Thalassocalyce

Thalassocalycida is a monospecific order of ctenophore, or comb jellies, known from the California Coast, Gulf of Mexico, and west north Atlantic.^[1] The order was erected by description of Thalassocalyce inconstans collected in slope water off the coast of New England.^[2] T. inconstans is a pelagic ctenophore typically occurring in upper-mesopelagic depths, but has been observed at depths up to 3,500 m in Monterey Canyon.^[3]

Due to their fragility, gelatinous zooplankton are inherently difficult to sample by traditional methods (i.e. net tows),^[4] and among ctenophorans Thalassocalyce inconstans is especially fragile having thin, flaccid tissues, likely contributing to broad under-sampling.^[2][3] The individual collection of undamaged specimens via SCUBA and novel instrumentation (e.g. Remotely Operated Vehicles, submersibles, and imaging systems) has allowed for more robust investigation of T. inconstans.^[2][5]

Morphology

Thalassocalycid morphology is unique within the Ctenophora, sharing certain similarities to both cydippid (i.e. canal structure) and lobates, and characterized by absence of auricles and muscular lobes. Rather, unmuscular lobes are fused into a continuous dome forming a medusa-like body plan. While morphologically and structurally distinct from the oral lobes of lobate ctenophores,^[2] the thin thalassocalycid bell is functionally analogous as mechanism for prey-capture. When fully expanded, the body forms a hemispherical bell, but a bi-radial (“two-globe”) morphology is apparent when the organisms is partially contracted. The function of fully developed tentacles that do not extend outside of the “bell” has not been resolved.^[3] Several lobate ctenophores, (e.g. Leucothea multicornis) pass through larval stages that are ‘mudisoform,’ but the retention of a medusoid-like bauplan in adults is unique to Thalassocalyce inconstans. Initial description of this species overturned suggestions that T. inconstans was a larval form of species already described.^[2] Auricle-free ctenophorans are known from the deep-sea, but these species have not yet been described due to sampling obstacles.^[3]

Foraging

Thalassocalyce inconstans is a predatory ctenophore that feeds on euphasiid krill and small crustaceans. A passive foraging mechanism is employed, whereby T. inconstans ‘fishes’ for prey with its bell expanded. Prey that come into contact with foraging (bell-expanded) T. inconstans stimulate the rapid contraction of the bell, and are trapped within a mucus-covered inner surface.^[3]

References