Most molluscan species have separate sexes and there are generally no external differences between males and females of the same species. However, the spider snail Lambis lambis (Strombidae), shows some dimorphism when the shells are fully grown, with females having larger shells with long upward-curved spines and males having smaller shells with short horizontally-pointing spines. Other notable examples are the very small yoyo clams (Galeommatidae), where dwarf males live inside the mantle cavity of the female.

Many bivalves (most notably Ostreidae, Tridacn-idae and Galeommatidae) change sex from male to female as they grow; that is, they are protandric hermaphrodites. All the sea slugs are functionally hermaphrodites, producing both male and female gametes and having both male and female reproductive organs in the same body. In other words, they are simultaneous hermaphrodites. However, they never fertilise themselves (although this is physically possible), and instead are able to mate with any other mature animal of the same species they encounter, both individuals acting as sperm donors and egg recipients at the time of copulation. The sea hares (Aplysiidae) (Fig. 24.5E) have an even more extreme kind of partnering where they form long mating chains; the individual at the rear of such a chain acts as a male to the one in front of it by delivering sperm. This second-to-last individual acts as a female by receiving the sperm and, simultaneously, as a male by delivering sperm to the partner in front of it, and so on through the chain. Indeed, there is one report in the literature of the animals at the front and rear of such a chain coming together to form a complete mating ring!

Some sea butterflies (Cavoliniidae) are believed to possess remarkable asexual reproduction. Under laboratory conditions, an unusual-looking skinny individual is formed by transverse fission of the body of a normal individual. The new individual then detaches itself from its shell as a naked animal, transporting only gonads, and grows independently into a fully-shelled separate individual.

Most molluscs simply shed their gametes (eggs and sperm) directly into the seawater where fertilisation occurs, but fertilisation is internal in some gastropods and all cephalopods. In those taxa with an intermediate stage, the resulting larva has a shell like a tiny cap, and lobes on its foot for swimming and capturing plankton for feeding. A larva of this type, termed a veliger, is typical of all molluscs. Veligers drift with the currents until they come across the adult food source at which time they break off their swimming lobes and become crawling juveniles. Those molluscs that have internal fertilisation lay their eggs in tough cases. Either, veligers hatch from these cases and join the plankton, or a miniature version of the adult crawls out. In the first case, the female produces a relatively large number of small eggs, each with a small amount of yolk (lecithotrophic development), and this is the most common pattern found in warmer waters like those of the GBR. In the second case, which is termed direct development, the female produces only a relatively few large eggs, each with a rich supply of yolk, and this occurs mainly in molluscs living in cold waters, although some examples are found on the GBR.

Sex reaches new heights in cephalopods, in terms of both mating and brood protection. The sexes are always separate. Many shallow water cephalopods come together in large numbers to spawn. They have elaborate courtship displays with the males 'dancing' and rapidly changing their colour and pattern at the same time to impress the females. The males of some octopuses display specially enlarged suckers to females as a sign of sexual maturity, with males of one species on the GBR, Octopus cyanea, waving a raised and coiled modified arm tip in the direction of the female during its courtship display. The sperm duct of cephalopods has become exceedingly specialised for the manufacture of sperm bundles (spermatophores). Each sper-matophore is a narrow, hard, torpedo-shaped tube containing a dense mass of sperm. While a penis is used in some species to transfer spermatophores directly to the female, in most cephalopods, the male uses a specially modified arm (the hectocotylus) to transfer spermatophores from the terminal opening of the genital duct to the female (the cephalopod equivalent of copulation), sometimes even leaving his arm holding the bunch of spermatophores inside the female's mantle cavity. Males of other cephalopods bite small holes in the female's skin into which they insert the spermatophores. These implanted spermatophores resemble small, white, parasitic worms under the skin. Cephalopod embryos are special among the molluscs from the moment after fertilisation. The eggs are comparatively large and yolky, and do not completely cleave so that the embryo is built up from a smaller disc of cells on the upper pole of the egg, and the larger part of the egg goes to form a yolk sac from which the young animal is nourished. Female octopuses brood their eggs until the embryos hatch, fiercely defending them from predators.

Although brood protection is best developed in octopuses, it is by no means unique to octopuses or just cephalopods. Females of several gastropods (Cyprae-idae, Ranellidae and Facelinidae) remain with their spawn masses until the young hatch out and it is a common sight to see a female cowrie 'sitting on her eggs' when one turns over a dead coral slab on the GBR. It is one of the reasons people should always replace rocks and coral slabs if they turn them over; if not, the female and all the embryos she is guarding will die from desiccation.

Females of a few bivalves (most notably Galeom-matidae) take brood protection to the extreme by incubating their eggs, and even the developing embryos, inside their mantle cavities. These little incubatory yoyo clams only produce a small number of embryos (up to a dozen) because of the considerable energy that needs to be invested in such maternal care.

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