Blue Crab Lab: A Recipe for Survival

Before it lives up to its scientific name — Callinectes sapidus or tasty, beautiful swimmer — the blue crab goes through a couple of ugly larval stages. Fortunately, its survival doesn’t depend on its being a beautiful baby.

Equally fortunate is the fact that Joanne Harcke, conservation and research coordinator for the North Carolina Aquarium on Roanoke Island, finds the ugly stages fascinating if not, in their own way, beautiful.

In a project funded by the North Carolina Blue Crab Research Program, Harcke focuses on raising blue crabs from eggs through larval stages. Ultimately, she hopes to develop procedures for growing blue crabs in hatcheries to help replenish wild stocks.

The Blue Crab Research Program is funded by the General Assembly and administered by North Carolina Sea Grant to address problems facing the blue crab industry — the largest of the state’s commercial fisheries. Some scientists are concerned that a drop in crab harvests in recent years may signal a decline in the species.

Stock enhancement research such as Harcke’s is relatively new with blue crabs. The University of Maryland and the Smithsonian Environmental Research Center report success in growing crabs for the same purpose. And, Harcke says, stock enhancement has been tried in Japan with another species of crab, Portunis trituberculatus, which is a swimming crab like Callinectes sapidus.

But until now, techniques for growing blue crabs have been elusive. Mortality is high, especially when the animals molt, or lose their shells — as frequently as every five days in the early stages. And once crabs reach a certain stage of development, they have a nasty habit, in captivity or under crowded conditions, of eating each other.

Harcke gets sponge crabs — females bearing eggs — from local crabbers and takes the crabs into her lab at the aquarium. When fertilized eggs hatch, they become larvae called zoea. Viewed under a microscope, they look like something that might be controlled by spraying, with sharp thorn-like structures between their eyes and on their backs.

After seven or eight molts, the zoea become megalopae, which are more shrimp-like in appearance.

Harcke’s goal is to find replicable ways to grow crabs to the megalops stage — just before they turn cannibalistic — so they could be released to the wild. However, in the lab, she has made tiny isolation chambers from short lengths of PVC pipe with screen mesh bottoms to protect juvenile crabs from each other until they reach adulthood.

Hatching and growing crabs is just the first, and still theoretical, step in stock enhancement. Once released, there is the question, not only of whether stock enhancement will work, but also how to tell if it works.

“One problem,” Harcke says, “is how do you tell hatchery-reared from wild crabs.” They look the same, she explains, and tagging would be difficult because they go through frequent molts.

Marc Turano, who oversees the Blue Crab Research Program for Sea Grant, says that while the issue of stock enhancement is debated among researchers, fisheries managers and others, he is enthusiastic about the success Harcke has had with raising crabs in the lab.

“Through her work, we have learned a lot about the life cycle – and we have learned a lot about the food preferences – of blue crabs,” he says.

People who enjoy visiting the Roanoke Island facility may be surprised to learn the aquarium is more than a nice place to visit.

“As members of the American Zoo and Aquarium Association, the North Carolina Aquariums are encouraged to conduct original research and collaborate with other researchers on both in situ and ex situ projects. We’re committed to establishing field conservation programs in North Carolina,” Harcke explains.

As to collaboration with others, this project gets a little help from crabbers as well.

Endurance and the Crab

Murray Bridges is one of the crabbers who supplies Harcke with sponge crabs. On the dock at his company, Endurance Seafood, near Colington, Bridges points out a crab beginning to shed. An opening has formed between top and bottom shells at the back end. Slowly, the peeler slides out of its former self — delicate legs last.

Now it is vulnerable – a soft-shell crab – and a more valuable commodity with an edible shell. Before long, its shell would harden in the wild, but this one will stay soft as it is packed in ice and readied for shipment to the Fulton Fish Market in New York.

Soft-shell crabs bring good money. If they didn’t, Bridges says, there wouldn’t be many soft-shell crabbers.

Tending peeler crabs is an around-the-clock job, indeed requiring endurance. In early spring, Bridges says, everybody’s still smiling, but when business picks up, moods change with the workload. In a busy season, he can have 100 shedding boxes going.

Bridges is skeptical about reports of a decline in crab abundance. He has been crabbing for 30 years now and says, “I really, in this area here, haven’t seen that much of a decline in crabs.”

“There are those that holler about a decrease in crabs, and I say ‘come out with me.’ ” Bridges and other crabbers who supply Harcke with sponge crabs know how to handle them gingerly. “Careful handling of female crabs from the time the crabbers remove them from the pots until they enter the hatchery is critical for the success of the project,” Harcke explains.

“They are carefully hand-selected to cut down on mortality,” she says. If sponge crabs become stressed, they can tear the eggs off prematurely, Harcke attests.

Even though he hasn’t seen a decrease in crab numbers, Bridges says, “I really support what she’s doing. I think it’s a great thing to start raising them.”

It’s not that Bridges doesn’t see fluctuations in crab catches — he just sees them as natural. He says 2002 saw a “big change” in harvests, but he attributes this to the drought, which caused salinity to go up to 18 to 20 parts per thousand. Normal is 12 for his location, he explains.

