Soft fangs

DEVIANT behaviour is not limited to humans: a sponge found in the Mediterranean Sea is a killer carnivore. Unlike other sponges that feed primarily by filtering small particles from seawater, this creature preys on small crustaceans (Nature, Vol 373, No 6512).

French scientists J Vacelet and N Boury-Esnault of the Centre D'Oceanologie de Marceille discovered this sponge -- which has an uncanny similarity with other sponges found at a depth of nearly 9 km -- in a cave only 17-23 m below sea level. Like the deep ocean, the environment within the cave had cold water, few nutrients and little light.

Sea sponges are filter feeders par excellence. Their entire body is organised to facilitate this sieving process. Water flows through the body in a unidirectional current, helped along by special pumping cells known as choanocytes.

It has long been suspected that this feeding strategy is modified in deep sea conditions. Scientists have suggested that other shallow water filter feeding creatures like bivalves, direct relatives of mussels, also become carnivorous in the deep sea.

Unlike shallow water sponges, the new find does not have pumping cells or a water filtering system. Instead, its body is covered with filaments which are sticky and have hooks much like Velcro fasteners, from which other small sea organisms can rarely escape.

Experiments with these sponges revealed that small creatures got caught in the non-moving filaments of the sponge, and were unable to free themselves. They remained struggling for several hours, indicating that the sponge did not produce any toxin or paralysing secretion. However, the cells of the filaments established contact with the surface of the prey and there was hectic movement of cells. The filaments grew shorter and thicker and new filaments grew over the prey, engulfing it. After a few days, the prey was completely digested.

Given these completely unsponge-like feeding characteristics, the scientists question whether this creature, which belongs to genus Asbestopluma, and its relatives in the deep sea living family Cladorhizidae should indeed be classified as sponges, with which they share only their sedentary nature. "This adaptation to a poor food environment has resulted in the loss of the diagnostic characteristics of phylum Porifera," argue Vacelet and Boury-Esnault, suggesting that it deserves a separate classification.