Clam hyper-extended? (56k run away)

[Crab Rangoon]

I got a call from my friend today that said "dude, you're (Gigas) clam thats always open real wide is split in half, man. The snails are swarming it, and its open way bigger than before, did it explode?"
:\
As soon as I get over there, I notice it looks as if the area below the mantle in the front (lower than the opening exposing the gills) has separated. The shell has been split at the end, but the mantle is by no means ripped, it just looks hurt.
WTF is the cause (as he has NOTHING but a Yellow Watchman Goby, Green Chromis, some snails and crabs in the tank he's holding my inverts), because he said nothing was aggitating it until this happened. While it is true that the clam was generally more spread open than the rest of my clams, it never looked bad, or had adverse effects, til this.

I'll post a pic in a minute, to see if I can make the problem more clear.

[Ross]

check for little white snails on the bottom of the clam.

[Crab Rangoon]

Here is the best I could get

[Instar]

Its possible for a snail to go in after a piece of wayward food or something small that died and smells
like its inside the shell. A snail goes in or falls in and the clam shuts, trapping the snail, but unable to completely close.
The reaction to close is even stronger then, and it strains the abductor. Other snails follow suit and find it a very nice
place to spend the day light hours but it injures the clam. You may have to elevate it above the sand to heal so the snails
and hermits will stay out now for a while. If it gets worse, there are lot of secondary infections as well as the pyrmidellae
snail possiblity. Bristle worms can also come up from the hinge if things are right. A friends tank, who knows what the
real cause is since its not possible to observe it at some other place. From the way its worded at the top, he isn't a salt
tank guy? A clam naturally has a split down below to blast water through to help it move around and bury in the sand
with its foot.
The fact that its always been wide open, perhaps too open, could mean it had health problems
and has lost control of its abductor muscle. If it was in poor health when you got it, that could give
rise to serious problems in this area. Open too wide with too wide an intake siphon is referred
to as "gapping" and indicates a potential problem.

[Crab Rangoon]

Yeah, this is a friend whom isn't all that educated about all things marine, but I got him into tank keeping.
I'm the one who sold him his tank, all his equipment, livestock, everything, as well as did all the setting up
and installing, so I know the tank as well as my own, and he lives across the street from me. Parameters
never swing around, and we haven't obsserved any negative hitch hikers, though we've looked plenty. I
thought about the snails and worms eating the clam from underneath, but there are 2 5" Croceas, this 4"
Gigas, a 3.5" Derasa, and a 8" Maxima in the tank as well, and they are all in awesome shape. I know about
the split/opening on the underside/shell of the clam, but never once (even of all the stock we've recieved
@ work) have I seen the separated this much, all the way to the foot of the clam. It's a good bit closed up
in the pic, as I'd just grabbed it and held it up for the picture, but when its open, it just looks way too wide.
Measuring across the gap @ the top in the middle, from shell tip to shell tip, this clam is almost 3.5" apart,
where the Croceas and Maxima are about 2-2.25" open, Derasa almost 2" open. So this clam is almost 2x
as wide open as other clams up to 2x its size :\ :(

[hobogato]

i have a gigas that stays open very wide also, and it has never seemed to be ill. i have had it several months, and it has grown some
and never given any indication of a problem. as you noted, it is about 2x as wide as a comparably sized maxima or crocea.

[Instar]

The comparison of openings is really relative. I have perfectly healthy clams of the same species and they all
open to different amounts. Some look like they were grown buried in the sand and others have their mantles extended
all the way around the open part and can open there. Yours is small for a gigas really and there is a lot of clams
weight in that tank. Are they getting enough noruishment? Light bulbs aged now? It not just about light. Its really next to
impossible to be very effective without being there. Perhaps you can find a clam person, other than you, near you?

[puredrive]

Its possible for a snail to go in after a piece of wayward food or something small that died and smells
like its inside the shell. A snail goes in or falls in and the clam shuts, trapping the snail, but unable to completely close.
The reaction to close is even stronger then, and it strains the abductor. Other snails follow suit and find it a very nice
place to spend the day light hours but it injures the clam. You may have to elevate it above the sand to heal so the snails
and hermits will stay out now for a while. If it gets worse, there are lot of secondary infections as well as the pyrmidellae
snail possiblity. Bristle worms can also come up from the hinge if things are right. A friends tank, who knows what the
real cause is since its not possible to observe it at some other place. From the way its worded at the top, he isn't a salt
tank guy? A clam naturally has a split down below to blast water through to help it move around and bury in the sand
with its foot.
The fact that its always been wide open, perhaps too open, could mean it had health problems
and has lost control of its abductor muscle. If it was in poor health when you got it, that could give
rise to serious problems in this area. Open too wide with too wide an intake siphon is referred
to as "gapping" and indicates a potential problem.

this is my tank...

i saw a few (4-5) snails having a feast when the clam was fully opened..then all them just went away after awhile.

the clam then closed itself..until a few hours later.


so the clam should make a full recovery?