Tridacna Clams for the Home Aquarium
by J. Charles Delbeek M.Sc.

In popular fiction, no picture of a coral reef environment is
complete without a picture of a giant clam maliciously capturing an
unwary diver it its cavernous maw. However, the giant clam
(Tridacna gigas) represents only one species, there are others
which are much more suitable for the home aquarium. Tridacna clams
belong to the order Bivalvia which contains all the animals we
commonly refer to as clams. Within this order are various families,
the Tridacna belong to the family Tridacnidae which contains six
species, Hippopus hippopous, Tridacna gigas, T. dersa, T.
squamosa, T. maxima and T. crocea all of which are found only in
the Pacific. One of the distinguishing characteristics of this
family is the greatly reduced foot and the increased importance of
the byssus gland in attachment. In this article we will only be
concerned with the last three species since they are the most
suitable for the aquarium and are the ones most frequently imported.

Tridacna clams are found throughout the Indo- Pacific and are
usually associated with coral reefs. The life span of these animals
can range from 8-200 years depending on the species and they can grow
at a rate of up to 5 cm/year (Achterkamp 1987a). One of the aspects
that sets Tridacna apart from the other bivalves is the presence of
symbiotic zooxanthellae in their mantle tissue. It is thought that
it is the presence of these algae which allow the Tridacna to do as
well as they have. Due to the presence of these zooxanthellae, the
Tridacna have undergone a number of physical and behavioral
adaptations. First of all, they are limited to shallow waters where
they can receive the maximum amount of light. In fact, some species
are found in water so shallow that they are exposed to the air during
periods of low tide. Physical adaptations include a large, fleshy
mantle which increases the surface area available for exposure to the
light. Similarly, the internal organs of Tridacna have been
twisted 180 degrees such that the heart, inhalant and exhalant
siphons, and the stomach lie near the top of the body, just below the
mantle. This allows the siphons to be on top, further increasing the
surface area. Tridacna have also light concentrating organs which
focus light onto pockets of zooxanthellae (Achterkamp 1987a). In the
Tridacna, the smaller individuals tend to be male, while the
females are larger, indicating that they change sex when they reach a
certain size. The larvae produced, spend up to 10 days in the
plankton then settle out into suitable environments at a length of
200 microns. At this point they don't have any zooxanthellae but
after 19-40 days they become noticeable; exactly how zooxanthellae
are incorporated by the clams is not entirely understood (Achterkamp
1987a).

Tridacna squamosa
This species is probably the hardiest species of Tridacna imported
with recorded life spans of 7-8 years in captivity being common
(Achterkamp 1987b). Commonly imported from the Singapore- Indonesia
region, they are characterized by broad, thick valves (shells)
covered with numerous raised ridges and a small byssus opening (the
byssus opening contains the byssus gland which produces numerous
threads which the animal uses to attach itself to the substrate)
(Fig. 1). Reaching a maximum length of 40cm, T. squamosa has a
large inhalant siphon fringed by numerous fleshy projections. The
mantle of T. squamosa is highly variable in color with green and
blue being common, however, brown with yellow markings is the most
common pattern (Achterkamp 1987b).

Tridacna crocea
Tridacna crocea is regularly imported from the West
Malaysia-Micronesia region of the Pacific but is less hardy than the
previous species commonly living for only up to two years in
captivity (Achterkamp 1987b). This is unfortunate since this species
has some of the most colorful members, ranging in color from blue,
green and brown in various combinations with iridescent spots. The
valves of T. crocea are smooth (except for the upper edge) and
reach a maximum length of 15 cm. The most distinguishing feature of
T. crocea is its extremely large byssus opening compared to the
other species (Fig. 2) (Achterkamp 1987b).

Tridacna maxima
This species is extremely widespread occurring from East Africa to
Polynesia. Tridacna maxima reaches a length of 35 cm and its
valves are much longer (3x) than their are broad, giving it an
elongated appearance (Fig. 3) (Achterkamp 1987b). Tridacna maxima
has also a large byssus opening intermediate in size between the
other two species.

Tridacna In the Aquarium
The placement and behavior of these clams differs between the
species. Tridacna squamosa lives in hollowed out areas in rock or
coral, therefore, it is best to place them between pieces of rock or
coral. However, these are very strong animals and they can easily
push these rocks apart, possibly destroying the display. The best
thing to do is to hole out a piece of rock and place them inside the
hole (Achterkamp 1987b). Be careful not to make the hole too small
or else they will not be able to open far enough; you should also
allow some room for growth. Tridacna crocea is the only species
which actively makes its own burrows through a rocking, cutting
movement which grinds a hole into the substrate. This makes them
difficult to collect if the rock cannot be taken with them. It is
best to avoid buying any that do not come embedded in their own rock.
The reason for this is that unless they were collected very carefully
the byssus gland was most surely damaged when the animal was removed
from its home and they will not live long after that has happened
(Achterkamp 1987b). To guarantee yourself healthy specimens, buy
them only if they come attached to their own substrate. Tridacna
maxima is rarely completely submerged and is therefore often covered
with various overgrowths. This species can be put directly onto any
substrate. With respect to water movement, none of the species
appreciates a strong current of water, especially T. crocea. The
light requirements of the three species differs in the degree of
intensity required; Tridacna crocea requires the greatest light
intensity followed by T. squamosa and T. maxima (Achterkamp
1987b). Tridacna crocea is the most demanding species with respect
to light and water quality requirements, this probably helps to
explain its short life span in captivity. Since these animals love
light one is often tempted to place them close to the surface, be
warned, however, that they can close their shells with some force and
can expel a surprising amount of water out of their siphons and out
of the tank! In some cases, they can also trap small, slow moving
fish such as mandarins, gobies or blennies.

Purchasing Tridacna
When buying these clams there are a few things to look for. First
check the inhalant siphon (the largest one) - it should be closed.
Any clam that has a constantly open inhalant siphon is not long for
this world (Achterkamp, 1987b). Smell it if it smells bad don't buy
it! Take a good look at the mantle, it should be colorful everywhere, 
there shouldn't be any white or clear areas, check also for rips and 
tears. See how the animal reacts to an external stimulus, a healthy 
specimen will respond by closing its shell with some force
(Achterkamp, 1987b). Check to make sure the byssus gland is undamaged.

I should also take this opportunity to point out that Tridacna
clams are becoming increasingly difficult to get due to export
restrictions in the country of origin as well as import restrictions
in Europe and the U.S., Canada may follow suit soon, so if you want
one of these beautiful animals you better get one soon. The reason
they are being banned is because the larger varieties are considered
food sources and their numbers are decreasing due to overfishing. It
is not entirely clear whether the aquarium trade has had any lasting
affect on these populations but its a lot easier to ban an entire
group of animals than individual species.