A Deep Sea Mutualism in Response to Predation

Sometimes we just need a little help to get by in life. A nudge, some encouragement or a simple pat on the back will suffice. Being stuck to a rock is not a real good way to avoid predators, unless that rock can move. Symbioses between sea anemones and snails have been well known for over a century, yet it is not entirely clear where the lines are drawn in this relationship. Are the anemones just happening to settle on the backs of snails as they would any hard substrate or is the anemone mafia running some protection racket?

In the latest issue of the Journal of Experimental Marine Biology and Ecology, Mercier and Hamel describe the association between snails and their hitchhikers from the western Canadian deep sea. The anemone Allantactis parasitica colonized the shells of up to 72% of Buccinum undatum, a predatory snail, in a single trawl (photo on left, scale bar=4.5 cm). Between 1 and 6 anemones were found on single gastropod. While living snail shells made up over three quarts of the anemone's habitat, about 20% were found on bare rock with the remainder on uninhabited shells. Unlike their shallow counterparts, none of the abundant deep sea hermit crabs carried anemones on their shells

Settlement

Strangely, when the anemone's larvae were offered shallow versus deep water individuals of the same species of snail, they always selected a deep water individuals. They suggested that larvae are attracted to a biofilm that grows on the mollusc's shell. Shallow water snails have cleaner shells. Biofilms have shown to be settlement cues for a wide variety of other encrusting fauna. What is a biofilm? Its nothing but a layer of small fouling organisms (i.e. bryozoans, sponges, bacterial layers) and other organic/inorganic matter.

But adults can move onto shells as well. Mercier and Hamel made a series of observation of adult sea anemones moving from bare rock to hitching a ride on a snail. Here's the low down:

So you're stranded in the middle of an undersea desert, going nowhere fast. Nothing but rocks, not alot food. A nice looking Buccinum comes your way.

"First, the tentacles of the sea anemone extended to reach the gastropod shell. This behaviour elicited a distinctive behaviour in the basibiont on three occasions: the gastropod tilted its shell toward the sea anemone (Fig. 6A - below)."

"Whether or not it displayed this initial tilting of the shell, the basibiont always remained motionless for the duration of the transfer. Within 5 min of initial contact, the sea anemone firmly grabbed the shell with its tentacles and detached from its original substratum (Fig. 6B - below)."

"then climbed on top of the gastropod (Fig. 6C - below). It generally took 50-60 min for the sea anemone to attach the pedal disc to the shell completely."

Why do snails wear anemones?

Sure they look pretty and you can collect all 6, but snails aren't really known as fashion connoisseurs. One thought is that anemones can help to defend the snails from becoming sea star food. Mercier and Hamel exposed the snails to shallow and deep water individuals of the sea star Lepasterias polaris which evoked a negative response from the snails. Snails adorned with anemones were able to thwart the sea star's approach. In fact, 90% of the time when the anemones were under observation, their tentacles were outspread, on guard and ready to defend their mobile home.

Its not a one-way street though. A different sea star, Crossaster papposus, seemed to prefer the anemone! The authors note 3 instances of predation on the anemone by this edacious echinoderm. C. papposus was deterred by the snail though. Hence, anemones were protected from their predator by sticking with the snail.

Life lesson learned from the spineless? Its a two-way street baby, we get by with a little help from our friends, but have to give a little as well.

MERCIER, A., HAMEL, J. (2008). Nature and role of newly described symbiotic associations between a sea anemone and gastropods at bathyal depths in the NW Atlantic. Journal of Experimental Marine Biology and Ecology, 358(1), 57-69. DOI: 10.1016/j.jembe.2008.01.011