Molluscs and nudibranchs
Extracted from The Sea that Never Sleeps by Douglas David Seifert, read the full feature in the DIVE night diving special
Many gastropod molluscs hide in the sediment by day and emerge at night, to hunt their prey – usually other shells. Although some shells eat worms, a few are herbivores, and cowries are grazers. However, the vast majority of the 30,000 species of gastropods are carnivores.
Some species bore through their victims’ shells and use a radula to rasp the flesh from within; other shells use their massive, muscular foot to engulf their prey and hold it until its digestive juices can render all the nutrients from the prey species.
Among the mighty gastropod hunters, soft-sediment habitats of the Indo-Pacific not only harbour the major harp shell, olive shell, sundial shell, moon snails, helmet snails, baler shells, and the Venus comb murex, among others, but also become a complex and unpredictable killing field, with mollusc feeding upon worms, mollusc feeding upon fish and mollusc feeding upon mollusc.
Cone shells, prized by shell collectors, are among the most active gastropod hunters. They utilise a tiny harpoon modified from their radula tooth and can fire it into the flesh of their prey, delivering an effective and fast-acting venom toxic enough to rapidly immobilise their prey. In one species, the geographic cone (Conus geographus), the venom is toxic enough to stop a human heart and has been responsible for the deaths of more than a dozen humans who mishandled the living shell. Studies have determined that it has a human fatality rate of 65 per cent!
The shell-less members of the Mollusca that can be found at night on soft-sediment habitats of the Indo-Pacific include some species of nudibranchs, which prefer to use the surface of the sediment as their hunting ground, rather than taking refuge within the sand or gravel matrix below. Perhaps the largest, at 50cm, and most strikingly coloured nudibranch, the Spanish dancer (Hexabranchus sanguineus) is seen most often at night. The Spanish dancer is a bright crimson colour with a texture of velvet and feeds upon sponges and soft coral. If disturbed or threatened, it can escape into the water column by undulating its body with contractions and expansions.
The moon-headed sidegill slug (Euselenops luniceps) only comes to the surface at night to hunt worms and molluscs and to reproduce. By day it remains concealed by the sediment labyrinth, unseen by diurnal eyes. Another nocturnal oddity is the massive sidegill slug (Pleurobranchus forskalii), which can have a blotchy pattern or be deep violet in colour and can grow to 30cm. It has a feathery gill located between its mantle and foot, is wholly nocturnal, and feeds upon sea anemones, sponges and tunicates.