Mutualism of the Month: The queen scallop and encrusting sponges

The queen scallop Aequipecten opercularis with its symbiotic sponge Suberites sp. Photo courtesy of James Lynott via flickr.com

The queen scallop Aequipecten opercularis with its symbiotic sponge Suberites sp. Photo courtesy of James Lynott via flickr.com

Every month we showcase a relationship between two or more species or groups of species that can be considered a mutualism: a relationship where both members benefit. This month’s mutualism is between the queen scallop Aequipecten opercularis and several species of sponges within the genus Suberites.

The queen scallop is a mid-sized edible scallop that gets to about 3 inches (7 cm) in size. This is one of the smallest species of scallops to be exploited commercially (fished). The queen scallop can be found from Norway to the Mediterranean, but is most common on gravel and sand beds in the North Sea (between Norway and the United Kingdom).

The sponges that associate with the queen scallop are "encrusting" sponges. They form thick sheets on top of rocks and shells where they filter feed floating debris, animals, and algae out of the water. Frequently these sponges are found growing attached to scallop shells.

What’s the benefit from associating with one another?

Each species provides protection from the other’s predators. One of the queen scallops’ main predators is the northern sea star Asterias rubens. Sea stars have powerful tube feet and mutable connective tissue, which make them excellent predators of bivalves like scallops. Mutable connective tissue is a specialized connective tissue, which can harden or soften at a moment’s notice. This allows for a sea star to use its tube feet to gain ground as it plays a life-and-death tug-of-war game with its prey, in this case, a scallop. When a queen scallop has sponge on it, grabbing hold becomes much more difficult for the sea star. Tube feet need a smooth surface to grab onto in order to start pulling the two shells apart and the sponge provides a rough, living surface, which the sea star can’t grab a hold of. This greatly reduces the predation risk northern sea stars pose to queen scallops.

The northern sea star Asterias rubens, a predator of the queen scallop. Photo courtesy of wikimedia.org.

The northern sea star Asterias rubens, a predator of the queen scallop. Photo courtesy of wikimedia.org.

In return for protecting the queen scallop from sea stars, the sponge receives protection from its predator: the sea slug Doris pseudoargus. This species of sea slug, which can get up to 5 inches in length (12 cm), is a specialist on sponges. When D. pseudoargus smells sponge, it closes in and consumes its non-mobile prey. If the sponge is attached to a scallop, it can get away. Scallops can swim and do so when threatened. The scallops swim away from D. pseudoargus, and by doing so protect their body armor.

The sponge-specialist sea slug Doris pseudoargus. Photo courtesy of James Lynott via flickr.com.

The sponge-specialist sea slug Doris pseudoargus. Photo courtesy of James Lynott via flickr.com.

This relationship is not obligatory for either species. Rather it’s considered a facultative mutualism. What this means is that both species facilitate one another’s survival, but neither species completely depends on the other.

For more information on the relationship between the queen scallop and its sponge associates, check out this interesting paper: Pond 1992. Don’t forget to check out the previous installments of mutualism of the month below and stay hungry!

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