Gastropods

Gastropods are a large taxonomic class of molluscs, commonly referred to as snails and slugs, many of which live in the marine environment. Unlike the bivalves, these shellfish are characterised by their long muscular foot and single external shell. The shell is often coiled or spiral shaped and can be simple or ornate. In all cases the shell is large enough for the animal to fully withdraw inside it for protection. In some species (for instance, limpets) the shell is only coiled in the larval phase, while other species (e.g. nudibranchs) lack shells altogether.

There are somewhere in the vicinity of 60,000 – 80,000 gastropods worldwide, of which its estimated 30,000 are marine. Over 2,500 marine gastropods have been recorded in New Zealand waters with around 80% exclusively found here. New Zealander’s are likely to recognise many common gastropod shellfish such as limpets, paua, cats-eyes, periwinkles and whelks, due to their importance both culturally and as a human food source. Gastropods perform an important role in the marine food chain, with many fish and birds consuming them as part of their natural diet. Gastropods employ a variety of feeding strategies including grazing, browsing, suspension feeding, scavenging, herbivory and carnivory, filling numerous ecological niches. Fossil records for gastropods date back to the late Cambrian era (around 514 million years ago).

Certain species of gastropod are known to respond to changes in fine terrestrial sediment (mud) and pollution in predictable ways, making them good indicators of estuarine health. Information on the most common gastropod species we monitor is given below.

Key features
The body of a gastropod can be divided in to 4 sections; the head, visceral hump, mantle, and foot. Gastropods have a well defined head with 2 or 4 sensory tentacles on which the eyes are positioned. However, smell is generally their most important sense for locating food sources. The radula is a toothed structure in the mouth used to scrape and cut food prior to ingestion. Some marine snails have modified radula which, in combination with an acid secretion, enables them to bore through shells of other creatures.
The visceral hump lies under the shell and contains the gastropods organs (viscera). The visceral hump is attached to shell by a series of muscles. The mantle is the fleshy shell lining often visible on the outside edge of the shell. It is here that new shell material is laid down, enabling growth. In carnivorous marine species of snail, the mantle is rolled in to a siphon, protruding about the head, which aids in chemical detection of prey. The mantle cavity refers to the space inside the shell where the body and head can be retracted to. Many gastropods can totally withdraw inside their shells, closing off the aperture (opening) with a door or lid known as the operculum. This acts as a defence barrier against predators. Most gastropods hide their gills within their protective shell, however the often brightly coloured, shell-less nudibranchs (sea slugs), display their gills as a rosette along their back.
The broad, flat muscular foot of a gastropod is used primarily for locomotion. Movement is often aided by a mucous secretion which can also be used as a food source for microbes.
Life cycle
Reproductive strategies vary widely between gastropod species; however unlike their terrestrial counterparts, there are often separate sexes. Fertilisation is done externally by releasing sperm and eggs in to the water column. Larvae remain in the water column as plankton and go through a sequence of development stages before finally settling on a suitable substrate and developing in to adult forms.

Typical internal structure of a gastropod (Source: http://intra.burltwpsch.org/users/jlee/Marine/Textbook/marine%20chapter%209.pdf)

Descriptions of species commonly found in Waikato estuaries

Common name: Speckled whelk
Latin name: Cominella adspersa
Scientific group or class: Gastropod
Description: Cominella adspersa is a large marine snail (up to 70mm in height) with a shell that is yellowish-brown and often patterned with reddish brown dots. The shell interior-opening is orange-yellow. Speckled whelks are highly mobile predators of shellfish as well as being scavengers.
Habitat: Sheltered to semi-exposed shores, intertidal and shallow subtidal sand and mud habitats as well as rocky platforms
Distribution: North Island, Northern South Island and Chatham Islands
Indicator species: Yes
Why is this a good indicator species? Cominella adspersa is found in muddy sediments (the optimum range is unknown). Where estuarine sediments change from a sandy to muddier type habitat the abundance of Cominella adspersa is expected to increase. This makes Cominella adspersa a good indicator species with which to assess changes in the input of sediment into our estuaries.
Common name: Limpet
Latin name: Notoacmea spp.
Scientific group or class: Gastropod
Description: Notoacmea are the most common soft-sediment limpets found in the North Island. They are variable in shape and colour and are large surface grazers .
Habitat: Attach to shells, stones and seagrass blades on intertidal sand flats
Distribution: New Zealand wide (North Island, South Island, Stewart Island and Chatham Islands)
Indicator species: Yes
Why is this a good indicator species? Notoacmea are highly sensitive to sediment mud content, with an optimum range of 0-5% and distribution range of 0-10%. Therefore, they are usually found in sandy habitats. Notoacmea can also be sensitive to pollution (particularly zinc). Where the sediment mud content increases (exceeding their optimum range) and/or becomes polluted, the abundance of Notoacmea is likely to decline. This makes Notoacmea good indicators with which to assess changes in the input of sediment and pollutants into our estuaries.

Gastropods

Key features

Life cycle

Descriptions of species commonly found in Waikato estuaries

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