On the floor - sediment dwellers

 The bobbit worms alarming mandibles stretch wide1

Extracted from The Sea that Never Sleeps by Douglas David Seifert, read the full feature in the DIVE night diving special 

The struggle to eat and avoid being eaten is more easily observed in the shallow, soft-sediment areas and these can be the most fascinating, most productive for a night dive.

A sandy, featureless, flat or slope into deep water is by day unremarkable and divers pass over them without a second glance. But by night, animals that have hidden in the sediment by day, rise up from the substrate and conduct their nocturnal hunting party.

Soft-bodied invertebrates, such as sea pens and sand anemones, rise up and capture the plankton smorgasbord floating past. Tube-dwelling anemones (Cerianthidae) look like filamentous flowers swaying in the gentle current, but they are not sea anemones at all. They are a much older member of the soft corals in the sub-class Hexacorallia. They live solitary lives, their bodies contained within a flexible tube made from mucus and tissues and the sediment itself. Only their
tentacles protrude above the sediment, each tentacle equipped with stinging nematocysts to capture and kill small planktonic animals, worms and tiny fish.

Predatory worms emerge from the substrate and hunt in the darkness, safe from fish predators of the day, and some even become fish predators by night. In addition to annelid worms, horseshoe worms (Phoronida), honeycomb worms (Sabellariidae) and ribbon worms (Nemertea), soft-sediment environments can be home to a vast number of polychaete worm species, including the infamous bobbit worm (Eunice aphroditois). This is a nocturnally-active, sand-dwelling, carnivorous marine polychaete worm that in its own little way is the stuff of nightmares.

The bobbit worm has a relatively large and rather alarming set of jaws comprised of a pair of enormous mandibles that stretch wide apart, like a pair of razor-sharp scythes, and six pairs of maxilla, to efficiently dispatch its prey of feckless fish. The bobbit worm’s ribbed, phallic body has an iridescent sheen of rainbow colours that shimmer when illuminated with the light of a night diver’s torch.

When it is encountered, perhaps only 6cm of the worm can be seen – jaws outstretched, waving slowly, languidly, in the night sea, attuned to stimuli in the waters that indicate the presence of prey.

The bobbit worm extends itself, bit by bit, from its sand burrow into the inky-black water column, awaiting a sleeping or passing fish. In a case of the hunter becoming the hunted, the fish that might have tried to eat the bobbit worm by day become the worm’s chosen prey.

The stimulus of the smell or vibration of a nearby fish triggers a lightning-fast strike with its jaws leading the body, and the fish is snatched from still waters and dragged beneath the sand, where the worm consumes and digests the fish.

Divers normally encounter bobbit worms the size and diameter of a human thumb, but a fully grown adult may be in excess of three metres in length!

Night DIVE5

 

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