Known to scientists as gastropods or univalves, snail-like shellfish are the most prolific of the seven mollusc groups. They are found in all marine habitats, from estuaries and coastal shores to the deepest depths of the ocean floor. The characteristic spiral shell ranges from the flattened ear shape of the native abalone or pāua (Haliotis iris), to the coiled spire of the common turret shell (Maoricolpus roseus). A few New Zealand snails, such as the spiny murex (Poirieria zelandica), have intricately sculptured shells, but there are few of the large, colourful shells found in other countries.
Most sea snails have a well-developed head, with tentacles and eyes. They withdraw their head and foot into the shell when inactive or threatened. For added protection many have a disc (operculum), attached to their foot, that covers the opening when the snail withdraws inside.
Sea snail sizes
At 24 centimetres long, the trumpet shell (Charonia lampas) is New Zealand’s largest sea snail. It is usually seen around rocky coasts in spring when it moves into shallow waters to breed. Take your pick for the smallest sea snails – around 75% of them are smaller than a little fingernail, and hundreds are the size of a pinhead.
Many of the sea snails commonly encountered around the rocky coast eat seaweed. They usually move slowly on their broad, flat foot, scraping away at low growths of seaweed with a rasp-like radula (tooth-lined tongue), or browsing on the fronds of larger seaweeds. Top shells (100 New Zealand species), limpets (50) and turban shells (25) are well represented among the coastal grazers.
Reaching up to 15 centimetres in length, black-footed pāua (Haliotis iris) are New Zealand’s largest herbivorous snails. These limpet-like creatures can attach themselves with such a powerful grip of their muscular foot that divers need a knife to pry them from subtidal rocks. Pāua feed at night on red seaweeds and decomposing pieces of brown seaweed that drift by.
Many of the carnivorous sea snails are instantly recognisable by their shell, which tapers at each end. When searching for food they extend a long tube (siphon), which acts as a sniffer for tasty chemicals in the incoming current of water.
Whelks, rock shells and oyster borers are common carnivores around New Zealand’s rocky coast, preying on barnacles and other molluscs. Whelks are one of the largest groupings of sea snails, with over 75 species recorded from New Zealand waters.
Small and deadly, oyster borers (Haustrum scobina) force open the protective plates of barnacles with their muscular foot, and insert their proboscis into the flesh to consume it. When attacking shellfish such as oysters, they drill the shell with their radula, taking anywhere between 45 minutes to two days to reach the succulent meat inside.
The beautifully marked Arabic volute (Alcithoe arabica) dwells under sandy sediments during the day, emerging at night to seek shellfish such as cockles, pipi, and Dosinia species.
Violet snails (Janthina species) float upside down in the open sea. They secrete a raft of bubbles that keeps them afloat as they drift with the currents, and feed on jellyfish and Portuguese man-of–war colonies. After strong storms their shells wash ashore, especially on Northland coasts.