Intro to Giant Clams

Basic Anatomy

Tridacnid clams have shells consisting of two halves, or valves, which are joined at the top. Muscles and ligaments on each side of the shell hold the shell closed and pull the shell open. Species of tridacnid clams can be easily identified by the shape, symmetry and texture of their shells.

All clams have a tissue structure called a mantle. One function of the mantle is to increase the surface area of the clam so that it receives maximum exposure to the light. It also precipitates calcium carbonate which forms the clam's shell. The mantle contains symbiotic algae (zooxanthellae) and light sensory organs which contribute to a clam's coloration and pattern. The zooxanthellae produce photosynthesis-derived food for its host while the light sensory organs serve to position the clam for maximum light exposure, to protect the clam against excessive light and UV radiation and to warn the clam of potential predators.

The mantle is whole except for where it is broken by two siphons; the inhalant siphon, which is often fringed with fine tentacles that strain larger particles out of the water, and the exhalent siphon which is tube like.


Photo of T. crocea showing the mantle, the inhalant siphon, the exhalent siphon and the gills (photo courtesy of A. Carriglio)


Clams are filter feeders. Water and food are drawn in through the inhalant siphon to the gills where food is caught. The gills also draw oxygen from the water. From the gills, the food is transported along a groove to an organ which pushes it into the clam's mouth. The exhalent siphon carries away the water. Clams get some of their food from filtering the water, however, they get a lot more of their nourishment from their symbiotic algae.

The byssal gland of a clam secretes filaments called byssal threads which hold the clam to the substrate. Because the larger clams, such as T. gigas, T. derasa, and H. hippopus get so large, they will eventually lose the gland and rely on their size and weight to hold them in place. T maxima and T. crocea are rock boring clams, therefore, their byssal glands excrete a substance that dissolves the substrate to which they are attached which helps them to burrow into it.


Lighting requirements


Some tridacnid clams require moderate to high lighting and some require intense lighting. Those that require moderate to high lighting are T. squamosa, T. derasa, H hippopus and T. gigas. Those that require intense lighting are T. maxima and T. crocea. If lighting in the aquarium is not sufficient, the clam may display higher than normal mantle extension. This usually means that the clam is trying to extend its mantle for maximum light exposure because of insufficient lighting.




A Guide to Buying and Caring for Tridacnid Clams



Make sure that you have enough light and space in your aquarium to support the clam long term.

Only purchase clams that respond quickly to shadow, touch or other stimulus by closing its shell.

The mantle should not have any tears or other damage to it. The mantle should extend past the edge of the shell (H. hippopus being the exception).

Check for signs of gaping (shell is open wider than what is considered normal, little or no mantle extension, intake siphon open wider than what is considered normal, slow closing response to stimulus). A gaping clam should not be purchased.

You should not see any torn or loose tissue hanging from the bottom of the clam. Some byssal threads may be visible, but no solid tissue should be hanging.

Place clams on the appropriate substrate and away from any corals or other inverts that may sting it. Do not place the clam where it is constantly blasted by strong and direct water currents. Because the exhalent siphons of a clam are capable of quickly expelling large amounts of water, you'll want keep this in mind if you decide to place your clam high up in the aquarium, close to your lighting system.

If you need to move a clam to another location in your aquarium, it is very important that you not tug at or pull on the clam if it is attached to the substrate. It's best to cut the byssal threads as close to the substrate as possible with a razor blade until the clam is no longer attached and then relocate it. One way to avoid having the clam attach to something that cannot be moved or attach in a place where you cannot reach the byssal threads, is to place a flat piece of live rock or a shell underneath the clam. Once the clam attaches itself to this piece of rock or shell, it can be easily moved. The rock/shell also prevents predation of the byssal muscle from worms and other sandbed fauna.

Certain angelfish, triggerfish, puffers, butterfly fish, wrasses and other fish have been know to eat clams as do certain crabs and shrimp. Therefore, spontaneous fish or invert purchases should be avoided if you are keeping clams or plan to keep them in the future.

Many clams can harbor parasitic boring (pyramid) snails which can wreak havoc in a tank full of tridacnid clams. Look for rice grain-sized, cream colored spots near the base or in the scutes of the clam, or, at night, along the upper edge of the shell. If the clam is attached to a rock, check by lifting the clam a short distance off the rock and look underneath. You are looking for small (0.08-0.2 inch / 2-5 mm long) snails. Remove all of these snails. If you have a quarantine tank, quarantine the clam until you are sure that all of the snails are removed. Also check for small, jelly like egg masses and remove them as well. Don't confuse the jelly-like mass some clams excrete around their byssus opening for these egg masses.

When introducing the clam to your aquarium, it's common for your fish and shrimp to check it out for any desirable food that may have come in on the clam's shell. This should only be a concern if the animal begins to bite and/or tear at the mantle.

The number one cause of a clam's demise is usually water quality.

Calcium is the main building block for clams and should be present in the water at levels of at least 280 mg/L for growth to occur. More rapid, natural growth is seen when calcium is in the range of 400-480 mg/L.

Strontium is incorporated in the shell along with calcium and should also be provided for optimum growth. The addition of iodide to the aquarium will also enhance growth and color in clams.

High pH and temperatures can cause problems. Do not let the aquarium exceed 82 degrees or a pH above 8.3. Maintain a dkh of 7.9.

Too high or low a salinity can cause the death of a clam. Try to keep specific gravity between 1.023 and 1.025.



