And now..onto the show...
So, I can only think that it must be just insane to have a sea cucumber's life.
I mean, you got FISH that live in your ANUS. As I wrote about awhile ago...
And NOW... a report about bivalves (i.e., "clams") that live in a sea cucumber's ESOPHAGUS!!!
A 1998 paper by Makoto Kato in the Canadian Journal of Zoology describes several different adaptations of the unusual bivalve endoparasite, Entovalva in three Indo-Pacific sea cucumbers, including Holothuria pardalis.
and Patinapta laevis and P. crosslandi from Guam/Indonesia and Tanzania respectively..
Here's a handy-dandy schematic of where exactly, the clams live..specifically the ESOPHAGUS, which is the muscular region just behind the mouth but just BEFORE the intestine.
So How Does this work??
The shell on Entovalva is actually wrapped INSIDE an extensive fleshy mantle that surrounds it (think of the fleshy covering in geoducks)
Note the Latin genus name "ento" for "inside" and "valva" for valve referring to the shell wrapped inside the mantle...(as seen below)
These critters actually live in male-female PAIRS..one set per animal!
The female is MUCH larger then the dwarf male (the smaller bit with the male symbol pointing to it in the image above). A feature which is apparently typical for Entovalva.
Another interesting feature is how these clams ATTACH. You know how mussels attach to substrates using these threads? Called BYSSAL threads??
Entovalva uses essentially the same kind of structure, except instead of some rock or bottom, it attatches to the ESOPHAGUS (i.e, the muscular wall) of the host! In this case... the sea cucumber Holothuria pardalis! (seen below)
Kato argues that there are adaptive reasons for this species to be living in the esophogus, rather then say, the guts and intestine where other parasites might live.
One reason? Is to avoid stomach enzymes and other unfriendly substances.. but ALSO to possibly avoid evisceration! That oh-so-delightful part of holothurian defenses that involves the deliberate expulsion of its guts and viscera as a defense mechanism against predators. Such as what its doing here...
Some workers have speculated that some cukes will also eviscerate to "clear out" the parasites in the body cavity.
This isn't something that is universal for all species..but in any case, for a clam to be stuck up in essentially the sea cucumber's esophagus you avoid this particular complication!
So (as I would ask if I had a clam living in my throat) What's it doing there??
Why? because if you're living inside a host, the surviving members are best evolved or adapted to their environment INSIDE of a gut or body cavity.
Internal parasites like nematode worms (below) are often featureless (presumably so as not to get caught on items passing through) and have begun to absorb food across their body surface. What sorts of structures have been "lost"? Eyes? If parasites live in the darkness of an intestine all their lives..who needs em? And if it ever had legs, well, it doesn't move much..so those would be gone as well.. etc., etc.
Unlike true parasites, Entovalva remains fully developed! Consider that in conjunction with the fact that most bivalves are filter feeders...that is, they suck water in, filter food and gases over their gills and spit it back out.
So, these bivalves LIVE in the region right behind the mouth. Water and food passes through this space..so, what happens is this: these bivalves take advantage of the water current and pick out food as the water flows from the mouth into the intestine. (at no apparent loss of nutrition to the host).
This is incidentally why these are considered "endo-symbiotic" rather then parasitic-no harm comes to the host.
The eggs and larvae of these bivalves are located in the mantle and get released FROM the mantle in the esophagus through the intestine and out the anus...
Where they get dropped into the sediment and are presumably picked up by the next appropriate sea cucumber host...
Ah..aint' life grand!
I've yet to hear of a single species that has BOTH the fish in the anus and the clams in the esophagus! But wow. Wouldn't THAT be awesome!