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Stephen M. Berberich

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Hatching a Plan for Blue Crabs
by Stephen M. Berberich   
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Last edited: Monday, July 25, 2011
Posted: Monday, July 25, 2011

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Of special interest to folks living in Chesapeake Bay country and the Carolinas.

They called her Princess.

Proud as new parents, Maryland blue crab researchers watched their prized female crab give birth. As she wriggled her spiny body rhythmically back and forth in the salty water, she released clouds of larvae so tiny they could not be seen with the naked eye, and seemed unfazed by the human faces peering into the aquarium-turned-maternity ward.


In the Chesapeake Bay, blue crabs (Callinectes sapidus) hatch in the spring and summer. A mother crab shakes off larvae, called zoea, that emerge from a spongy apron of eggs attached to the underside of her hard-shelled body. But Princess' laboratory hatching happened in winter, on February 14, 2002. Her admiring researchers had been adjusting the water conditions in her tank for several weeks to encourage her to hatch early, at an experimental blue crab hatchery at the University of Maryland's Center of Marine Biotechnology (COMB) in Baltimore.

Blue crab numbers are declining in the Bay, and suffered an 85 percent drop in population since 1990. Increasing development and agricultural runoff are part of the problem. When nutrients from eroding soil and fertilizer wash into the Bay, algal blooms become more prevalent, killing off marine vegetation that provides blue crab habitat. A high demand for crabmeat is another factor: More than one-third of the United States' blue crab harvest comes from the Chesapeake, generating about $50 million per year, the most valuable of all blue crab fisheries.

Fewer crabs could mean hard times for the local fishing industry and disruption of natural ecological cycles in the Bay. The decline has raised a high tide of federal, state, and private funding for the COMB hatchery and Smithsonian-led ecology studies to save the blue crab. When watermen hauled in Princess with their harvest of commercial crab pots the previous fall, they donated her and several other healthy females to the Smithsonian Environmental Research Center (SERC) in Edgewater, Maryland, where scientists selected the best specimens for the COMB hatchery.

Ordinarily, a sweet name like "Princess" wouldn't fit a blue crab. She is one of the most fearsome looking species in the Bay, with large pinching claws and an aggressive, cantankerous attitude. But Yonathan Zohar, a leading fish reproductive endocrinologist and COMB director, put great significance on her Valentine's Day hatching. It was the first at the new hatchery and the climax of two months of anxious experiments. Certainly his researchers had cause to mix some sentiment with science when they nicknamed her.

Blue Crab Enhancement Project
Princess and her first brood of zoea that day began what is now the largest tag-release-and-recovery crab study in the world, says Zohar. His COMB team nurtures blue crab hatchlings on a gourmet diet that includes minute shrimp and select species of algae. The larvae are raised in giant tanks of clean, disease-free water in a completely closed, pollution-free system a few yards from Baltimore's Inner Harbor.

Outdoor studies of Princess' hatchlings began May 6, 2002, when partnering scientists at SERC released thousands of her babies, each about the size of a quarter, into Chesapeake Bay waters. By the 2004 season, COMB was on a roll, producing more than 60,000 juveniles in captivity. Then, a Smithsonian team led by estuarine ecologist Anson Hines at SERC released half of the hatchlings into small coves of Boathouse Creek on the Rhode River, several miles south of Annapolis, Maryland. A second team of scientists at the Virginia Institute of Marine Sciences (VIMS), many of them former students of Hines', released the other half of the hatchlings for duplicate studies into coves of the York River near Williamsburg, Virginia.

The entire scientific effort is called the Blue Crab Enhancement Project and is featured in the Blue Crab and the Bay exhibit at the Smithsonian's National Zoo.

No one knows for sure yet if the enhancement project will help boost the numbers of blue crabs in the Chesapeake Bay. But Hines, a 25-year veteran of ecology studies of crabs and other benthic fauna, is optimistic. "Whether we are able to enhance the population on the scale of the Chesapeake Bay or not, this research is providing very valuable information about [blue crab] habitats, movement patterns, survivorship, reproductive success, and juvenile growth and development," he says.

