantism, is thought to have arisen from a combination of cold-driven low metabolic rates and high oxygen availability in the polar oceans (the 'oxygen-temperature hypothesis'). If the oxygen-temperature hypothesis indeed underlies polar gigantism, then polar giants may be particularly susceptible to warming temperatures. We tested the effects of temperature on performance using two genera of giant Antarctic sea spiders (Pycnogonida), Colossendeis and Ammothea, across a range of body sizes. We tested performance at four temperatures spanning ambient (-1.8°C) to 9°C. Individuals from both genera were highly sensitive to elevated temperature, but we found no evidence that large-bodied pycnogonids were more affected by elevated temperatures than small individuals; thus, these results do not support the predictions of the oxygen-temperature hypothesis. When we compared two species, Colossendeis megalonyx and Ammothea glacialis, C. megalonyx maintained performance at considerably higher temperatures. Analysis of the cuticle showed that as body size increases, porosity increases as well, especially in C. megalonyx, which may compensate for the increasing metabolic demand and longer diffusion distances of larger animals by facilitating diffusive oxygen supply.

Upper limits to body size imposed by respiratory-structural trade-offs in Antarctic pycnogonids.

Across metazoa, surfaces for respiratory gas exchange are diverse, and the size of those surfaces scales with body size. In vertebrates with lungs and gills, surface area and thickness of the respiratory barrier set upper limits to rates of metabolism. Conversely, some organisms and life stages rely on cutaneous respiration, where the respiratory surface (skin, cuticle, eggshell) serves two primary functions: gas exchange and structural support. The surface must be thin and porous enough to transport gases but strong enough to withstand external forces. Here, we measured the scaling of surface area and cuticle thickness in Antarctic pycnogonids, a group that relies on cutaneous respiration. Surface area and cuticle thickness scaled isometrically, which may reflect the dual roles of cuticle in gas exchange and structural support. Unlike in vertebrates, the combined scaling of these variables did not match the scaling of metabolism. To resolve this mismatch, larger pycnogonids maintain steeper oxygen gradients and higher effective diffusion coefficients of oxygen in the cuticle. Interactions among scaling components lead to hard upper limits in body size, which pycnogonids could evade only with some other evolutionary innovation in how they exchange gases.

Genetic signature of Last Glacial Maximum regional refugia in a circum-Antarctic sea spider.

The evolutionary history of Antarctic organisms is becoming increasingly important to understand and manage population trajectories under rapid environmental change. The Antarctic sea spider Nymphon australe, with an apparently large population size compared with other sea spider species, is an ideal target to look for molecular signatures of past climatic events. We analysed mitochondrial DNA of specimens collected from the Antarctic continent and two Antarctic islands (AI) to infer past population processes and understand current genetic structure. Demographic history analyses suggest populations survived in refugia during the Last Glacial Maximum. The high genetic diversity found in the Antarctic Peninsula and East Antarctic (EA) seems related to multiple demographic contraction–expansion events associated with deep-sea refugia, while the low genetic diversity in the Weddell Sea points to a more recent expansion from a shelf refugium. We suggest the genetic structure of N. australe from AI reflects recent colonization from the continent. At a local level, EA populations reveal generally low genetic differentiation, geographically and bathymetrically, suggesting limited restrictions to dispersal. Results highlight regional differences in demographic histories and how these relate to the variation in intensity of glaciation–deglaciation events around Antarctica, critical for the study of local evolutionary processes. These are valuable data for understanding the remarkable success of Antarctic pycnogonids, and how environmental changes have shaped the evolution and diversification of Southern Ocean benthic biodiversity.

Morphological and genetic data clarify the taxonomic status of Colossendeis robusta and C. glacialis (Pycnogonida) and reveal overlooked diversity

Colossendeis robusta Hoek, 1881, originally described from the Kerguelen shelf, is considered as one of the most widespread Antarctic pycnogonids. However, the taxonomic status of this and similar species has long been unclear, as synonymy of C. glacialis Hodgson, 1907 and several other species with C. robusta has been proposed. Here we test the synonymy of C. robusta and C. glacialis with two independent molecular markers as well as comprehensive morphometric measurements and SEM data of the ovigeral spine configuration. We show that C. robusta and C. glacialis are clearly distinct species, and our results also indicate the existence of another previously unrecognized Antarctic species, C. bouvetensis sp.n., as well as an Antarctic lineage closely related to the endemic Kerguelen group C. robusta s.str. We find evidence for strong regional differentiation within each species. Our results suggest that diversity of Antarctic pycnogonids is still underestimated.

