Spurilla australis

July 2, 1998
From: Bill Rudman

PHOTOS: Upper: Head of Spurilla australis showing the network of brown ducts (ringed) in which the zooxanthellae are "farmed". Pilot Beach, Laurieton, NSW, November 1979. Lower: Head of dark colour form with half-swallowed sea anemone Cricophorus nutrix, in its mouth. Split Solitary Is, Coffs Harbour, NSW, February 1987. Photos: Bill Rudman

In response to Michael Phan's question on Berghia verrucicornis, here is some information on a closely related Australasian species. Spurilla australis is a member of the aeolid family Aeolidiidae, whose members all feed on sea anemones and related cnidarians. Its particular food is a small brown sea anemone Cricophorus nutrix which is usually found living attached to subtidal brown algae. Consequently, Spurilla australis is normally found crawling on or amongst brown algae. Sea anemones usually attach themselves firmly to a rock or other firm object with sticky secretions and the muscular actions of their "pedal disc" and are usually very hard to dislodge. Cricophorus nutrix however has evolved an escape behaviour. When disturbed it rapidly releases its hold and drops off the algae. This is a useful way of escaping from the attack of crawling predators such as nudibranchs. On a number of occasions I have watched Spurilla australis feeding on Cricophorus and it seems to have found a way of beating the anemone. As it approaches and "feels" the anemone with its long oral tentacles, it lines itself up with a tentacle on each side of the anemone then wraps the tentacles around the anemone so that as the anemone loosens its hold on the algae it is prevented from falling off. Spurilla then pulls the anemone to its mouth and engulfs it.

Some aeolidiids, including Spurilla australis and Berghia verrucicornis, feed on sea anemones which have single-celled plants (zooxanthellae) living in their bodies. This "living together" or symbiosis is quite common in cnidarians, the most well-known example being the reef-corals which could not produce their massive reefs without the assistance of the plants that grow in their tissues. When the aeolids eat the sea anemones they store the still living plants in special ducts connected to their digestive systems, where they are kept alive and "milked" of the sugars they produce when they photosynthesise. In the photo alongside of the head of Spurilla australis, the brown network pattern behind the head tentacles (rhinophores) are a network of ducts in the skin which contain brown zooxanthellae. This brown network penetrates the skin of all parts of the body which are open to sunlight. In this way the zooxanthellae are "farmed" in the best parts of the body for plant growth. For some reason some animals either lose their zooxanthellae or never retain any. Animals without any plants are very pale in colour, as in the animal on the right in the top photo.

Bill Rudman

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