Gaping, Mantle Parts & Feeding


Active Member
Gaping in Clams.
What causes a clam to gape (hold it’s incurrent siphon or even sometimes outcurrent siphon open, or both) is sometimes a hard thing to figure out. BY NO MEANS will a clam for sure die if it is gaping. There are several of other signs though which added to gaping will almost 95% of the time result in a clam’s death. These other signs are the mantle being retracted into the shell, generalized bleaching, and the clam being nearly completely closed accompanied by gaping (there are several other signs, but these are the biggies).
The reason I say 95% is because I have treated several of clams for pinched mantle (not mine). Usually during the 2-7 days following a treatment, the clam will keep the mantle retracted in to the shell while it also holds its incurrent siphon WIDE open; then the clam usually recovers.
The Maxima below in the first picture is probably about 2 years old and has been gaping for nearly 11 months. Gaping in clams that are usually perfectly healthy is most of the time caused by environmental factors. High temp, salinity to high or to low, intense lighting (more then the clam is use to), water quality or even irritation of pests. One Maxima that I had gaped VERY badly for about 8 months because it had Xenia growing right under the lip of the mantle. If you notice a clam is gaping, there is usually always a reason why – and hopefully a remedy.
However if you have a clam that is gaping and everything else looks fine, like the mantle is fully extended and doesn’t appear “pinched” or there are no signs of infection - it usually never means death as long as you can find out and correct what is wrong with the clam or water in an expedited amount of time. I am going to try to get a good picture of a byssal organ later to post so hopefully it will show one intact and in good condition. Some clams just gape because they can too; even though everything else checks out fine – these tend to be larger clams.
Incurrent and Outcurrent Siphons: Feeding too

Below is a picture of a Squamosa (2nd picture) that when fully opened is about the size of a football. I outlined its incurrent and outcurrent siphon.
When you place a NEW clam, you always want the incurrent siphon to be below the outcurrent siphon or have the flow blowing water across the incurrent siphon first and then the outcurrent siphon. The reason why, is because the clam releases waste through the outcurrent siphon. You DEFINETELY don’t want the waste blowing back into the clam's “mouth” incurrent siphon. Quick reference is that the outcurrent siphon looks like a small smoke stack on top of the clams’ mantle towards the center. NEVER try to move a clam that is attached.
If you can get a good look inside of a clam’s incurrent siphon you might be able to see the gills. These are two white spongy organs that hang on the inside of the clam from the top of the mantle cavity (in front of the heart, kidneys, digestive tract, etc.). These gills, as far as I can tell, act like water filters. When a clam eats it will take in most forms of zooplankton and most forms of phytoplankton. The plankton should usually be considerably smaller though (nanoplankton), ranging from about 2-25 microns. The larger the clam gets the larger particulate matter it may be able to take in. Some products have the micron size written on the packaging. If a clam is ever fed and begins to “hiccup” it is usually closing off one of its siphons so that is can force particles that are too large, back out.
I myself believe that clams have to eat; and there is one main reason why. A clams shell is made to basically fall open so the clam does not use a lot of energy to keep it open, but does burn off or expel what it gains throughout the day doing whatever clams do. Clams will sometimes store food (plankton) in their gut though. Only one reason seems to make sense to me. Zooxanthellae produce a HUGE amount of “food” for the clam to use during the day.
This AWESOME strength that is needed to hold a shell shut, the energy has to come from somewhere. If the Zooxanthellae aren’t producing “food” for the clam, where do these nutrients come from? The Zooxanthellae are most likely borrowing a small amount of the nitrogen that a clam may be taking up while it is shut at night. When a clam is holding itself closed for the evening its metabolic rate and energy consumption would probably be through the roof. This may be why clams take in and store plankton in their digestive tract. A clam probably needs more nutrients during the evening then any other part of the day. This could also be why the plankton in their digestive tract seems to deteriorate more during the nighttime rather then the day.
The third picture is just of a clam with an odd form of coloration.



Great write-up. We always hear about gaping, but it is good to know what causes it and what can be done about it. Thanks for sharing your knowledge.


Active Member
Thanks. I think I might try to do one about infections next and even test "alternative" cures for them that may not stress out the clams that much. The wall that I seem to be running into is finding any clams with infections. I have the testing tanks set up (just little ten gallons) and even several different forms of antibiotics. I just can't find the clams
After that I hope to get into lighting.


Active Member
I think I might have to do parasites next. Seems to be more emailing interest about those. I just need to find some parasites.