By releasing COMB hatchery juveniles as ¾-inch-wide juveniles rather than as zoea, blue crab researchers can skip the difficult task of tracking the crabs through the early, vulnerable stages of their life cycle. The COMB juveniles are also older than their wild counterparts when they are released into the shallows of the York and Rhode Rivers, and are therefore better equipped to dodge predators.

While some SERC and VIMS researchers monitor survival of COMB's crabs, others are assessing the Bay's potential carrying capacity for more blue crab releases. The results may help Maryland and Virginia set better crab harvesting regulations, perhaps based on whole ecosystem data, says Rom Lipcius, a marine conservation biologist at VIMS. The stark reality is that pregnant females that manage to survive a tough gauntlet of crab pots and reach spawning waters are no longer producing enough offspring to replenish the population, according to VIMS' annual trawling survey.

The scientists realize that to really help blue crabs, they have to get up close and personal with them.

The Life of a Princess
Blue crabs are bottom-feeding crustaceans that live primarily along the eastern coasts of North and South America, from Nova Scotia to northern Argentina. Their top shell, or carapace, is a gray-green or bluish green. Their characteristic bright blue claws are big and powerful tools, used for gathering food, digging for shelter, fighting, and for males, sexual posturing. They also have retractable eyes on little stalks that give them a nearly 360 degree range of vision.

Like most members of Portunidae, the swimming crab family, blue crabs evolved in tropical systems, but this species found a highly suitable home in the Chesapeake. Although not at the geographic center of blue crabs' range, the Bay is the largest estuary on the East Coast. Crab-friendly coves and salt marsh grasses line 4,600 miles of tidal shoreline, and the average water depth is only 18 feet, except for the mid-rib channel—an ancient riverbed—that is about 80 feet deep. Watermen and scientists agree that the Chesapeake was an ideal habitat for blue crabs to thrive before human development.

Blue crabs' genus name, Callinectes, means "beautiful swimmer" in Greek, and the species name, sapidus, means "savory" in Latin. Around the Bay, they are known at restaurants and dinner tables for their succulent crabmeat; but in the water, they are defined by remarkable swimming skills and migrating patterns. A crab may spend part of the year in the Bay's northernmost flats of the Susquehanna River near Pennsylvania, and in the same season travel up to 200 miles south to saltier Virginia waters near the ocean to spawn.

A blue crab walks, tiptoes, and sprints sideways along the bottom on three pairs of pointed legs. A fourth pair, which are front legs, are modified into large claws on either side of the mouth. A fifth pair on the posterior side are swimming legs that widen into oar-shaped flippers. They can rotate at 20 to 40 revolutions per minute, allowing crabs to hover like helicopters over prey, or zip away to escape predators.

When spawning, a female releases one to eight million zoea, each 1/100 of an inch wide—the size of a single onion skin cell—at hatching. But they don't stay in the Bay. The larvae drift as tiny orphans into the open ocean, floating among plankton during the next four to seven weeks. They feed on microscopic plants and animals, and may even nip the skin of seashore waders. As larvae, they will molt as many as seven times, growing just a bit in between each molt.

Under a microscope, zoea look like tiny shrimp with huge eyes until they molt into a more crayfish-shaped megalop stage. Then, their carapace is only 1.5 mm across—one-sixth the size of a dog flea. They retain a shrimp-like tail for a while, but are already armed with claws. After one final molt to a true crab shape, they settle into the lower Bay grass beds.

Scientists once thought that blue crab larvae in the ocean ride a wedge of heavy salinity, hugging the bottom, back into the Bay. It's a habit of many estuarine fish larvae. But instead, "What appears to happen," says Hines, "is that they undergo a selective tidal stream transport on flood tides. The post-larval megalop stage swim vertically up to transport into the Bay on flood tides. Then, they drop down to the bottom on ebb tides to wait for another flood tide." Hines says the babies then sort of "walk their way into the grass beds." After several weeks of feeding to build up their swimming strength, they grow to a width of ¾ inch, and begin migrating up the long Chesapeake estuary. The salinity of the water in the ocean is 30 parts salt per thousand (ppt), but by the time they reach the headwaters of the Bay and its major rivers, it is only about three ppt.

