Crustaceans are an apt topic for this month as one of our favorite local epicurean delights is coming into its brief season. Warming waters are bringing blue crabs that over wintered off shore and in deep holes in the sounds back into the creeks. Soft-shell crab season is upon us once again and you’re all invited to join us for the 4th Annual Soft-Shell Crab Festival and Earth Day festivities in the Old Village of Port Royal on April 21st.
In past years this month’s column has been dedicated to the blue crab. For you who missed those or would like a refresher course, they are numbers 6,7,16 and 18 in Curator’s Corner on our web site. We’ll broaden the field this year and with Ruppert, Fox and Barnes* and the internet’s Wikipedia as our primary guides, explore the wider world of the arthropod sub phylum known as crustacea.
Right off the bat we have a conflict with Ruppert et al saying there are 42,000 described species and Wikipedia claiming 52,000. Still another source says there are 30,500 known species and yet another gives the number of 38,000. It makes one wonder if any of these folks are talking to one another. The lack of agreement over the numbers aside, sources consulted were in general agreement on the following information.
As referenced two columns ago (Science vs. Assumptions; #38) crustacea belong to one of the two high level taxa of arthropods called Mandibulata. Evolution has waved its wand to give us species ranging in size from the impressive Japanese spider crab with chelipeds (claw arms) up to five feet in length down to creatures of less than a millimeter.
Through the eons the descendants of the first manipulates have been transformed morphologically and dispersed ecologically to rival any phylum the in the sheer diversity of species alive today. Some ventured from the original marine environment to establish fresh water lineage while others abandoned the waters altogether for a terrestrial life.
They swim, walk, crawl and burrow. Some live between grains of sand while others (barnacles) build permanent fortress homes. They filter feed, graze, scavenge, parasitize and hunt live prey. One formidably armed land crab husks and cracks open coconuts.
One element responsible for the huge diversity among species was the presence of a large number of appendages that through adaptive specialization over time allowed for the exploitation of both niche and macro environments.
As an example of specialization in a niche environment there is the taxon Rempipedia. The first of the 10 known species was discovered in only 26 years ago in undersea caves of the Caribbean. As a possible link to the earliest crustaceans thought to have been marine bottom crawlers and swimmers, the creature looks much like a segmented leggy worm. I counted 34 pair of swimming appendages in an illustration of one.
The crustaceans we are most familiar with are of the class Malacostraca. They are the decapods, those with ten appendages like crabs, shrimp and lobsters which account for approximately one forth of known species of crustaceans.
Space does not permit the further exploration of this ecologically and economically important order at this time. We will, however, delve deeper into the subject of decapods in a future column.