Gastropoda: Life History & Ecology


Gastropods live in every conceivable habitat on Earth. They occupy all marine habitats ranging from the deepest ocean basins to the supralittoral, as well as freshwater habitats, and other inland aquatic habitats including salt lakes. They are also the only terrestrial mollusks, being found in virtually all habitats ranging from high mountains, deserts and rainforest and from the tropics to high latitudes.

Gastropod feeding habits include grazers, browsers, suspension feeders, scavengers, detritivores and carnivores. Carnivory in some taxa may simply involve grazing on colonial animals, while others engage in hunting their prey. Some gastropod carnivores drill holes in their shelled prey, this method of entry having being acquired independently in several groups, as is also the case with carnivory itself. Some gastropods feed suctorially and have lost the radula, though most species make use of a radula in some aspect of their feeding behavior.

Most aquatic gastropods are benthic and mainly epifaunal but some are planktonic - a few such as the violet snails (Janthinidae) and the sea lizards (Glaucus) drift on the surface of the ocean where they feed on floating siphonophores, while others (heteropods and Gymnosomata) are active predators swimming in the plankton. Some snails (such as the whelk Syrinx aruanus) reach about 600mm in length. There is also a very large (and poorly known) fauna of microgastropods that live in marine, freshwater and terrestrial environments. It is amongst these tiny snails (0.5-4 mm) where many of the undescribed species lie.

Most gastropods have separate sexes but some groups (mainly the Heterobranchia) are hermaphroditic. Most hermaphroditic forms do not normally engage in self-fertilization. Basal gastropods release their gametes into the water column where they undergo development, derived gastropods use a penis to copulate or exchange spermatophores and produce eggs surrounded by protective capsules or jelly.

The first gastropod larval stage is typically a trochophore that transforms into a veliger and then settles and undergoes metamorphosis to form a juvenile snail. While many marine species undergo larval development, there are also numerous marine taxa that have direct development, this mode being the norm in freshwater and terrestrial taxa. Brooding of developing embryos is widely distributed throughout the gastropods, as are sporadic occurrences of hermaphrodism in the non-heterobranch taxa.

The basal groups have non-feeding larvae while veligers of many neritopsines, caenogastropods and heterobranchs are planktotrophic. Egg size is reflected in the initial size of the juvenile shell or protoconch and this feature has been useful in distinguishing feeding and non-feeding taxa in both Recent and fossil taxa.