Ok, this may be a little long for my usual posts but here it goes:
Clams are generally regarded in the hobby as very difficult to keep, however most of the deaths may very well result from shipping stress, and not to the animals being particulally hard to keep. Once clams are established in a system (more than 2 months or so), they can be very durable inhabitants and are actually able to tolerate wide ranges in temperature as well as levels of other water quality parameters that are deemed questionable for other invertebrates such as corals. There are 6 species of clams that are commonly found in the hobby:
Tridacna gigas: The true “Giant Clam”, these monsters can grow to over 1.5 meters in length and growth is extremely fast. While they are hardy, the adult size makes them poor choices for anything but the largest systems. These are generally tolerant of a wide range of lighting and prefer to sit on the sand bed.
Tridacna derasa: The next largest clam in the family, derasa clams also have exceptionally fast growth when they have the correct environment. These clams are tolerant of the lowest lighting, commonly thriving under PC lights. Like gigas, these clams prefer to rest on a sand bed when they are fully grown and their byssal threads that will hold them to rocks as juveniles will detach as they grow.
Tridacna squamosa: The third and final of the sand clams, this clam also happens to grow to the third largest size and has an exceptional growth rate, just like T. gigas and T. derasa. Like the other two sand clams, T. squamosa is tolerant of lower light conditions than some of its relatives.
Tridacna maxima: maxima clams are the first of the “rock clams”, which maintain a byssal thread connection to hard substrate thought their life. maxima can grow to a decent size, however the growth rate is slower than that of the “sand clams”. maxima clams generally require higher light, but under the correct conditions can thrive. Identification of maxima clams is relatively easy from their scalloped shell, which is more distinct in this species than any of the others.
Tridacna crocea: The smallest of the clams, crocea clams are also a “rock clam”. While they are relatively small compared to their other family members, they make up for the size in relatively bright coloration. They have a moderate growth rate and are fairly light dependent.
Hippopus hippopus: The hippopus, or horses hoof clam, is a fairly new appearance in the aquacultured clam industry. Unlike the Tridacna sp. clams, the hippopus clams do not extend their mantle past their shell. If I remember correctly, they are a “rock clam” and medium on the light scale.
In our tank, we have a derasa clam that is approx. 5 years old. He has grown significantly since we have had him the past year or so, however like most clams the majority of the growth occurs when they are younger, so if you buy a small clam don’t expect it will stay that way for long. As long as calcium levels are within an acceptable range, and lighting is adequate for the species in question, they should be fine and are great additions to any system.
Just as a side note: smaller clams do NOT need supplemental feeding. There is a common misconception that smaller clams need to be fed phytoplankton, however small clams can survive just fine without it. It does increase the growth rate of smaller clams, however is not necessary.