Most of the larvae are eaten by predatory fish such as croakers, bass, red drum, striped bass, or eels; by swamp dwelling birds such as herons and egrets; or by adult blue crabs. Many of the lucky few that survive to maturity are scooped out of the water in crab pots and, if they measure at least five inches wide, could wind up a person's dinner.

Case in point: Princess. She was a one-in-a-million crab simply because she reached adulthood. That's how many blue crab larvae survive hatching, washing out to sea, returning with tides to estuarine grass beds, and then migrating up the Bay. Rare survivors like Princess are not just lucky. They are smart.

As baby Princess began her trek north, she fed on tiny animals such as rotifers, worm larvae, and copepods. As she grew, she fed more on small snails and soft-shell clams. But she stopped eating each time she had to molt and focused instead on hiding out in the "woods." Scientists discovered that woody debris is a very important molting habitat for juveniles—just one of the pieces to fit into the crab ecology puzzle. For example, the tapering depths of the 16-mile, wooded shoreline of the Smithsonian property on the Rhode River keep juvenile crabs safe from predation by adult crabs and fish. In contrast, juveniles are easy pickings along bulkheads and rip-rap rocks of private, developed shorelines.

While still a small juvenile, Princess sought cover in woody debris to molt every three to five days. When she reached two inches wide, she shed her rigid exoskeleton every ten to 15 days to accommodate the growth of her internal tissues. At the beginning of each molt, Princess formed a new, soft shell under her hard shell. When the new shell was fully formed, Princess absorbed water, swelled, and cracked open her old shell across the back, fracturing the claw shells too.

Backing out of her old shell was slow and arduous, but in this process blue crabs deploy a bit of clever biology. Their tissues salvage inorganic salts from the old shell and deposit them into the new shell to thicken and harden it.

Throughout the summer, Princess continued to swim north, stopping in the shallows periodically to molt. By September, her carapace had grown to more than four inches wide, and she molted only once a month. She was ready for her final molt, just north of Annapolis. But this time, she wanted to be noticed. A female's final molt is her pubertal, or mating, molt.

Cruising down from deeper water in the north was a large male—watermen call crabs like him jimmies. He was drawn to Princess' sexual pheromone, which acts like a mating perfume, and perhaps he noticed that the tips of her claws had turned a bright orange red when she matured, almost as though she had a manicure.

When Jimmie found Princess, she was not properly dressed, though; she was still in a hard shell. Blue crabs can't have sex unless the female is in a soft-shell state and his shell is hard. However, their nuptials began anyway, with a strange sexual ritual.

Usually, when blue crabs meet, nasty behavior ensues: mortal combat over prey, cannibalistic devouring of juveniles, or duels between big-clawed males over a receptive female. Sex is an absolute contrast, a gentle and attentive encounter. Jimmie hovered over his sook, as watermen say, his face just above hers. He then cradled her up to his body with his walking legs and held her close for two or three days to allow her to shed her hard shell. A male may continue to swim vigorously for miles in order to keep his mate out of harm's way. He may help her shed her hard shell.

When Princess molted and her shell was soft, the pair copulated for many hours. She received hundreds of sperm packets from Jimmie, perhaps a total of a billion male sex cells, and stored them in sac-like receptacles for later fertilizing. But Jimmie was not about to turn her loose. Although he would later cruise the Bay's mid latitudes for new mates, he continued to faithfully cradle and carry his partner for another two or three days until her new shell fully hardened.

Can we nominate Jimmie as the ideal husband? "No, that's the old dogma," says Lipcius. "What the male is mostly doing is protecting his sperm investment." Hines adds, "The impression you get is that the adaptive advantage is for males to protect the soft female from predation and so that she can't be mated by another male." This is important because, if she mates with another male after Jimmie, the sperm of the two males could be mixed, and Jimmie's genetic legacy would be diluted.

