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Monday, January 29, 2018

Drilling Sea Snails

When I think of snails, I generally think of two things: the little buggers that arrive in my aquarium stuck to live plants and slowly multiply to such extremes that I have to get rid of them less they take over the tank - even though they are excellent at cleaning algae; and escargot - those delicious snails slathered in butter.

What I don't think about, however, is the predatory nature of some snails, in particular the rapa whelk, a Japanese sea snail of the Muicadae family.

I was reading a newspaper article of a young woman (and friends) traversing the old Marco Polo route through Europe and Asia, and in a stop in Turkey, she learned that the rapa whelk is an invasive species in the Black Sea that has virtually eliminated bivalve diversity there, es evidenced by the beaches littered with punctured seashells.

The Japanese rapa whelk drills into the carapace (shell) and then injects a digestive enzyme, and as the fleshy meat liquefies, it slurps it out. This is opposed to the larger whelks that pry open shells with their one arm and then eat the insides.


Usually making its home in the Sea of Japan, Yellow Sea, East China Sea and the Bohai Sea, and has now been found in Chesapeake Bay in the U.S., the Japanese (or Asian) sea snail (the Rapana venosa) has a pretty, classical sea shell that is rounded and heavy with a very short spire - those coils that come to a point.

It has a large body whorl on the main part of the shell, with a wide opening that is roundish.

While its colors vary from gray to reddish-brown with dark brown dashes on the spiral ribs, some have a black/dark blue vein-like coloration patterns throughout the inner portions of the shell, usually originating from each individual tooth at the outer lip.

The height of the shell can reach up to 180 millimeters (about seven inches).

How bad is the spread of the Japanese rapa whelk? Well, the Rapana venosa is considered among the 100 worst alien species in Europe in the DAISIE European Invasive Alien Species Gateway, one of two marine gastropods on the list. It is considered as about the 52nd the worst alien species in Europe.

These sea snails are very hardy, moving to warmer waters during the water, but are quite adept at living in stagnant, oxygen deficient waters - even polluted waters.

It's a burrower, too, preferring sandy bottoms to bury itself.

A fertile creature, the the rapa whelk is distinctly male or female.

A female Japanese sea snail seen here laying eggs - those banana-like objects on the sea floor.
As for eating, as mentioned the Japanese rapa whelk will drill and secrete an enzyme to suck out the meat of other mollusks, but it has other means of attacking, depending on the species and or size of the food source.

It could secrete a thick mucus that may or may not contain biotoxins to weaken the food source - just like the Alien did in that classic movie where in space no one can hear you scream.

It attacks and eats such fare as oysters and mussels, clams and the northern quahog, but along with the drilling, some species will smother their prey by wrapping around the hinged region of the shell and feed by introducing their proboscis between the opened valves.

The Rapana venosa seen here eating a crab.
This invasive rapa whelk survives because of its tough shell that helps it NOT be a source of food for other whelks, while allowing it to ravage at will.

One creature that will kill and eat the rapa whelk, is the star fish, but it only eats smaller ones, avoiding the larger ones.

While the sea snail does exterminate the local sea creatures, it is at least a delicacy in Japan, where it has now become a cash crop for fisherman robbed by the snail's attacks on their more native fishing such as native mussel crops.

I know, I know... a slow day if all I'm doing is talking about sea snails... but a 7AM hockey game on Saturday and one at 8 something on Sunday have left this brain liquefied. I need to sleep in and imagine I am catching up on my sleep. I know that scientifically that doesn't work, but there is the impression in my brain that it does.

Andrew Joseph

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