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Science
Volume 40 Issue 18 • Sept. 2-8, 2010
now in our 40th season

Feeling Crabby?

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

As summer fun ends and fall obligations like school and work loom large, I thought it might be fun to talk about some of our more aggressive and feisty marine and estuarine creatures.  At the Nantucket Field Station, we have two aquariums full of creatures captured in Folger’s Marsh and in the harbor that are today’s subjects:  the spider and blue crabs.  These little guys and gals have been the source of much amusement, some horror, and can’t help but be educational, so before they get released back into the wild, let’s learn a bit more about them.

True crabs are decapods ("ten footed") crustaceans of the infraorder Brachyura, which typically have a very short projecting "tail" (Greek “brachys” stands for “short” and “οura” which means “tail”), which is the area where the reduced abdomen is entirely hidden under the thorax.  Other animals, such as hermit crabs, king crabs, porcelain crabs, horseshoe crabs and crab lice, are not true crabs.  There are many other crabs that call Nantucket’s waters their home; both native species and interlopers paddle, skulk, scamper, skitter, or crab-walk around.  Common species include the lady crab, black-fingered mud crabs, spider crabs, blue crabs, and fiddler crabs, less common are the rock crab and the shame faced crab.  Invasive species include the green crab and the Asian/Japanese shore crab.  Out of all the crabs you’ll find, I think the blue crabs and spider crabs are some of the most intriguing; they seem to represent the yin and yang of the crab kingdom.

Both the constellation Cancer and the astrological sign Cancer are named after the crab, and depicted as a crab.  John Bevis first observed the Crab Nebula and its resemblance to the animal in 1731.  The word “cancer” evolved from the Greek word “carcinos” used by Hippocrates (460-370 BC) to become a Latin word when used by the Roman doctor Celsus (28 BC - 50 AC).  The term was applied to tumors because these physicians felt tumors looked a bit like a crab.

Many crabs exhibit sexual dimorphism which readers of this column will recall means the creatures show some difference physically between the sexes.  A common example is a larger claw for males to attract mates and deter rival male crabs.  Crabs exhibit physical differences in their “apron” or abdominal area due to the fact that the females carry the eggs under themselves where they can protect them with both claws.  So to determine the sex of a crab without buying one a drink, you need to flip them over and look at their bottom shell or “pleon.”  All crabs has a somewhat triangular abdomen but the females have rounder larger, broader abdomens while the males abdomen is narrow and pointed to form a “lighthouse” or “T” or a “Washington Monument” shape.  The patriotic mnemonic helps significantly; if the abdomen looks like the Washington Monument, the crab is male; if it looks like the U.S. Capitol, it is female.  You can draw all the conclusions you want from that.  Moving along...  Something I just learned this year is that female blue crabs have red tips (i.e., they “paint their fingernails”) on their claws while the males have blue tips, which is good to know as opposed to trying to flip over a crab to see its apron when it is not in the mood.  The front “claws” or pincers are called the ”chelipeds” from the Greek word “chele” and Latin word “chela” (always competing for etymological supremacy) for claw which evolved to be “chelae”.

Crabs are omnivores and well eat anything available from algae to detritus to other crabs to worms, oysters, clams, and other bottom dwelling invertebrates.  They do best on a mixed and varied diet.  They are preyed upon by a variety of species from fish to other crabs to whelks.

The scientific/Latin name for the Atlantic Blue Crab is Callinectes sapidus, from the Greek words “calli” or "beautiful," “nectes” for "swimmer," and the Latin word “sapidus” which means “tasty” or "savory."  Blue crabs are found in the waters of the western Atlantic Ocean, the Pacific Coast of Central America and the Gulf of Mexico.  They have been introduced to parts of the Eastern Atlantic, in the northern and eastern Mediterranean, and also in Japan. Blue crabs are extremely important to the scallop fishery on Nantucket because they tend to predate on green crabs.  There is evidence that blue crabs in eastern North America are able to control populations of the invasive green crab, Carcinus maenas; numbers of the two species are negatively correlated, and C. maenas is not found in the Chesapeake Bay, where blue crabs are most frequent.

All crabs have exoskeletons, which mean they have to molt to get larger. Female blue crabs mate only once in their lives, when they become sexually mature immediately following their pubertal molt (immediately following this molt, the female is known as a "sook" and her abdomen looks more rounded versus a triangle) When approaching this pubertal molt, females release a pheromone in their urine which attracts males.  Male crabs vie for females and will carry and protect them, called "cradle carrying," until molting occurs.  Following this molt, when the female's shell is soft, the pair will mate. During mating, the female captures and stores the male's sperm in sac-like receptacles so that she can fertilize her eggs at a later time.  Once the female's shell has hardened, the male will release her and she will migrate to higher salinity waters to spawn. 

The frequency of spawning depends of latitude, with two spawning periods (spring and summer) in the Chesapeake Bay.  They live typically for three years.  In the wintertime, crabs are dormant and stay buried in the warmest, siltiest areas they can find.  Mating occurs primarily in relatively low-salinity waters in the upper areas of estuaries and lower portions of rivers.  Mating takes place in areas where female crabs normally go to molt—shallow areas with marsh lined banks or beds of submergent vegetation.  Mating for blue crabs is a very involved and drawn out process and very fascinating.  For more information go to http://www.bluecrab.info/mating.html.  We had a pair mating in our tanks this year and it was a pretty amazing sight.  And we also got to see some cannibalistic behavior which is common for blue crabs. Cannibalized blue crabs make up as much as 13% of a crab's diet.  Blue crabs in poor health, missing important appendages, heavily fouled with other organisms, and those during or immediately following molt are more likely to be cannibalized.

