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ABSTRACT We describe, for the first time, egg masses and larval developmental mode of a recently described Antarctic philinoid snail, Waegelea antarctica. Egg masses resembled the gelatinous, attached masses of many temperate philinoid species and contained very large offspring that hatched as developmentally advanced veligers with many juvenile features. Like other Antarctic heterobranch egg masses, development in the masses of W. antarctica appeared to be largely synchronous despite low internal oxygen levels. Hatched larvae could both swim and crawl, and we did not observe metamorphosis over several days. Molecular barcoding using cytochrome c oxidase subunit I (COI) showed an almost perfect (<0.002% difference) match between our specimens from McMurdo Sound in the Ross Sea and a single sequence from a specimen collected >8,000 km away in the Weddell Sea, suggesting either high realized larval dispersal or a recent range expansion. We also describe the egg mass of the related Antarctophiline alata (identified using COI barcoding) from the Ross Sea, which differed from published descriptions in having considerably smaller embryos. 

Antarctic animals share many traits that are attributed to evolution in a stable, extremely cold climate. Among invertebrates, development is exceptionally slow, making observational studies of development logistically challenging, particularly when conducted under natural conditions in the field. Using multiple deployments to McMurdo Station, Antarctica, we characterized the development, in the field, of an unidentified buccinoidean gastropod species with encapsulated development. Thirteen egg capsules collected at Granite Harbor, McMurdo Sound, Ross Sea, were attached to natural rock and outplanted at a depth of ~25 m at the base of the McMurdo Intake Jetty on 2 December 2007, photographed on 5 October 2011 and 6 September 2012 and then returned to the laboratory on 27 November 2015. In 2015, four capsules were open and empty, five were open and contained a single large hatchling and the remaining four capsules were intact but not open, each containing a single large juvenile snail. To identify the developing embryos, we sequenced mitochondrial cytochrome c oxidase subunit I (COI) from two hatchlings and compared those sequences with those from adults collected near the egg mass, as well as with sequences of other buccinoideans from GenBank. Based on the close match between hatchling and adult COI sequences (hatchling sequences differed from those of an adult at only 2 of 658 nucleotide positions), we identified the embryos as Antarctodomus thielei (Powell, 1958)). The egg mass morphology and development of this species have not been previously described. Our study shows that A. thielei has a development time of more than 8 years, which is the longest measured for any gastropod.