Pycnogonida from the Bellingshausen and Amundsen seas: taxonomy and biodiversity

The Bellingshausen and Amundsen Seas are among the least studied Antarctic areas. Pycnogonids constitute a common and conspicuous component of the Antarctic marine fauna. Antarctic pycnogonids have been widely studied and are usually more abundant than elsewhere. Therefore, they represent a key taxon to understand the zoogeographic and bathymetric distributions of the fauna from these two poorly sampled seas. Furthermore, we aim to compare the diversity and composition of the pycnogonids in these areas with those in other Antarctic zones. Three main surveys were carried out in these regions (Bentart 2003 and 2006 and Biopearl II 2008). In total, 879 pycnogonids belonging to 65 species were recorded in 49 stations. Two new species are described: Heteronymphon krappi n.sp. and Nymphon nakamurai n.sp. Ammothea magniceps and A. hesperidensis are recorded for the second time in the Antarctic. The most abundant family is the Nymphonidae (60.5 %), and Nymphon australe is the most abundant species (25.5 %). The biogeographic analysis revealed 39 species in the Bellingshausen Sea (16 new records) and 19 species in the Amundsen Sea (18 new records). The circumpolar pattern is the most common found. The Bellingshausen Sea seems to be a poor area in terms of abundance and species richness compared to the Amundsen Sea and other Antarctic zones. Faunal similarity was clustered into three main groups: the shallow-water stations, the outer continental shelf stations and the slope stations. The abundance of individuals likely responds to the varying amount of organic matter that reaches each bathymetric zone.

The complete mitochondrial genome of the Antarctic sea spider Ammothea carolinensis (Chelicerata; Pycnogonida)

Mitochondria are responsible for the oxidative phosphorylation process. Accordingly, putatively adaptive changes in their genomic features have been variously associated with major eco-physiological shifts in animal evolution, including increased metabolic rates and heat adaptation. Antarctic pycnogonids offer an interesting system to test whether the selective pressure for heat production and increased aerobic metabolism may be driving genomic changes like: (a) unusual compositional biases at the nucleotide and amino acid level, possibly related to cold adaptation; (b) an accelerated rate of mutations/genomic rearrangements, possibly related to the mutagenic effects of oxygen intermediates. The complete mitochondrial genome (mtDNA) of the Antarctic sea spider Ammothea carolinensis Leach, 1814 (Arthropoda: Pycnogonida), the type species for the genus Ammothea, has been determined and is here compared to known genomes from Antarctic and temperate species. We describe a marked heterogeneity in base composition skewness parameters as well as a strong signature of purifying selection toward an increase in thymines at second codon positions, possibly associated with an increased stability of hydrophobic inter-membrane domains. We further observe a fairly high rate of genomic changes, including a possible hot spot of recombination at the level of tRNA-Q. Nevertheless, these features do not seem to be restricted to the two Antarctic pycnogonids analyzed, as to suggest a causal relationship between cold adaptation and genomic changes, and are better interpreted as basal features shared by the entire group. The relevance of the newly determined sequence for the phylogeny of pycnogonids, including its base composition and genomic rearrangements, is further discussed.

Cryptic mitochondrial lineages in the widespread pycnogonid Colossendeis megalonyx Hoek, 1881 from Antarctic and Subantarctic waters