So, Jimmie eventually split. Princess was single and pregnant. And instinctively, she headed home to Mom, or to where she left her mother, near the ocean. She followed the top edge of the Bay's deep main channel on the Eastern Shore side, longing for salty spawning waters to the south. When she got hungry one day, she crawled into a crabber's pot for a bite of fish bait, got yanked up suddenly, and was on her way to COMB.

Underwater Espionage
Today, blue crab scientists understand long-held secrets of the blue crab—intimate details of feeding, predation, mating, and migrating. To know so much, you'd think they've been living on the bottom with the crabs. In a way, they have.

Hines and zoologist Thomas G. Wolcott of North Carolina State University are pioneers in using ultrasonic telemetry to spy on crabs where they live, starting with adults more than a decade ago, and moving to juveniles in recent years. They designed waterproof transmitters weighing only a few grams that they attach to crabs' backs. A tagged crab transmits a signal that scientists pick up with an underwater hydrophone and a receiver. "So we began to 'see' what their daily movements are and where they are feeding," says Hines.

They first experimented with a feeding tag, a set of electrodes attached to the crabs' mandibles. When a crab's mouth muscles contract, an electronic signal is transmitted. Hines explains, "We could then tell where the crab was by a location signal—a steady beep—and then superimposed on that was a signal for the contraction of that muscle. So we could tell where and when it was feeding."

Crab chewing patterns emerged. A crab eating a clam, for example, will twist it around to chip away pieces of its shell. The eavesdropping scientists captured the chewing pattern for clams: intermittent, with lots of pauses. If a crab eats a worm, the chewing pattern is steady slurping. Or, if it eats a small fish—soft parts on the outside and hard parts inside—the chewing pattern contains more pauses.

Another tag tracked the crabs' molting behavior. "We wanted to know where they went to molt exactly," says Hines. "They wouldn't go into the marsh, but were molting in the very shallow water at the edge of the marsh."

The scientists also learned to deploy "multi-channel tags," Hines says, to monitor two or more behaviors at once, such as feeding and fighting. They placed tiny reed switches and magnets on the upper claws. When a crab gets into an antagonistic fighting display, it spreads its claws in "a classic spread posture," he says, bringing the magnet close to the switch and changing its signal.

By combining data from feeding, molting, and fighting tags, the scientists mapped crabs' daily movements. "What we found was that the crabs typically meander around a couple hundred meters, probably walking, for many hours to a few days," Hines explains. "Then they suddenly move in a very oriented fashion at high speed, say 400 meters per hour, probably swimming part of that time. And then they will settle and meander again. This repeats over and over again." The team concluded that blue crabs linger near higher densities of prey, particularly clams. But why do they leave quickly when there are plenty of clams left?

"Well, this is where we brought in the fighting tag," says Hines. "First a crab would begin to tear open its prey, which then exudes a chemical plume in the water that attracts other crabs. As more crabs built up at feeding sites, they fight. At some point, the crab just gave up feeding and fled to avoid the antagonistic interactions, to find a new prey patch." Studies with electronics, backed by laboratory experiments, solidified blue crabs' position as a keystone species in the Chesapeake. "[We] started looking at blue crabs in the system as top predators on benthic prey and how they regulate the abundance of clams, worms, and small crustaceans in these soft sediments of the Rhode River and the Bay," says Hines. But are there enough clams, snails, and other crab food in the Bay's shallows to maintain greater numbers of crab releases?

Counting on a Recovery
One warm, sunny day in July 2004, COMB researchers drove to SERC and VIMS with 11,500 baby crabs for release. At SERC's boat dock, SERC research technician Midge Kramer directed students to sort and count crab juveniles in shallow water tanks. Little gray crabs scampered into and out of black plastic meshing in the tanks, the laboratory equivalent of wooded shorelines. Earlier, volunteers had stamped a tiny metal tag to each juvenile.

