Heteropod (Atlanta peronii)

This wholly planktonic gastropod mollusc is a member of the superfamily Heteropoda. Along with janthinids, they are the only prosobranch gastropods to live entirely in the water column. Planktonic life is achieved by a great reduction in shell, presence of a swimming fin and a largely transparent body that makes them less obvious targets for predators.

The genus Atlanta, family Atlantidae, is thought to be one of the most primitive heteropods. This species, Atlanta peronii, may reach up to 11 mm in shell diameter. The soft body is housed in a calcareous, planispiral shell that has a glossy and smooth surface ornamented with growth lines. The animal swims upside down with the fin-like foot held above the body and the blade-like keel around the periphery of the compressed shell aids in keeping the organism in a vertical orientation in the water column.

Atlanta is a voracious predator. Its large, highly developed eyes are tubular and projecting, moved by special muscles, and used for scanning prey. Such keen eyesight enables them to detect prey at a distance and, once located, they attack it from below. The captured victim is held in place by a sucker on the long and slender ventral fin while the radula, with prominent chitinous, razor-sharp teeth, shreds away pieces. Atlanta commonly actively predates medusae, small fish, pteropods and other heteropods.

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