Seafriends: neritacea

  • Class Gastropoda
  • .
    -- Seafriends home -- back to classification -- Revised: 20010429,

    Nerita melanotragus
    (L: Nereus= god of the sea, belonging to the sea; melano= black; tragos= goat) (E A Smith, 1884)
    Size: 25 mm. Maori name: matangarahu

    Description: The family is typically a tropical group. A related species in the West-Indies is the well known 'Bleeding Tooth'.
    There is something fascinating about the black dull surface when wet or dry, and the contrast with the curiously porcelain pellucid blue-white inner surface. The columella lip inside the opening extends inwards and part of the way across the opening as a white partition, reducing the size and shape of the mouth opening to a half-circle. The operculum is rather beautiful: granulated and marked in orange and mauve, semi-circular in shape, with a curious process on its inner surface.

    Habitat: In the Hauraki Gulf, Nerita ranges from the littoral fringe down to the bottom of the barnacle zone, and in the far north, where it reaches its maximum, it extends plentifully to low tide.

    Biology: Like the periwinkles, Nerita can when necessary breathe in damp air, and it lives by scraping up algae and organic debris with its strong, broad radula. The eggs are deposited under stones in high tidal pools, clustered in tough capsules, forming small greyish-white patches.
    Its main predators are the white (Dicathais orbita) and dark (Haustrum haustorium) rock shells, which rarely leave the water of nearby rockpools.
    Nerita is essentially a 'crash helmet' with a low profile and a wide foot, offering secure attachment. This shape offers least resistance to waves and moving debris like sand and rocks. When suddenly prized from the rock, and turned, it draws its operculum close, releasing a large droplet of water, its 'survival package' against drying out. Its gills, while wetted by water, are capable of breathing air, thus enabling it to live high up the shore, out of reach of its predators. Neritas are often found huddling together in moist recesses, usually facing away from the sun. In this manner they can withstand large waves, while being close enough to exchange spawn. This behaviour also traps more moisture in between them.

    Distribution: Nerita is a warm water snail, common on Australian coasts from Queensland to Perth, and at the Kermadec and Norfolk Islands. Even at Auckland, its range is far more limited than in Northland, and it is there almost absent from harbours. It is found at Taranaki and Hokianga, but is elsewhere almost lacking on the west coast. South of East cape it is a rare shell, and at Cook Strait is hard to find even one.

    Nerita melanotragus
    The drawing shows a normal-sized specimen (16mm) of Nerita melanotragus and an unusually large one (28mm).
    (Morton & Miller, 1968)
    Nerita melanotragus
    Acluster of black neritas huddling together in a damp spot. Also visible on left in the foreground the black topshell Melagraphia aethiops.