Physiology of Finfish and Shellfish (2+1)

2.1.35. Gastropods with filamentous gills

In the most primitive gastropods, the gills are bipectinate, having an overall shape similar to a birds feather, with narrow filaments projecting either side of a central stalk..

Gastropods such as abalone and keyhole limpets have two gills, which is also believed to be the arrangement in the earliest fossil gastropods. The water current to supply these gills is evacuated through a slit or notch in the upper surface of the shell, below which the anus opens.

In most other gastropods, the right gill has become lost. In groups such as the turban shells, however, the gill still retains its primitive bipectinate form. In these animals, the water current is oblique entering the mantle cavity on the left side of the head flowing over the gills and then being flushed out on the right side. The anus is also on the right side of the body, so that waste matter is efficiently carried away.

Bipectinate gills must be supported by membranes that can become fouled with debris and sediment, restricting such gastropods to relatively clean water environments, such as water flowing over solid rock. In living gastropods, a unipectinate arrangement is more common, allowing species to invade muddy or sandy environments. This type of gill is firmly anchored to the mantle wall along its length, with a single row of filaments projecting down into the water system.

Unipectinate gills are found in a wide range, including marine, freshwater and even terrestrial forms. Examples include periwinkles, conches and whelks. The water current is oblique as it is in the turban shells, but many have developed a siphon from the margin of the mantle. The siphon sucks in water to the mantle cavity and may be long enough to extend through the substrate in burrowing species. In one amphibious group, the Ampullaridae, the mantle cavity is divided into two, with a unipectinate gill on one side, and a lung on the other side.