Nerite Shells are very common inhabitants of rocky shores on the Queensland coast in Australia; and they generally have a wide Indo-Pacific distribution. There are several species but the specimens shown here are Nerita costata Gmelin and were photographed at Port Douglas.
They have a characteristic appearance with thick black rounded ridges spiralling around the shell whorls, with lighter coloured furrows between them. The spire is blunt. The aperture opening is roughly semicircular with a specific arrangement of protruberances or ‘teeth’. The odd-shell-out in pictures 1 and 2 is a Mulberry Shell or Granulated Drupe (Morula granulata Duclos) which has an odd look with spirals of dark rounded bumps vaguely resembling a bunch of grapes or similar.
Mud Creepers are a common sight on the tidal mud flats at Cairns in Queensland, Australia. Also called Telescope Shells, Mudwhelks, Telescopic Creepers, or Mangrove Mud Whelk. The Latin name is Telescopium telescopium L. They are so emblematic of the place that they have been glorified by a fantastic piece of sculpture by Dominic Johns on the Esplanade.
The reality, however, is that this large gastropod marine mollusc – living on the glutinous muds exposed by the ebbing tide – is not the most attractive of seashore creatures, in fact, a bit creepy. It is dark in colour, about 6 inches long, and the shell is very thick and heavy, The protruding muscular foot and tubular siphon of the animal are hard to distinguish from the mud itself but I believe the flesh is edible.
This sea snail seems to struggle as it drags its weighty shell across the mud in strange irregular movements without the supporting medium of water. The furrows incidentally ploughed by the shells leave networks of trails on the mud. In life the shells are often caked with mud but empty shells washed up on the beach show there is actually a great striped pattern.
Most seashells with a coiled shape are Gastropod molluscs – like the common winkles and whelks. However, the small, loosely-coiled seashell from Myall Beach at Cape Tribulation in Queensland, Australia, shown here belongs to the Cephalopod molluscs. The most commonly-occurring Cephalopod molluscs at the present time are animals without external shells. Examples of these include the squid, and cuttlefish - which have a reduced internal skeleton – like the familiar cuttlefish bones found so frequently on beaches. Octopuses are also Cephalods.
We know from the fossil record that, in the distant past, there were many cephalopods with external coiled shells; and these were called ammonites. There are several earlier posts in this blog about fossil ammonites. There is a similar loose coil shaped Cretaceous ammonite called Aegocrioceras quadratum (Crick) found in the UK.
There are a still a few modern Cephalopod species which retain an ancestral-style external coiled shell such as the ones found as Ammonites. These include the Nautilus and and Paper Nautilus group of species. Less commonly known is the single species of the single genus belonging to the family Spirulidae – shown in the photographs here. This is the Common Spirula or Ram’s Horn (Spirula spirula L.). You can see through the fragile translucent shell to the internal septa or partitions that divide the shell up into the compartments that are so familiar from ammonite fossils. The octopus-like animal itself would have in life mainly occupied the outermost compartment, with its swimming tentacles protruding from the circular opening.
Apparently the shell of the Common Spirula is found on beaches all over the world but the living animal is rarely seen because it is a pelagic deep water form. This particular shell specimen has washed ashore with minute goose barnacles or stalked barnacles attached to it. The small balls of sand on the beach with the shell are made by the many burrowing Bubbler Crabs that inhabit this tropical shore.