Colossendeis megalonyx Hoek, 1881 is a widespread and abundant pycnogonid in the Southern Ocean which has also been reported from the South Atlantic and South Pacific Oceans. Its strictly benthic lifestyle is expected to promote genetic differentiation among populations and ultimately facilitate speciation. On the other hand, the reported eurybathy and unknown larval stages of this species may allow Colossendeis megalonyx to maintain genetic continuity between isolated shallow-water habitats by active dispersal through the deep sea or by passive rafting on floating substrates. Thus, it remains unknown whether and to which extent geographically separated populations of Colossendeis megalonyx maintain gene flow in the Southern Ocean. We sampled 96 specimens of Colossendeis megalonyx from three stations in the Atlantic Sector of the Southern Ocean and one station from the South American continental shelf (Burdwood Bank). The genetic structure of nominal Colossendeis megalonyx as well as its phylogenetic position within the genus Colossendeis were assessed using a fragment of the cytochrome c oxidase subunit 1 gene. Our data strongly support that nominal Colossendeis megalonyx consists of at least five cryptic and one pseudocryptic mitochondrial lineages, four of which appear to be geographically restricted. Two lineages occurred at locations separated by more than 1,000 km in the Antarctic, thus indicating high levels of gene flow or recent colonization. No haplotype sharing across the Polar Frontal Zone was observed. Our results strongly suggest that cryptic speciation occurred within the genus Colossendeis. The wide biogeographic distribution range of Colossendeis megalonyx and perhaps that of other Antarctic pycnogonids should therefore be regarded with caution.

Oxygen hypothesis of polar gigantism not supported by performance of Antarctic pycnogonids in hypoxia.

Compared to temperate and tropical relatives, some high-latitude marine species are large-bodied, a phenomenon known as polar gigantism. A leading hypothesis on the physiological basis of gigantism posits that, in polar water, high oxygen availability coupled to low metabolic rates relieves constraints on oxygen transport and allows the evolution of large body size. Here, we test the oxygen hypothesis using Antarctic pycnogonids, which have been evolving in very cold conditions (-1.8-0 degrees C) for several million years and contain spectacular examples of gigantism. Pycnogonids from 12 species, spanning three orders of magnitude in body mass, were collected from McMurdo Sound, Antarctica. Individual sea spiders were forced into activity and their performance was measured at different experimental levels of dissolved oxygen (DO). The oxygen hypothesis predicts that, all else being equal, large pycnogonids should perform disproportionately poorly in hypoxia, an outcome that would appear as a statistically significant interaction between body size and oxygen level. In fact, although we found large effects of DO on performance, and substantial interspecific variability in oxygen sensitivity, there was no evidence for sizexDO interactions. These data do not support the oxygen hypothesis of Antarctic pycnogonid gigantism and suggest that explanations must be sought in other ecological or evolutionary processes.

Locomotory mechanisms in Antarctic pycnogonids

Patterns of walking, modes of joint movement, and individual limb diversity were analysed with the aid of cine film of several living Antarctic pycnogonids, including the 8-legged Colossendeis australis, C. angusta, Pallenopsis patagonica, and Nymphon sp., the 10-legged Decolopoda australis, and the 12-legged Dodecolopoda mawsoni. Appendage musculature of several of these species and also of the 10-legged Pentapycnon charcoti and Pentanymphon antarcticurn was dissected. At least two distinct morphotypes were identified: a short-legged, crawling variety (P. charcoti); and the more typical long-legged, large bodied, walking forms. No gross differences in musculature of joints were noted in the species examined. All joints are, at least superficially, hinge joints. The coxa-body joint is largely immobile, the coxa 1-coxa 2 joint alone exhibits promotion-remotion and all other joints are flexion-extension joints. The 8-legged forms move in an imprecise manner, there being irregularity of leg raising and lowering and where legs touch down in relation to the body and to other legs. The 10- and 12-legged forms exhibit more precise patterns of metachronal leg movements. Although legs move in a basic promotion-remotion, extension-flexion mode, there is a certain degree of twisting of a leg as it is picked up, brought forward, and set down; models indicating how such joint movement occurs were constructed. The possibility that hydrostatic pressure is employed in extension is considered and is found to be remote. Lateral placement of legs, orientated in almost all directions in the horizontal plane of the trunk, achieves a versatility of movement similar to that in crabs. Comments on pycnogonid taxonomic affinities are offered.

Digestion in pycnogonids: a study of some polar forms

The process of digestion was studied in some polar pycnogonids, mainly the Antarctic forms Nymphon australe and Nymphon orcadense. Our findings did not support the theory of the digestive process proposed by Schlottke (1933) and this led to a re-examination of his ideas. We propose a new scheme for digestion in pycnogonids and fit Schlottke's observations into this framework. It is suggested that Antarctic pycnogonids may possess behavioural adaptations peculiar to their supply of food. Some directions for further study are indicated.