Mutualism of the month: Filter buddies: the story of a sponge and its clams

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 a species of sponge (Spongia sp.) and a bivalve (Vulsella vulsella).

A sponge (with clams) after its canals were filled with red resin and then the tissues dissolved away by acid. This leaves only the structure of the canals. The complexity of the filtration canals is amazing. Photo source.

Sponges (not to be confused with the stuff you wash your dishes with) are a group of filter-feeding animals, which have vast canal systems running throughout their bodies enabling the removal of oxygen and food from the water. Water enters through microscopic openings called ostia and then out through larger openings you can see called oscula, driven by the beating of hair-like appendages attached to specialized cells called choanocytes. Captured by the choanocytes during filtering, particles of food are digested and passed on to another group of cells called amoebocytes, which transport the digested food to all the other cells within the sponge. Sponges are superb filter-feeders, removing particles as small as bacteria out of the water column! Similarly, bivalves, such as clams, mussels, oysters, and scallops, are also excellent filter-feeders, employing comb-like gills, called ctenidia, to filter out particles from the water column.

The clam Vulsella vulsella grows embedded within the body of the sponge in a novel "filtering mutualism". The red line on the sponge in the left image corresponds with the red line on the top of the clam. The black lines are both 1 cm for scale. Photo source.

The bivalve Vulsella vulsella lives embedded within the body of the sponge Spongia sp., with only a small bit of the shell protruding outside the sponge body. The sponge provides protection for the clam by making the clam inaccessible to its predators as well as secreting poisonous chemicals. In return, the clam provides the sponge with a hard substratum to grow on, allowing for a more complex three-dimensional structure. The extra height, allowed by having a skeleton of clam shells, gives the sponge and bivalves access to water higher within the water column where food is less likely to have been filtered by competing filter-feeders. However, an important question remains:

How do these two filter-feeders not compete with one another in this arrangement?

The answer is that sponges are capable of consuming much smaller prey items than bivalves. This difference in food selectivity allows for some very interesting relationships to develop without the threat of competition for food. For example, check out February’s Mutualism of the Month between the Queen scallop and several species of encrusting sponge, where the scallop and sponge protect one another from their predators.

A clam within the body of the sponge. Water exits the excurrent siphons of the clam out the right side into incurrent siphons of the sponge (black canals) and through to choanocyte chambers (dotted areas) where food is filtered out. Filtered water exits the sponge via excurrent canals (white). Photo source.

Scientists at Kyoto University in Japan have recently found that these are not the only benefits between Spongia and Vulsella vulsella. The sponges utilize the clams’ filtering capacity by building its incurrent (intake) siphons around the excurrent (exhaust) siphons of the clams. After being filtered by the clams, the water is filtered a second time by the sponges. This greatly increases the volume of water the sponges get access to and reduces the amount of energy that needs to be put into bringing in water for the sponges. Up to 80% of the water being filtered by the sponge is first brought in by the clams; the sponges are highly dependent on the clams for their water intake.

They postulate that the clams also benefit from this arrangement. They suggest that the sponges are also enhancing the amount of water the clams get access to through their own water movement. While the sponge isn’t directly pumping water into the embedded clams, it is moving water around the sponge-bivalve complex. This brings fresh, unfiltered water to both the sponge and the clams, increasing food availability for both participants.

This is a novel type of mutualism; only a few “filtering mutualisms”, where both species promote an increase in the filtration rate of the other species, have been described. To read the original scientific article from the open-access journal PLOS One click here.

Don’t forget to check out more Mutualisms of the Month and all the other great articles on FTDM! Stay hungry!

More Mutualisms of the Month