Crabs hatch into a zoea stage which bears no resemblance at all to the adult.  Crab zoea will eat a variety of plankton (both plant and animal).  There are usually seven zoeal stages and one postlarval, or megalopal, stage although if the temperature and salinity change, there may be fewer stages. The zoea can only get larger by molting.  On occasion, an eighth zoeal stage is observed.  The next stage is the megalopa stage where the crab starts to kind of look crab-like, although their abdomen is extended which makes them  look like a little shrimp or copepod.  The megalops stage lasts 6 to 20 days, after which the megalops molts into the "first crab" stage, with proportions and appearance more like those of an adult.

The blue crab forms the basis for a lucrative fishery one of the top ten largest crustacean fisheries in the world according to the FAO, or Food and Agriculture Organization of the United Nations which keeps data on many commercially important fisheries.  A large component of the fishery in Maryland and many Southern states is the soft shelled crab which has shed its shell.  Soft crab season is marked by the first full moon in May and continues through early fall.  The harvest of blue crabs has declined along the Eastern seaboard which is reflected in the global capture totals which came down from an average of 120 million tonnes in the mid 1990’s to about 70 million tonnes in 2007.

According to a September 2008 article by David A. Fahrenthold in the Washington Post, the federal government declared the blue crab fishery in the Chesapeake Bay to be “failed” in order to release federal funding for rehabilitation and restoration and research.  “The crabs' numbers have fallen by more than 70 percent since the 1990s, scientists say, because of such factors as heavy fishing, pollution and the slow warming of bay waters.”  Work by researchers at North Carolina State University in conjunction with aquaculture specialists at University of Maryland's Center of Marine Biotechnology have been concentrating on breeding and harvesting blue crabs from freshwater ponds, instead of from the sea in order to help protect the wild populations.

Although they can live in a variety of salty and semi-fresh or brackish waters, blue crabs tend to be a bit more sensitive to temperature and pH (acidity).  When air temperatures drop below 50°F (10°C), adult crabs leave shallow, inshore waters and seek deeper areas where they bury themselves and remain in a state of torpor throughout the winter.  Blue crab growth is regulated by water temperature.  Growth occurs when water temperatures are above 59°F (15°C). Water temperature above 91°F (33°C) is lethal.  Blue crabs are susceptible to sudden drops in temperature.  They do best in normal estuarine and salt water pH of 6-8; if the pH drops below 6 due to a huge influx of rain, blue crabs can die.

Spider crabs, although they look much fiercer than blue crabs, are relatively mellow (for crabs) and much less aggressive to their fellow tank mates. There are two species found in the waters surrounding Nantucket and up and down the North Atlantic coast, the Portly spider crab (Libinia emarginata, Leach 1815) and the long-nosed spider crab (Libinia dubia H. Milne Edwards 1834).  I am sure the portly spider crab, which is also called the common spider crab and nine-spined spider crab, would call itself “big boned.”  L. emarginata is triangular in outline, with a carapace about 4 inches (100 mm) long and a leg span of 12 inches (300 mm).  The whole crab is khaki colored (mud colored), and the carapace is covered in spines and tubercles, and is often clothed in debris and small invertebrates.  For L. emarginata, mating takes place, and eggs are produced from June to September.  The eggs are initially a bright orange-red, but turn brown during development, which takes around 25 days.  The eggs then hatch as zoea larvae, and the female can produce another brood of eggs within 12 hours, unlike many other crabs, where females only mate directly after molting.  It is a bit tough to tell these two apart, but the portly spider crab is rounder and the long nosed spider crab does have a longish nose or rostrum area.  Less is known about spider crabs because they are not a commercial species.  The portly spider crab is also one of the few crabs to walk forward as opposed to the sideways motion which is easier for most crabs.  The blue crab has modified back legs into swimmerets or paddle legs which help them swim where the spider crab has pointed appendages.  Remembering their order of Decapoda, they both have 10 “legs” which are modified to serve a variety of needs.

The native range of L. dubia extends from Cape Cod to southern Texas, Bahamas and Cuba, so these crabs are at the northern end of their range. Both spider crab species have been called “decorator crabs” for the tendency, especially as juveniles, to attach bits of seaweed, like sea lettuce to their shells for camouflage.  Using hooked, Velcro-like setae on the surface of the cara pace, the crabs attach bits of algae and invertebrates for camouflage.  This behavior is most common in juveniles, and the shells of adult crabs are usually found clean because they are now too large to fit in most predators’ mouths. Under the decorative covering, the carapace of L. dubia is rounded, bearing approximately six spines down either side and along the median line on the dorsal surface.  A forked rostrum extends between the eyes, and the overall color of the body is yellowish- brown.  Long, thin walking legs originating from the rounded body give the crab the spidery appearance for which it is named.  These legs culminate in curved points, allowing the crab to cling to various surfaces from rocks to docks.  Although the longnose spider crab is primarily a benthic species, it has been associated with several pelagic (mid level swimming) organisms, including, the loggerhead sea turtle, the cannonball jelly, the sea nettle, the sea wasp, and the moon jelly.  I do not know why, but they always remind me of sheep in a tank because they tend to be pretty gentle versus the much more aggressive blue and lady crabs.  They also tend to be scavengers and algae eaters although they have been shown to eat the tissue of host jellyfish.

Crabs have inspired many field station student research projects who have investigated the self-decorating habits of spider crabs, the habitat preference of lady crabs (sandy versus silty bottoms) and the swimming rituals of blue crabs.  Anyone who has observed crabs like the blue crab or lady crab will instinctively understand where the term “crabby” comes from.  Hope you are not feeling crabby and enjoy the Labor Day weekend.  We’ll release our blue crabs back into the estuary this week although my husband keeps thinking they should be released into a steam bath.

 

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