Asteroidea

Antarctic Asteroidea: Morphological trends between deep-sea versus shallow taxa

Depth has played an important role in the diversification of different groups of deep-sea Asteroidea. For example, the widely occurring Atlantic species Zoroaster fulgens Thomson, 1873 was found to display different morphotypes present at different depths (Howell et al. 2004). Mah (1998, 1999), Mah & Foltz (2011b), and Zhang et al. (2019) have all shown phylogenetic analyses of the Brisingida which have shown stemward, shallow-water clades, including the Antarctic Odinella Fisher, 1940 relative to derived clades, such as the Freyellidae that have diversified into bathyal and abyssal habitats.

Although data collection on bathymetric distributions of Antarctic asteroid species remains ongoing, some trends are evident. Observations of USAP specimens suggest morphological differentiation with depth appear evident within different groups. Based on genera of the Goniasteridae and the Asterinidae documented here and in Mah (2011) there are several aspects of skeletal robustness appear more strongly developed in shallow-water settings versus those which occur in bathyal and abyssal settings. Within the Goniasteridae, Chitonaster Sladen, 1889 and Notioceramus show more heavily calcified granules, thicker spines and a more heavily developed skeleton compared to deeper-water species. Mah (2011: fig. 3) outlined this for Chitonaster, showing that the shallow-water Chitonaster trangae Mah, 2011 displayed the presence of papulae, larger abactinal plates, thicker spines, and more heavily calcified granules versus those Chitonaster species below 2000 m, which showed smaller abactinal plates, absent papulae and finer spines. Similarly, Notioceramus anomalus shows more heavily calcified granules and marginal plates and thicker furrow spines relative to the bathyal/abyssal species of Notioceramus documented herein.

Similarly, for the Asterinidae, Kampylaster species in shallow settings (<1000 m) demonstrate more robust skeletal characters than those present in deeper-water settings. Kampylaster incurvatus in shallower habitats display rounder, more strongly developed granules, and more heavily developed skeletal plates than in species from deeper-water settings (e.g., Kampylaster claireae n. sp.). Although known from a single specimen, the deep-water Paralophaster paucispinus n. sp. shows far fewer spines per paxilla than species of Paralophaster from shallower depths.

In contrast, several species occur across wide depth distributions but don’t appear to show appreciable morphological differences. Odontaster meridionalis was documented from between 0 and 2907 m as reported here and the astropectinid Psilaster charcoti (Koehler, 1906) has been documented from between 30 and 3246 m (A.M. Clark 1962).

Gut contents & feeding biology from specimen collections

Specimens reported herein include two specimens with gut contents which provide further evidence for Antarctic/high-latitude predation by solasterids. Among gut content findings were the first evidence of predation by Paralophaster. Three specimens of Paralophaster antarcticus, USNM 1136644, 1418098, and CASIZ 174645 showed disarticulated comatulid ossicle remains within the gut cavity. Ossicles in 1136644 were unidentifiable, 1418098 and CASIZ 174645 were fragments of the antedonid crinoid Florometra mawsoni A.H. Clark, 1937, identified based on the small, fine spinelets on the distal edge of the brachials. A second specimen in USNM 1418098 contained a molt from an amphipod in its gut.

One specimen of Paralophaster godfroyi, USNM E 41734 showed unidentifiable disarticulated comatulid crinoid ossicles in its gut cavity. No prior record of predation by Paralophaster could be located, but predatory habits appear to be similar to those of Lophaster, which has been observed feeding on comatulid crinoids (Mah, unpublished data). Predation on other asteroids, especially cannibalistic behavior has never been reported for Paralophaster or Lophaster.

A specimen of Lophaster stellans, USNM 1662078 (Fig. 24C) was also observed with gut contents that appear to be disarticulated echinoderm ossicles, apparently composed of paxillae and adambulacral ossicles from a solasterid asteroid, perhaps Lophaster or Paralophaster.

One specimen of Solaster regularis, USNM 1137298 (Fig. 24A, B) from Dawson Island, in the Magellan Strait, at 2–9 m, demonstrates a novel observation of feeding for this species, with what appears to be a partially digested asteriid, Anasterias antarcticus in the mouth being digested. Fisher (1940) reported remains of the pterasterid Pteraster sp. from gut contents of an individual of this species collected from the Falkland Islands. No other reports of feeding by Solaster regularis could be located. Although Solaster regularis is poorly understood, other Solaster species are known for their predatory behavior, feeding on a range of echinoderms including other sea stars (Jangoux 1982).

Several specimens of Crossaster penicillatus contained fish vertebrae, various organic debris and sponge spicules, suggesting that they feed in part on detritus.