The complicated life of shrimp in the Southern Atlantic Bight.
Story By Joe Staton
Joe Stanton is chair of the Department of Natural Sciences and a professor of biology and marine science at the University of South Carolina Beaufort.
The common term “shrimp” applies to a diverse group of crustaceans that include many commercial species for consumption, as well as other species important to our local ecosystem.
There are ghost shrimp, which burrow a foot or more down into the sandy intertidal zone of our beaches. Also small skeleton shrimp (under 0.1 inches long, which are amphipods — not really shrimp), that live in and among the fouling communities on the pilings of our floating docks.
Snapping (or pistol) shrimp are so numerous in our salt marshes of the Lowcountry that you can hear them snapping with their “pistol” claws (chela) in a chorus of high-intensity clicks that call to mind the largest of Rice Krispie bowls crackling. Each click is a sonic stun gun, cracking like veritable bullwhip tips, breaking the sound barrier to immobilize their microscopic prey.
Some species of snapping shrimp have a sociobiology not unlike that of bees, forming a colony of shrimp living symbiotically in large sponges with a queen and worker shrimp. Other areas of the near arctic oceans have krill, a small shrimp species that grows in such stupendous numbers each season that it can sustain the largest animals on the planet, the baleen whales that depend on that food biomass to breed and sustain themselves.
Most of us using the term “shrimp” are envisioning “eatin’ shrimp,” as commercial species are called in the southern U.S. When we say shrimp, we mean the mass-market, pre-cleaned tails of commercially caught or farm-raised species that we can easily get at a chain restaurant or at the local grocery store.
Most people who really love shrimp think of wild-caught, local shrimp in the Southern Atlantic Bight (southern coastal NC, SC, GA and northern FL) in the group Dendrobranchiata, the suborder of Decapods (“ten-legged”) crustaceans that include white shrimp (Litopenaeus setiferus), brown shrimp (Farfantepenaeus aztecus) and the less common pink shrimp (Farfantepenaeus duorarum). These are the most encountered native species in the Southern Bight. These shrimp species have some common elements to their life cycle, but let’s focus on the white shrimp as an example.
White shrimp have a complex life cycle. Most of us learned Monarch butterflies’ life cycle from grade school: egg hatches into caterpillar, which grows and molts (ecdysis, the shedding of the cuticle), until a large caterpillar, dangling from its hind “feet,” splits its skin to become a hardened chrysalis (dormant house for metamorphosis), and eventually a butterfly emerges.
This can happen in your backyard within 100 square feet if you grow milkweed, their larval food plant. White shrimp, however, make this look simple by comparison. They have a vastly more complex life cycle and require a greatly varied diet at multiple larval stages that often require nursery grounds tens to hundreds of square miles.
White shrimp mate off the continental shelf, and these related species release their eggs to float free in the water column (called “demersal” eggs, typical of all dendrobranchiate shrimp). This is different from other shrimps, crabs and lobsters that all cement their eggs on their swimmerets (or “pleopods”).
These free-floating eggs hatch into planktonic nauplii (singular, nauplius). Naupliar larvae are a common larval form among many shrimp, lobster and barnacle species. The infamous “sea-monkeys” advertised in the back of childhood comics are the naupliar larvae of fairy shrimp, whose desiccated eggs rest in suspended animation until you add water at your home (mimicking the seasonal rehydrating rains that do the same, in nature). These nauplii are attracted to light and stay afloat in the upper shelf waters, feeding on microalgae (phytoplankton).
Nauplii undergo a series of five molts as they feed and grow into larger and larger nauplii, since their maximum size for any one stage is constrained by the volume contained by their exoskeleton. All arthropods must molt to grow, and shrimps are no exception. The nauplii then molt into three larger larval stages, called protozoea, which look increasingly more like a shrimp.
Protozoea, in turn, molt into three sequentially larger mysis stages, all occurring in the surface waters of the continental shelf. The naupliar forms and all subsequent stages look substantially different from the adult white shrimp, so much so that early biologists often described many marine larvae as separate species — being unable to mimic the conditions necessary to successfully grow them to adulthood back in the laboratory.
Up until this point, all initial larval stages are pelagic, shelf-dwelling vegetarians, but then the shrimp metamorphose into post larvae. These stages radically retool their digestive systems and behavior, becoming repelled by light, diving near the bottom (demersal) and switching to carnivory. These stages ride tidal pulses, at depth, into the estuaries of the Southern Atlantic Bight, where they molt through two post-larval stages into juvenile shrimp.
Juveniles are the targets of many fall cast-netters, who fish for them every season to fill their coolers and freezers to overflowing. Those that survive migrate to the shelf in the more open waters where they can be trawled up to end up in local seafood markets, roadside stands or posh restaurants touting fresh, local-caught, wild shrimp — the sine qua non for shrimp connoisseurs everywhere.
So, our local delicacy is a result of a complex ballet of biochemistry, development and behavior that has become exquisitely tuned to encompass the marine and estuarine systems we love. Not that long ago, many locals sported bumper-stickers stating, “No estuaries, no seafood,” and this could not be truer for local commercial shrimp species. It is in all our interest to protect the local environment, especially in the coastal zone, where the shrimp are counting on us, as are all the other wild creatures that consume the different shrimp life phases which help to sustain the complex marine and estuarine food web from the bottom to the top.
Authors note: When I was a kid, I really hated seafood. My mom and dad met many years ago in Nags Head, North Carolina, back when Andy Griffith was an unknown actor playing Sir Walter Raleigh in “The Lost Colony” (one of the longest running outdoor dramas in the country). For many summers, we got dragged to Manteo, not to hang out at the beach or slide down Jockey’s Ridge but to visit their old friends and eat as much seafood as possible in a week’s time. Over the years since, I found a few dishes I could stand, starting slowly with deviled crab — my gateway seafood. Eventually my dad converted me, and I became a true seafood lover — especially of delicious fresh shrimp.