Well, if you weren't before GET READY!!! Cause' THAT is what we're talkin' bout TODAY!
What ARE you talking about??? Okay, so in the tropical coral reefs of the Pacific and Atlantic you've got these BIG sea cucumbers (up to about 1 to 2 FEET long!) that digest sediment like earthworms, picking up sediment and digesting the organic particles-like this one:
And others like Bohadschia argus and other species...
PLUS you've got these fish in the family Carapidae-common name The Pearlfishes.. such as those in the genus Carapus...
AND these fish live in the cloacas (aka the nearby rectal space) and body cavities of sea cucumbers!
but its also pretty dang cool! Let's see how this relationship works!
Info on this came from Parmentier & Vandewalle (2005), Parmentier & Das (2004), and Trott (1981). There's a LOT of complexity to this relationship and undoubtedly I will be adding more to the relatively boiled down version here...
So, How Does This uh...relationship work??? First. Physical dynamics.
Sea cucumbers are big fleshy bags full of water and guts. As illustrated here, courtesy of the Tree of Life...
If we simplify that a bit to this (drawn by Echinoblog Art Department!):
And then ADD in RED where the fish live...
So, these fish basically LIVE in the cloaca (i.e., the ASS space) of sea cucumbers! Sometimes they're just too big and so, they stick the big forward part of their body OUT of the anal opening! Bear in mind though, that it doesn't take much for these fish to get into the body coelom or other parts of the sea cucumber host!!
This is what's classically considered a COMMENSAL or as we'll, see sometimes even a parasitic relationship.
The truly commensal pearlfish species apparently feed on crustaceans, using the holothurian's cloacal space as protection...
For additional reasons outlined below, its thought that living within sea cucumbers is adaptive because it confers protection for the pearlfish.
- One or sometimes two animals (male and female when that is the case) will geneally live in a single host sea cucumber.
- Pearlfish live in the cloaca but can also live in the actual body cavity (i.e., coelom) and what's called the respiratory tree-a bunch of tubular branches that comes off the cloaca (in the diagram above in blue). Sea cucumbers use this structure to extract oxygen.
- Pearlfish seek out the cloacal opening of the host and then work their way INTO the anus sometimes head first or sometimes tail first, working themselves backwards into the anus of the sea cucumber. This latter tail first method is used 80% of the time.
- Larger pearlfish are found in larger host sea cucumbers.
- Only SOME species of pearlfish are commensals in certain species of sea cucumbers. Some are free living and others are not...
are actually PARASITIC on sea cucumbers. So, not only do these pearlfishes LIVE in the cloaca/body space/whatever, these little buggers will actually EAT tissues from inside the cucumber, including the gonads, various viscera, and body wall muscles-basically ANY tissue that seems convenient... Yikes! Talk about a bad houseguest!
...sometimes they ALSO live in the huge body cavity of the giant cushion stars (Culctia novaeguineae) or even crown of thorns! (Acanthaster planci)
Which as we've seen in prior posts...have just a PALATIAL coelom filled with water and guts-easily accessible via the tube foot groove!!
Parasitic pearlfish species inside starfish will devour tube feet, stomach tissue, gonads, digestive organs, and etc... in the same way they would on sea cucumbers...
But WHAT do any of these fish GET OUT OF THE HOST ANYWAY??? Some thoughts by Parmentier & Vandewalle (2005) include...
- The commensal Carapus doesn't get food from the host-so what benefit comes?? It turns out that that Carapus spp. may use their host not only as habitat but MAY also as a place to REPRODUCE!! (remember that when two are present you have males and females present!)
- Its suggested that males and females do teh Sex INSIDE the sea cucumber. The eggs are released via the water currents through the respiratory tree and anus into the water. The juveniles are carried by currents to the open ocean before hatching.
It turns out that some pearlfish, such as Carapus actually have increased resistance to the sticky toxins produced by Culverian tubules, which many tropical sea cucumber species spurt out and use to defend themselves...like this one...
but the Culverian tubules only seem to react when threats to the external surface of the holothurian are at play!
The intrusion of pearlfish into the cloaca of the sea cucumber? DOES NOT SET OFF the defensive response!!
Is it possible that the sea cucumber hosts are chosen because of their built in defense systems against predators??? What fish wouldn't want to live in a giant sticky-guts shooting house??
One final word, if these fish do so well, then why aren't all species of holothurians (aka sea cucumbers) occupied by pearlfish??
To put it simply, there may simply NOT be the space that permits oxygen respiration for all species...Some pearlfish species are thought to be KEPT out of occupying some large sea cucumber species.
How might that be accomplished?
Two Words: ANAL TEETH.
These structures protect the anus and the cloacal space!
AND as a personal bit of good advice? Don't look up the term "Anal Teeth" on Google Image without the safeties on! It can be....traumatic...