AHABS... Issue Five
Caridean Shrimps In Marine Aquaria
Ronald L. Shimek, Ph. D.
Marine Aquarium “Bugs”
… Are probably not Mr. Bunny and friends wearing SCUBA gear!
In a true statement that is coming close to entering into the apocrypha of modern science, the noted twentieth century British biologist (and atheist), J. B. S. Haldane was apparently asked by a seminary student what Haldane could conclude about the Creator by his studying of biology. Haldane's response,
“If one could conclude as to the nature of the Creator from a study of his creation it would appear that God has a special fondness for stars and beetles,”
has entered quotation lore as an indicator as to the nature of, I suppose, both stellar objects and living creatures. As to the former, one of the paradoxes in nature is, “Why is the night sky black?” The paradox is that everywhere one looks with a telescope there are stars, and the more powerful the telescope, the more stars are visible in between stars that have already been seen. It seems the night sky should at least give a pretty good illusion of being completely illuminated by stars. Well, obviously, it doesn't and the reason that is so is quite interesting, but if you, dear reader, want to find out what that reason is , you will have to search in places other than this article.
As regards the famous coleopterans of Haldane's quote; the story is more relevant. If all of the animal species that have been described are enumerated and categorized, the total number will be somewhere between 1,000,000 and 1,500,000 species. Of that number fully two thirds are beetles. If you want to discuss a “typical,” and relatively large (or, in other words, not microscopic) animal of our planet, that animal is undoubtedly a beetle. You might ask yourself what is it about “beetleness” that makes it so easy to their myriad forms to proliferate and diversify. That question can be kicked around in any number of ways, but one of the really more interesting things about being a beetle is that there are none of them living in truly oceanic or marine environments. There are a lot of beetle species in deserts, there a lot in each and every tree in each and every rain forest, there a lot in prairies, they are common in houses, and there untold numbers crawling around the intertidal zones of the world, but they stop at the water's edge of the lowest low tide. In the archetypical beetle we have an animal design that is so wonderfully adaptable that they are literally everywhere on land; yet, they are nowhere to be found in the seas. In fact, of that huge group of exoskeleton-clad animals possessing six jointed legs only a scant few dozen insects, by all accounts less than 50 species, are found living full-time in or on the oceans. Those few insect species living in the marine realm are either gnats...
A Discussion Of Some Of The “Bigger” Bugs
Ronald L. Shimek, Ph. D.
Order In The Court
The court of life, that is… There seems to be a human desire when considering a diverse but obviously related assemblage of animals to want to put them in some sort of order. This is particularly true of the crustaceans. Perhaps more than most animal groups, there seems to be a range of morphology and behavior coupled with an “obvious” polarity that allows us to use the terms “basic” and “derived,” or “simple” and “complex.” In many cases, these terms are often considered to be equivalent terms for “primitive” and “advanced,” but it is necessary to consider that if all of these animals are derived from a common ancestor, they have all been subject to an equivalent length of evolutionary history and although some traits may appear to be shared with what we perceive as an ancestral type, the present day organisms are by no more “primitive” than any other living species.
With animals such as the crustaceans, and the vertebrates, too, for that matter, there is a tendency to want to create an “evolutionary ladder” with the more “advanced” animals on the higher rungs. This is regrettable, for within any group such an arrangement gives a misrepresentative interpretation of relationships. Natural selection in these animals has created more of an evolutionary bush than a tree, with many different lineages coming from the ancestral types resulting in a wide array of equally derived but very different morphologies trying to place them in a order of “advancement” is not only silly, it is fruitless.
Because any animals now living has the same length of evolutionary history, placing them in some sort of order based on presumed advances is simply an exercise in data manipulation. Consequently, I will treat the animals in this discussion functionally rather taxonomically. I will concentrate this article on discussing several two main crustacean groups: the brine shrimps (and their fresh-water equivalent, the fairy shrimps) and the decapods, these are the true shrimps, true crabs and hermit crabs (which I guess are untrue crabs...).
Before I discuss these specific groups, a short discussion of crustacean morphology is in order to show how and why these groups are related. Crustaceans are characterized by specialization of their appendages for various functions. While there are several discrete lineages , the patterns and relationships between them may difficult to discern either on first glance or without the proverbial “program” listing the players in this game of life. Be that as it may, the appendage morphologies of all of the modern crustacean groups are distinctive...
Crabs, The Other Shrimps.
Ronald L. Shimek, Ph. D.
Crabs are animals both familiar and enigmatic to many hobbyists. Virtually everybody knows what a crab looks like, but at the same time, that definition of a crab seems to fall apart under scrutiny, as all sorts of widely different animals are described as crabs. This ambiguity of description is not surprising. One of my former professors, the late Dr. Paul Illg, a noted authority on crustaceans, once said, “The crab habitus (or body form) is commonly found among many groups of crustaceans, and it can be very difficult to distinguish them.”
There are horseshoe crabs, Alaskan king crabs, hermit crabs, and mole crabs, none of which are found within the group of animals that a biologist will refer to as crabs or brachyurans. Sometimes called, “the true crabs,” to distinguish them from all the “pretenders,” the brachyurans are amongst the most highly derived crustaceans. By that, I mean that they are linked to THE ancestral crustacean by a long evolutionary history, and are very different from that ancestor. In this regard they are similar to humans, birds, and other highly derived vertebrates, all of which are very dissimilar to the wormlike animals that were our distant progenitors. Although crab share many characteristics with other crustaceans, particularly their close cousins, the shrimps, crabs really are a group of amazing animals unto themselves, all with similar body architectures.
Shells, Shapes, and Skeletons
As I have indicated in the article on shrimps, small crustaceans may reach astounding population sizes. However, these small animals typically are miniscule and found in the mid-oceanic regions; they are seldom seen by the casual observer. To really observe and appreciate these animals, one needs specialized collecting gear, coupled with microscopes for the necessary examination of the collected materials. The crustaceans that people typically see are the larger ones, such as the crabs, shrimps and hermit crabs.
Interestingly enough, though, virtually all of the larger crustaceans belong to the same major taxonomic group. This group, called the “Class Malacostraca,” contains the crabs, shrimps, krill, amphipods, mysids, isopods, and a number of smaller groups, most of which contain only small animals. Almost all large crustaceans are crabs and shrimps...