From a flat-bottom boat in the afternoon, SERC researchers poured thousands of the tagged juveniles into a cove at Boat House Creek on the Rhode River. Every week until the fall, workers at SERC and VIMS dragged seine nets and small dredge sleds to sample the crab populations and check on their tagged crabs. The researchers waved an electronic wand over each slug of mud, grasses, and small benthic creatures they unearthed, and it beeped when it came near the metal tag of a COMB crab. It even found a tag in the belly of a croaker.

The data gathered indicate just how few crabs are left in the Bay. Kramer says, "When I first came here to SERC, we would get 50 to 75 juvenile crabs in one seine. Now, we get one juvenile after five seines." But she does find it encouraging that the scientists at both rivers are recording increases in crab numbers of ten to 200 percent. Some spring-released COMB crab juveniles are five inches wide by fall, and are ready to breed. The preliminary conclusion is that released crabs are surviving predation. But again, are they getting enough to eat?

On July 30, 2004, VIMS ecologist Rochelle Seitz and two students drifted their small research boat into calm gray waters in York River coves to count snails. In full wet suits, the students kneeled beside PVC-pipe squares they had positioned on the shallow shores using GIS mapping, and counted the number of snails inside. The concern is that because the blue crab has declined, there could be an abundance of snails. Too many snails have been shown to destroy salt marshes in Georgia and Louisiana by spreading a grass-killing fungus disease.

Nearby, another VIMS team swings a small crane over the water and dips a heavy dredge sled of chains to scrape the bottom. With each muddy lift of the sled, they sift for releases from COMB and wild crabs in Timberline Creek. They also count another favorite crab food, small clams Macoma balthica and Mya arenaria.

Seitz explains, "We are studying the food variability and getting closer to the issue of carrying capacity. For example, if you release thousands of crabs, they might wipe out the clams," and cause an ecological imbalance, she says. "But on the other hand, if they balance out, and the clams survive the addition of more crabs, then you can carry more crabs." Gut contents of crabs at the York River revealed about 50 percent clam material, she says. "So, this research is measuring bottom-up controls for crab ecology." It's also showing that coves may be just as effective nurseries as grass beds. "We estimate that we have as many crabs in the upper river coves as we have in the grass beds," she adds.

With very little precedent, Zohar and his COMB team had spectacular success in developing a blue crab hatchery program, according to leading fishery experts meeting in Baltimore last year. And beyond 2005, finding and identifying the released crabs may be easier. COMB researchers will soon identify a genetic marker, from their completed mitochondria genome map of the blue crab, that will distinguish crabs from the bloodlines of Princess and other mothers of the hatchery project.

And, because ecological studies suggest that there is a higher carrying capacity in the Bay for blue crabs, Zohar is ramping up, establishing grow-out nurseries at two coves in the Bay's middle latitudes. According to waterman Mick Blackistone, who is coordinating the nurseries, "We should be releasing at least a half million crabs by next year." COMB will continue to rear larvae up to the megalop stage, 300,000 in each massive tank. But, "when they start putting on their clothes [shells], we will ship them to outside nurseries for scaled-up releases," explains Zohar.

Sitting on the boat dock at SERC, post-doctoral researcher Eric Johnson, a recent addition from North Carolina State University, is also optimistic. "If you can use this opportunity to augment, you have a chance," he says. His N.C. State colleagues are collaborating with Blue Crab Enhancement Project scientists on ecology studies in another traditional blue crab habitat, the Pamlico Sound near the Outer Banks, where crabs have also declined by 80 percent in recent years. "As we are releasing the juveniles into the wild, at some point we will see if they become mature...But there is tremendous mortality out there that hopefully we can bypass. So the question is: Are the hatchery crabs to be fish food—or are they surviving?"

Stephen Berberich lives in Waldorf, Maryland. He has written about wildlife conservation, science, and the environment, and contributed to a story on the National Zoo's molecular lab that was published in the D.C. area's Journal newspapers.

Web Site: Smithsonian Puclications

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