Caulobothrium sp.

Scallops are highly prized as seafood because of their tasty adductor muscle and roe, but humans are not the only ones with a taste for scallops. These bivalves are on the menu for a wide range of marine animals including various crabs, snails, seastars, marine mammals, and fishes. And many parasites make use of these predator-prey interactions to complete their life cycles.

Scallops are an important part of the Peruvian aquaculture, but little is known about their parasites there. In the study we're looking at today, researchers collected samples of scallops from a scallop ranch in Sechura Bay over the course of three years between 2013 to 2015, to examine them for parasites. They ended up looking through a total of 890 scallops, and the parasite that they encountered most frequently were whitish cysts that turned out to be tapeworm larvae belonging to the genus Caulobothrium.

SEM and light microscopy photos of tapeworm larvae. The lower left photo shows the tapeworm's scolex
Photos from Fig. 1 and 2 of the paper

Those tapeworm larvae were embedded in the scallops' gonads, and their numbers ranged from just twenty to over two hundred per scallop. While the number of infected scallops varied each year, they were nevertheless consistently high, with about eighty to ninety percent of scallops harbouring tapeworms. While this level of prevalence may seem unusually high, this is actually comparable to previous studies on tapeworms in scallops from other regions, so this is nothing too out of the ordinary.

Ultimately, those tapeworms are waiting for a rendezvous with the final host which, based on what is known about other species of Caulobothrium around the world, is the most likely a ray of some sort. Tapeworm species in the Caulobothrium genus have been reported from eagle rays in the waters of United States and Chile, as well as stingrays on the coast of Australia. On the coast of Peru, the adult stages of Caulobothrium have been found in the gut of both eagle rays and cownose rays, and given the circumstances, it is likely that the tapeworms found in the scallop gonads represented the larval stage of those worms.

Rays have specialised jaws armed with heavy, rounded teeth that allow them to crunch through the shell of bivalves such as scallops, and this tapeworm make use of their taste for shellfish to complete their life cycle.

Tapeworm larvae are not the only parasites with an affinity for scallop roe. Flukes in the Bucephalidae family also infect the gonads of scallops and turn them into parasite factories that churn out streams of parasite larvae. Much like those flukes, the presence of so many tapeworm larvae in the scallop gonads can impair the scallop's reproductive capacity, which as you can imagine, would be a concern for scallop aquaculture since they can potentially reduce the number of scallop larvae produced during spawning season.

In terms of infected scallops' edibility, Caulobothrium is known for being host specialists which can only infect rays, so there is no real risk of these tapeworms infecting humans, but on an aesthetic level to most would-be consumers, scallops with tapeworm-filled roe simply look too gross to eat.

The life cycles of most marine tapeworms are not well understood, and of the over one thousands species of tapeworms which have been described from sharks and rays, the full life cycle is only known for a measly FOUR species. Finding and documenting the larval stage of such tapeworms in marine animals such as scallops can help us put together the biological puzzles that are their complicated life cycles, and work out the roles these parasite play in marine ecosystems.

Reference:
Castro, T., Mateo, D. R., Greenwood, S. J., & Mateo, E. C. (2019). First report of the metacestode Caulobothrium sp. in the Peruvian scallop Argopecten purpuratus from Sechura Bay, Piura, Peru. Parasitology Research 118: 2369–237.