Then there was the year of the hurricanes, when fresh water flooded the estuaries and, Bridges says, the crabs were pushed out by salinity changes.

As temperatures warm each spring, female peelers are targeted by crabbers.

Female blue crabs about to undergo a final molt seek out mature males, called jimmies, for mating and protection. A jimmy cradles the female, protecting her until she sheds. Soft-shell crabbers capitalize on this process by using jimmies as bait. Bridges usually catches 10 to 15 per pot — sometimes many more with a good jimmy.

This is the only time in the female’s life that she is capable of reproduction and the last time she will molt. She holds the sperm until conditions are right for forming the sponge, or egg mass.

A female that survives to be a sponge crab could end up in Harcke’s lab where, Harcke hopes, the chances of survival for released eggs will be much improved.

At the Crab Lab

At the aquarium on Roanoke Island, visitors mill about like ocean currents, peeking at alligators, gazing at ocean life, having close encounters with sea creatures at the “touch tank” and, via closed-circuit TV, spying on the osprey tending its raggedy nest outside.

Away from all the activity is a utilitarian little room aquarium-goers won’t see. Along one wall, a row of recycled bottles gurgles with green and brown stuff, backlit by fluorescent bulbs.

In an aquarium sized for a kid’s room or a high school science lab, Number One squats, wary, behind grass made of green curling ribbon.

Number One is a one-of-a-kind for the aquarium — the first from the batch of blue crabs raised last year.

In a larger aquarium, two full-grown females face each other. Last year, they provided eggs for the project.

Female crabs that have mated can retain some sperm for a second batch of eggs, Harcke says. She hopes these two may develop eggs again this year.

Scientists believe only about one of each million or so crab larvae will grow to adulthood in the wild. One has to wonder at the biological inefficiency of such a system. But Harcke explains the role of these crab larvae, called zoea, in the ecosystem.

“Everything has a niche,” Harcke says. Unable to swim, zoea float passively with ocean currents to areas of higher salinity. Drawn toward sunlight, they become plankton – sacrificial pawns in a game of survival, providing food for fishes and other sea animals.

Then a dramatic change occurs. When zoea molt into megalopae, they can swim — though not yet beautifully — and propel themselves away from the light to which they were at first drawn. They also move toward areas of lesser salinity, traveling back to the estuaries.

Harcke explains that the endurance of the megalopae is heightened by osmoregulation, which allows them to tolerate higher degrees of salinity until they get into more favorable conditions in the estuaries, where they will at last become something that looks like minute, pinhead-size crabs.

Each dramatic metamorphosis brings new needs and food preferences. It’s a challenge for the scientist attempting to provide sustenance for the blue crab from larvae to adulthood.

Working on the Food Chain

Here’s food for thought for anyone, say, eating crab cakes in a seafood restaurant while staring out at a sound like the Roanoke: We know how we like to eat crabs — in creamy bisques, as dips, as stuffing for mushrooms or flounder. Some of us like them whole, right after they shed, as soft-shell crabs. And then there are crab cakes.

But what do we know about what they prefer to eat? Crabbers know how to bait a crab pot, but what about how to feed crabs that are newly hatched, planktonic members of the food chain themselves?

Harcke has learned a few things about the subject, and it wasn’t easy.

“I’m doing more work raising food than growing crabs,” Harcke jokes.

Her goal is to increase the eggs’ one-in-a-million odds for survival. Keeping the zoea out of predators’ reach is the first step, but they also have to be fed. Figuring out what they eat in the wild was educated guesswork.

The feeding experiments were meticulous — 250 zoea were individually lifted by pipette to one-liter beakers of artificial seawater, where they were given a variety of foods. At the end of five days, they were gently sieved out, examined under a microscope, counted, and determined to be dead or alive.

Harcke started with food that was easier to come by. In beaker after beaker, zoea were fed commercially available algae paste in various forms. They responded with what might be articulated as “give me something else or give me death.”

Survival was zero until live algae were introduced. They are the bottles of green Tetraselmas sp., and brown Chaetocerous sp. and Isochrysis sp. bubbling away in the lab, constantly aerated and grown using established culture techniques.

Combinations of live feed and algae paste were compared so that feeding procedures could be established. “It’s like setting up a buffet,” Harcke says.

Just as humans need a variety of foods for balanced nutrition, so do blue crab larvae.

In addition to the algae buffet, Harcke is growing zooplankton called rotifers. To the naked eye, they look like tiny dots. In a small container of seawater, they look like carbonation bubbles — except that the rotifers are very small and don’t zip straight up. Under a microscope, they are round and randomly mobile.

So far, survival from the best buffet combination was greater than 20 percent. Less than impressed? Think of the one-in-a-million odds the newly hatched crab larvae get in nature.

This coming year, Harcke will add copepods — tiny crustaceans to make food for other crustaceans – in hopes of boosting survival even more for the lab crabs and, possibly, for those in the wild.

This article was published in the High Season 2003 issue of Coastwatch.

For contact information and reprint requests, visit ncseagrant.ncsu.edu/coastwatch/contact/.