Photo of T. Gigas showing well defined ribs (notice lack of scutes).
(Photo courtesy of A. Carriglio)




Photo of T. maxima showing well defined ribs and tightly spaced, well defined scutes.
(Photo courtesy of JP Dias at www.justphish.com)



Photo of H. Hippopus showing distinct asymmetrical shell and well defined ribs.
(notice lack of scutes) (photo courtesy of JP Dias at www.justphish.com)



Photo of T. crocea showing well defined ribs and tightly spaced, less noticeable scutes.
(photo courtesy of JP Dias at www.justphish.com)



References

Identifying the Giant Clams, James W. Fatherree, M. Sc., Reefkeeping On-line Magazine. Identifying The Tridacnid Clams - by James Fatherree - Reefkeeping.com

Fish & Chips, A Monthly Marine Newsletter, Elizabeth M. Lukan 8/17/99.
Fish & Chips August 1999

Giant Clams: A Comprehensive Guide to the Identification and Care of Tridacnid Clams, Daniel Knop, Ricordea Publishing, July 1996.

 

Tridacnid Identification Guide

tridacnid Identification Guide


T. Maxima



Mantle Color/Characteristics
Blue, purple, gray, brown, and yellow are common. Black specimens exist but are somewhat rare. Spotted, striped, or blotched. Patches of solid color are more common than multi-colored patterns.

Shell
Asymmetrical. 5 very distinct ribs. Moderately defined, tightly spaced scutes.

Approx. Adult Size
16"

Byssal
Medium to large byssal opening.Semi-rock boring.

Siphon
Small, simple tentacles on inhalant.

Lighting Requirement
Intense.

Placement
Preferably on rockwork. Brightly colored specimens require brighter lighting than brown ones. Brown T. maximas should be placed lower in the aquarium to prevent them from getting shocked by strong lighting.

Feeding
Phytoplankton only when clam is 3" or smaller. Light and supplemental phyto when larger.


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T. Crocea



Mantle Color/Characteristics
Blue, purple, yellow, green, gold, orange, and brown. Lines, spots, or blotches of yellow, blue, and green can also be seen.

Shell
Asymmetrical5 to 6 moderately defined ribs. Scutes are tightly spaced but not well defined.

Approx. Adult Size
6"

Byssal
Large byssal opening. Rock boring.

Siphon
Small, simple tentacles on inhalant.

Lighting Requirement
Intense.

Placement
Preferably on rockwork. Clams with bright coloring (especially blue) should be placed high in the aquarium.

Feeding
Phytoplankton only when clam is 3" or smaller. Light and supplemental phyto when larger.



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T. Derasa



Mantle Color/Characteristics
Wavy striped patterns or spots. Can be found in black, white, blue, yellow, and orange color combinations, but most mantles have a golden color. Some have bright green or blue lines.

Shell
Symmetrical. . 5 to 7 moderately defined ribs that lack scutes.

Approx. Adult Size
20"

Byssal
Narrow/small byssal opening.

Siphon
Large complex tentacles on inhalant.

Lighting Requirement
Strong.

Placement
On rockwork or on sand.

Feeding
Phytoplankton only when clam is 3" or smaller. Light and supplemental phyto when larger.



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T. Squamosa



Mantle Color/Characteristics
Beige, brown, or gold. Some can have blue or green blotches and some have stripes running parallel to the shell.

Shell
Symmetrical4-5 large, very defined ribs. Large, sharply defined, widely spaced scutes.

Approx. Adult Size
16"

Byssal
Medium to non-existent byssal opening.

Siphon
Large complex tentacles on inhalant.

Lighting Requirement
Strong.

Placement
On rockwork or on sand

Feeding
Phytoplankton only when clam is 3" or smaller. Light and supplemental phyto when larger.




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T. Gigas



Mantle Color/Characteristics
Blue, green, golden-brown, or yellow in color. Iridescent spots may also be present, especially near the edge of the mantle.

Shell
Asymmetrical4 to 5 defined ribs. Juveniles have some scutes on their shells but adults do not.

Approx. Adult Size
Up to 4'

Byssal
Small to nonexistent byssal opening.

Siphon
No tentacles on inhalant.

Lighting Requirement
Strong.

Placement
On sand.

Feeding
Phytoplankton only when clam is 3" or smaller. Light and supplemental phyto when larger.




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T. hippopus



Mantle Color/Characteristics
Brown-green with some deep yellow striping. The mantle of H. hippopus does not extend past the shell edge as do the other tridacnids.

Shell
Asymmetrical and very distinct.Typically has 7 to 8 well defined ribs which lack scutes.

Approx. Adult Size
14"

Byssal
Small byssal opening.

Siphon
No tentacles in inhalant.

Lighting Requirement
Strong.

Placement
On sand.

Feeding
Phytoplankton only when clam is 3" or smaller. Light and supplemental phyto when larger.










Editing/adding in progress...

 

wow. great post amcarrig. i have had my clam for a long time. but i learned a lot from that. thanks

 

I have a quick question. In your opinion. Is lighting that keeps an acropora's colors bright and vibrant enough light for most clams?

 

Nice! Karma for the great info. ;D

 

Yes. I have kept clams under VHO lighting for years but IME, they do much better under MH.

 

WOW this an amazing resource.

Nice going

J

 

I think I would go as far to call this thread a workshop... Well done!! Well organized, informative, and attractive. Great photos from "Just Phish" Thanks for the thread. Karma to ya.

 

Am

Nice post have you seen the book

Giant Clams: A Comprehensive Guide to the Identification and Care of Tridacnid Clams, by Daniel Knop, 255 p

 

I own it and used it as a reference for the article