No. 42–Decapods, Part I
Publication of Estuarium Notes began four years ago this month with a basic explanation of what estuaries are. The intent was, and continues to be, to better acquaint the reader with the most dominant feature in our local environment and inspire a sense of stewardship for it and the myriad creatures it sustains. It has since evolved and become an exercise in the continuing education of the author.
In what began as a leisurely stroll of pointing out features in a landscape I had become somewhat familiar with, it became evident over time that personal observation and experience would carry one lacking any formal marine science background only so far. It was that realization while standing in the marsh surrounded by mysteries that ultimately led to a trail of muddy boot prints to the library door.
Following on last month’s column, the ancestor of today’s decapods was a free swimming shrimp like creature according to Rupert, Fox and Barnes.* They further state that the classification of decapods is based on the three body types that, through the evolutionary process, are represented today in shrimp-like, lobster-like and crab-like creatures.
Most resembling the ancestors, the various shrimp taxa are the oldest decapods on the evolutionary scale and retain the ability to swim to varying degrees. The propulsion for swimming is provided by the rhythmic beating of feathery appendages (pleopods) on the underside of the abdomen (tail).
The best swimmers have light, flexible, uncalcified exoskeletons and small chelipeds (pincers). These include the white (Penaeus setiferus) and brown shrimp (Penaeus azetecus) we pop into pots, fry, grill, and sauté.
There are also other species of shrimp sharing the estuary with their better known cousins. The prickly sensation one feels standing bare legged in river and creek shallows is compliments of grass shrimp munching on dead epidermal cells. Youngsters visiting the Estuarium get a kick out of reaching down into our tickle tank of hungry grass shrimp. They’re excellent swimmers also.
The ubiquitous popping sounds heard coming from the marsh on quiet evenings are courtesy of the pistol shrimp. With this taxon, discussed more fully in a previous column, swimming efficiency has been sacrificed for a heavier exoskeleton and larger chelipeds.
The rock shrimp (Sicyonia brevirostris) that inhabit our offshore waters are the most heavily armored members of the shrimp family and take there name from their tough exoskeleton. Living at depths of 140 ft. or more, they possess only rudimentary pleopods and can’t swim a stroke. It is a commercially viable species in some regions of its greater range but is seldom sought locally at the present time. The meat is much firmer than Penaeideans and has a taste something between shrimp and lobster.
That gives us a good segue to next month’s column when we will continue with the lobster and crab-like decapods.
*Invertebrate Zoology, Seventh Edition