No. 43–Decapods, Part II

Now we come to the lobster-like group of decapods which is the intermediate evolutionary stage that led ultimately to the crabs. Though shortened in relation to body length, the abdomen is retained but the pleopods have devolved from swimming appendages and are used primarily for ventilation.* This grouping, along with lobsters, includes burrowing shrimp and fresh water crayfish.

For most of the country, the most common image that comes to mind when the word lobster is used is of the American lobster Homarus americanus aka, Maine lobster. Ranging from Newfoundland to North Carolina, they require a rocky habitat which provides cover from predators. While they will scavenge feed, that’s how they wind up in traps, they are primarily predators of the night feeding on bivalves, live fish, and crabs and assorted other bottom dwellers.

When they think of lobster, residents of Florida and the Gulf Coast are more apt to think of the Caribbean spiny lobster Panulirus argus. There are striking differences between the two species. P. argus is more slightly built, sports spiny projections on its carapace and lacks the powerful chlipeds of its distant cousin. The two share roughly the same feeding regime but Homarus grow much larger, up to 3 ft. and may live up to 70 years. The spiny lobster grows to less than 1/3 that size and is thought to live between 10 and 20 years.

I found it interesting to learn that Homarus is more closely related to the crawdads I caught and ate as a kid in Oklahoma than it is to the spiny lobster.

The crab-like decapods are of two taxa, Anamura and Brachyura. Anamurans represent the next transitional group from shrimp to crab with some looking more like crabs than others. The mole crab Emerita talpodia, aka Atlantic sand crab, which inhabits the intertidal zones of local beaches, bares little resemblance to what one would normally think of as a crab. The porcelain crab, Petrolisthes armatus, is also found locally and is considerably more crab-like.

P. armatus lives concealed beneath intertidal rubble and shells and it is an invader. It appears to have now completely displaced our native species which I knew by the common name of olive porcelain crab. P. armatus appeared in the mid ‘90s and it has been several years since I last saw an olive.

The most recognizable and numerous of the anomurans are the hermit crabs which will rate a future column all their own. Hermit crabs do not build shells; they inhabit shells that were made by the snails that formerly occupied them. It’s surprising how many people are unaware of that.

Over 4,500 species of Brachyura (true crabs) are known. They are the final evolutionary stage, to date, in the decapods line. The abdomen is reduced to a mere flap folded tightly beneath the body. The male has lost all but two highly specialized pereopods used for mating while the female retains a full compliment to which eggs attach for brooding.

Evolution has made the Brachyurans the most highly specialized, successful and widely dispersed of all the decapods. They range from the sea to the estuary to fresh water to land. A little time spent randomly browsing their realm on line will prove to be both entertaining and enlightening. Google “crab photos”, you won’t believe what’s out there.

*Ruppert, Fox and Barnes – Invertebrate Zoology, Seventh Edition