Molluscs

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No sandcastle is complete without shell decorations 

 

This group of marine animals provide endless enjoyment for beach goers, for children fossicking for sandcastles decorations through to serious amateur collectors of the calcareous remnants of chitons, snails and bivalves. Again, the Wellington south coast provides a place of considerable variation and diversity of species, forms and colours for this largest marine animal group. Calcareous remnants of shell fish have been important in fossil studies, to identify past ages and sequences in the life of the planet.

Molluscs are all mantled (whether forming a shell or not), have a soft, unsegmented body and most have a rasping ‘tongue-like’ radula with which to graze on microalgae and bacteria, or on other molluscs. Most (except for bivalves) have a head, eyes and sensory tentacles. They are widely distributed, terrestrial and marine, and the latter group exist from the high littoral to oceanic deeps. Most molluscs have gills for respirations and some for filter feeding.

On the South Coast we can find:

Chitons

Characterised by eight plates held together by a girdle around the edge, and occupying the high intertidal zone, chitons are often the most easily visible and identifiable live molluscs on rocky chores. When threatened they can roll up like a slater for protection. They have a barely discernible head, and have eyes, or ocular perception, through small holes in the plates.

Green chiton Photo S Journee

Green chiton Photo S Journee

Bivalves

These are a large group of molluscs which are filter feeders, some of which anchor to rocky reefs or seabed cobbles, such as rock oysters and mussels. Others burrow into sands and sediments, e.g. Venus shells, clams and scallops. Bivalves which are prominent on the Wellington coast include paua (also known as abalone). An extreme form of bivalve is represented by species of Teredo shipworms, which bore into unprotected wood pilings and hulls. Scallops can swim, to escape predators using a water jet for propulsion.

Gastropods

These are characterised by snails (shells) and slugs (little or no shell). Shells may be coiled or uncoiled and include limpets, top shells, and whelks. Some are carnivorous, such as whelks, which feed on other molluscs or worms. Cat’s eye turban shells graze algae, as do two species of paua, blackfoot (the larger of the two) and yellowfoot.  Paua are flat gastropods, with minimal coiling. Common species in sandy habitats include volutes, which graze on shallow burrowing bivalves, and ostrich foot (which have ornamental uses overseas). Ostrich feet have been in this region since Gondwanaland broke up. Violet snails float on a raft of bubbles and are thus able to prey on jellyfish. Limpets may be confused with chitons, clinging to rocks in the intertidal zone, but they have an entire shell, not plates.

Ostrich foot snail

Ostrich foot snail

Gold rimmed nudibranchs Photo: D Stent

Gold rimmed nudibranchs Photo: D Stent

 

The slugs include species with an array of colours from drab black and brown to multi-coloured nudibranchs. Vestigial shells exist on some as shields for vital organs. Sea hares are slugs with antenna which appear to be ‘ears’. They lay copious spaghetti-like knitting of eggs.

Sea slug Photo: M Francis

Sea slug Photo: M Francis

Paua cleaning algae from boulder

Paua cleaning algae from boulder

Cephalopods

The most developed of the molluscs, or certainly the most mobile! These include the squid and octopus.

Source: Mainly  Beu and Marshall, ‘Taputeranga Marine Reserve’, Gardner and Bedll, VUW 2008 Photos MH unless otherwise captioned.