Kyoto, Japan -- Sea cucumbers spend their lives prowling the ocean floor, scavenging for food and generally minding their own business. We can see snails leading similar lives, slimy but not bothering anyone.
Yet some species of tiny sea snails are a bother: they are common parasites of sea cucumbers. Extensive taxonomic research has been conducted on these host-parasite interactions in Japan, where sea cucumbers are a seafood delicacy -- for humans.
Despite these previous studies, however, local species richness still contains some unknowns. Parasites of the sea cucumber species Holothuria atra have been thoroughly investigated, but those of Holothuria leucospilota have not. This is likely because this latter species discharges Cuvierian tubules as a defense mechanism when stressed, making them difficult to dissect.
Motivated to learn more, a team of researchers at Kyoto University has now conducted a comprehensive sampling of Melanella snails parasitizing H. leucospilota in Shirahama, a beach resort in central Wakayama prefecture. They anesthetized the sea cucumbers using menthol to prevent discharge of the tubules, and after examining both internal and external parasites, the scientists were able to differentiate the parasitic snails based on their shell morphology and mitochondrial DNA.
The team found four species of snail parasites: two on the exterior and two more inside the host's body cavity. Molecular analysis identified one of the external parasites as M. spina, the first ever case recorded in Japan. The two inside remain unidentified.
"Our results represent the first record of eulimids exploiting the internal habitat of H. leucospilota," says corresponding author Tomoyuki Nakano. "To the best of our knowledge, it is also the first record of a single holothurian species, like sea cucumbers, being utilized by both external and internal Melanella parasites."
The researchers noted that the infection rates of the two internal Melanella species must be extremely low in Shirahama. Though the team found two parasitic snails inside sea cucumber bodies in 2022, the following two years they did not encounter these parasites in any of the sea cucumbers studied.
The snails' invasion routes into the hosts' body cavities, meanwhile, remain unknown, but the team has some promising clues. Both snails were found around the mouth, with previous research suggesting a strong tendency of other Melanella species to attach to the outer surface around their hosts' mouths. It is possible the tiny snails were taken into the body cavity -- even passively along with sediment -- when the host retracted its mouth.
This seems likely, but it is not the only possibility. Another specimen of one of the unidentified Melanella species partly buried its shell into the hosts body wall, suggesting it may be able to invade the host by penetrating its body. This mystery of the parasitic snails' invasion route is what the research team hopes to decipher next.
