(out of the water) in comparison to shells, it is no wonder it has taken many years for opisthobranchs to become popular. Yet this subclass of gastropod molluscs has members with remarkable lifestyles and features, colours and shapes which rival the science fiction beings of outer space.
These molluscs may have reduced external shells, or no shells at all in their adult form.
They may have exposed gills, hidden gills, or no gills at all, depending on the respiratory functions of the skin, or other parts of the body (such as pustules or cerata).
Most have a well-defined head, one pair of rhinophores and at least one pair of oral tentacles.
The eyes are reduced, or non-existent in the majority of species, with most only acting as light receptors. Scientists have separated this subclass into eight different orders.
There are no rhinophores on the head which has an expanded head shield used for burrowing beneath the substrate in search of prey. Most are carnivores feeding on a range of smaller invertebrates including polychaete worms, molluscs and forams. Some are thought to nibble sponges and others eat filamentous algae.
They all emit copious amounts of slime, a few can swim and a number are nocturnal.
They have an oral tentacle on each side of the mouth and smaller cylindrical rhinophores on the head. Eyes as such are non-existent, only minute, black-pigmented, photo-receptors are present.
The body develops posteriorly to a globular shape and a short or long tail may be present. Well developed parapodial flaps enfold the body, which has a thin fragile open-mouthed shell beneath the mantle and purple "ink" glands.
Most species eat green, brown or red algae and seagrass and lay large masses of spaghetti-like egg strings.
They have two rhinophores on the head and the more primitive species (bivalve gastropods etc) may have a thin external shell which they can retract into by various degrees.
Many sapsuckers (especially the green ones) retain microscopic algae components from their food. This process is called chloroplast symbiosis.
Various different kinds of sapsuckers may only be found on one species of algae and are thought to be species specific. Others eat several species of green algae, red algae and diatoms as well as fish and opisthobranch eggs.
The gill is housed along the right side of the body between the well developed foot and overlapping mantle.
Some species have flattened internal shells(Pleurobranchaceans), the Umbraculaceans having a flattened
limpet-like external shell.
Shape and ornamentation is extremely diverse, ranging from smooth, firm, flat ovals (Platydoris) to long skinny serpent-like slugs covered in stinging cerata (Pteraeolidia). Size is variable, ranging from a few millimetres to around 300mm.
Some species may grow to different sizes at different locations depending on suitability and availability of food. Giant forms of the Spanish dancer exist in Indonesia, Fiji and Papua New Guinea but these are generally only observed at night.
Many species are so well camouflaged that detection is difficult, others appear to mimic their food source and may be discreet and inconspicuous. In contrast, the brightly-coloured flamboyant species blatantly advertise their presence to visual predators as a warning deterrent.
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