Island Science

The Beautiful and Vicious Lady Crab

~ by Dr. Sarah D. Oktay, Director, University of Massachusetts Nantucket Field Station ~

There are many 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, purple marsh 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. Today we will concentrate on the lady crab, who in reality is no lady. 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.

crab-oktayLady crabs are very beautiful and extremely aggressive. They are basically the “James Bond woman” of crabs. Their scientific name is Ovalipes ocellatus and they are found in the waters off eastern North America. They are also known as calico crabs or ocellated crab; ocellated means to have eye like markings of spots and true to its name lady crabs have a shell covered in clusters of purple spots. They can be found all the way up in Canada to down to Georgia, and their favorite food is mainly molluscs, such as the Atlantic surf clam. In their profile online they claim to enjoy long walks on the beach.

The lady crab was first described by J. F. W. Herbst in 1799 who named it Cancer ocellatus, Cancer being a title given to crabs. O. ocellatus was first described by Johann Friedrich Wilhelm Herbst in 1799, as Cancer ocellatus. In 1898, Mary Jane Rathbun moved the species to her new genus Ovalipes during her discoveries described in the “The Brachyura collected by the U. S. Fish Commission steamer Albatross on the voyage from Norfolk, Virginia, to San Francisco, California, 1887-1888”. I bet that was a fun journey. 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.

The lady crab is a brightly colored, aggressive, swimming crab. In the water and under direct sunlight, this crab’s coloring appears iridescent. The species is called the lady crab because of the beautiful color patterns on the shell, obviously there are male and female lady crabs or this would be a very short article. The spots have a similarity to leopard spots or even appear as if someone put on purple lipstick and then kissed the crab, at least that is my story and I am sticking to it. When you see sunbleached molts or dead crabs on the beach you’ll notice that the bright purple color has faded to a pink or coral tone.

The carapace of O. ocellatus is slightly wider than long; full grown ones will be around 8.9 cm (3.5 in) wide, and 7.5 cm (3.0 in) long. This distinguishes it from other crabs in the family Portunidae, which often have elongated lateral spines which make them longer from side to side. Lady crabs have very sharp, powerful pinchers which are whitish in color with purple-spotted tips and jagged teeth. The last pair of legs are modified into paddles and are adapted for swimming. Lady crabs are excellent swimmers. Three sharp points are present between the eye sockets of the lady crab, as well as five sharp points along the carapace that turn toward the eye sockets. The number of points along the carapace helps to distinguish this crab from other similar crabs. The tail of this crab is tucked underneath the body and lies against the abdomen. Many crabs exhibit sexual dimorphism which readers of this column will recall means the creatures show some differences physically between the sexes. A common example is a larger claw for males to attract mates and deter rival male crabs. Male fiddler crabs have one claw larger than the other claw, but lady crabs do not. 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….. that shape is not just coincidental, the female carries the eggs under a flap on her abdomen and when the eggs hatch they are planktonic. Young crabs hatch in the early summer months during which they become food for many fish. As they grow, they pass through two main stages (five stages overall) called the zoea and megalopa before becoming an adult and settling down into the benthos (bottom) usually in early fall. O. ocellatus goes through a grand total of five larval stages, lasting a total of 18 days at 25 °C (77 °F) and a salinity of 30‰, and 26 days at 20 °C (68 °F) and 30‰. The warmer the water the faster the process. Nantucket Harbor is usually around 30-31.5 ‰ and for the latter part of the summer, around 25 °C or warmer.

The meat of lady crabs is not considered as tasty as that of other crabs, so they are not harvested commercially. Unfortunately, the fact that they are not a commercial species means that not nearly as much information is available on lady crabs versus the enormously valuable and tasty blue crabs.

Lady crabs make up for their second class status with their aggressive disposition and sharp claws. Many people who encounter them even consider these crabs to be more “crabby” than their better known cousin the blue crab. Part of the aggression stems from the fact that like other swimming crabs, the lady crab does not have a very thick or rigid carapace. This means it has less protection than some other crabs, which have harder shells or exoskeletons. Because it is not very well protected, it makes up for this by its speed, feistiness, and camouflage. Lady crabs are often seen partially buried in sand with only their eyestalks protruding. If you go snorkeling in Nantucket Harbor that is precisely what you will see. This species of crab is most often found in sandy substrates in quite shallow water, i.e. the surf zone. Normally, this is a very difficult habitat, because of strong wave action and constantly shifting sands. But the lady crab stabilizes itself and allows itself to ambush more food by burrowing just beneath the sand surface. As waves toss the sand around, the crab quickly shifts position and digs back under the surface. The lady crab will dart out of its hiding place using its powerful paddles to swim after its prey. Like most other crabs, lady crabs are scavengers, eating both dead and live fish, crabs, and other invertebrates. They can rapidly consume small clams and prey upon hard clams. Oyster toadfish, tautog, striped bass, American lobsters, whelk, and other crabs prey upon lady crabs. Crabs are omnivores and well eat anything available from algae to detritus to other crabs to worms, oysters, clams, and other bottom dwelling invertebrates ( http://www.edc.uri.edu/restoration/html/gallery/invert/lady.htm).

Ecologically, almost all crabs function as scavengers, feeding on dead or dying animals or organic debris of any kind. They are in fact like the vultures or hyenas of the sea. But they don’t sit around waiting for things to die. Many species including the lady crab are also active predators, capable of killing small fish and breaking open the shells of different mollusks. Lady crabs are somewhat messy eaters, ripping apart their prey with their pincers and sweeping the bits into their mouth using a series of appendages called maxillipeds and maxillae. A pair of comb-like mandibles guard the mouth and chop the food into tiny pieces, which are then swallowed. A short esophagus leads to a gastric mill analogous to the gizzard of other animals, where food is ground up.

All crabs have exoskeletons, which mean they have to molt to get larger.  These molts often confuse people as they appear to be dead crabs. These crabs appear to stop molting when they reach 4 in wide. Adults that have stopped molting may be covered in growths of barnacles or seaweed. Lady crabs prefer sandy bottoms and higher salinity parts of estuaries versus other crabs that can be found in muddy bottom areas. Look for them next time you are out swimming in the harbor. We will often have huge swaths of dead lady crabs eaten by seagulls that can be found in the wrack line entangled in the eelgrass in what I call the “lady crab apocalypse” in early summer. Here at the field station we have been evaluating their population in relation to blue crabs and other native and nonnative crabs. Blue crabs are less plentiful but we were able to mark and recapture several very large ones in Folger’s Marsh this summer. Later this fall I will gather up some crab data on a variety of species and post a link here. I hope everyone is enjoying the warm weather before summer turns into fall. Last but not least, check out the former Nantucket Field Station Junior ranger (in 2015 a junior scientist and intern) Thomas Glover’s blog post from a year or two ago about lady crabs: http://jrrangerfieldnotes.blogspot.com/2013/08/lady-crabs-by-thomas-g.html.

I wrote about crabs back in 2010 and you can find that article here http://yesterdaysisland.com/2010/science/18.php More information on these beauties is also available at http://eol.org/pages/1037670/overview.