Just Being Crabby…

Maia McGuireFlorida Sea Grant Extension Agent

Arthropods• “Jointed foot” animals• Have exoskeleton made of cuticle/chitin• Animals have to shed their skeleton as they

grow (molting)• Have highly-developed brain, some have well-

developed eyes (color vision)• Most are gonochoric

Horseshoe crabs

• Most closely related to spiders and scorpions• Very ancient species• 4 species of horseshoe crabs worldwide, but

only one in North America (from Maine to the Yucatan peninsula)

Horseshoe crab anatomy

• Have 6 pairs of legs (1 pair modified as feeding appendages); first pair of walking legs is modified in males

• Several pairs of claws• “Book gills” help in swimming

Horseshoe crab feeding

• Scavengers—crawl along the sea floor in shallow areas looking for clams and worms or other food items

• Use senses of touch and smell to find food

Horseshoe crab biology

• Well-developed circulatory system• Copper-based blood (hemocyanin)• Compound eyes, as well as other light and chemical-

sensing organs• Gonochoric• Mature males have modified

“claws” on first pair of legs—used for clasping female’s shell

Horseshoe crab reproduction

• Spawning is in late spring• Females crawl out of the water onto the

beach; males grab onto females and fertilize eggs as they are laid

• Pastel green eggs are important food for shorebirds (red knots)

• Eggs hatch in about 2 weeks• Lifespan is about 19 years

Horseshoe crabs and people

• “Blue blood” of horseshoe crabs is used to test human medicines for bacterial contamination (it will clot)

• Vision research has been conducted using horseshoe crabs’ optical nerves

• Chitin from horseshoe crab shells is used to coat surgical sutures

www.horseshoecrab.org

Activity

• Horseshoe crab models (DE Sea Grant; NJ Sea Grant)

Crustaceans

• Subphylum of Arthropods• Crabs, lobsters and shrimp• 5 pairs of legs; 2 pairs of antennae• Back shell is often called the “carapace”• Cuticle may be calcified• Gonochoric; most have planktonic larvae that

go through several molts• Generally have compound eyes

Crustaceans molt

• Crustaceans cannot add to their shell—when they outgrow it, they separate the shell at the back of the carapace and crawl out of it.

• They have a new, soft shell which they stretch by taking in water.

• They hide for a few days until the new shell hardens, then release the water and have room to grow.

www.1cro.com

Molt or dead?

• Molts have clear eye capsules (dead crabs have black eyes)

• You can separate the carapace from the rest of the shell on moist molts (once they have dried out, this will not be possible

• Old dead crabs will STINK! Molts will not smell.

Crabs—male or female?Images from www.bluecrab.info

Crabs on the beach—ghost crab

Ghost crabs• In the same family as fiddler crabs• Get to be about 2” across the carapace• Feed on coquina clams and mole crabs; also

eat sea turtle eggs and hatchlings• Can change color to provide camouflage• Must keep their gills moist• Nocturnal

Ghost crab burrows

• May be up to 4 feet deep• Crabs may close the entrance to the burrow

on hot days• Burrows are usually above the high tide line

Crabs on the beach—mole crabs

• Sometimes also called “sand fleas”• Live buried in the sand in the swash zone• Get to be about 1” in length• Pale grey in color, egg-shaped• No claws• Feed by filtering plankton using their

antennae, which they stick out of the sand.www.jaxshells.org

Crab shells on the beach

Fiddler crabs• Different species—some found on the beach,

some in the saltmarsh• Males have one large claw, one small• Males wave their claw to attract females• Large claw can be on the right or left

Fiddler crabs

• Live in burrows that can be about a foot deep• Burrows may be inter-connected• If frightened, a fiddler crab will withdraw into

any nearby burrow• Crabs use a large ball of mud to seal up the

burrow at high tide• Small mud balls outside the burrows are the result of feeding activity

midcoast.tamu.edu

Activity

• Crabs (Monterey Bay Aquarium)• Walk like a Decapod-ian (UGA, CrabEcology)

Barnacles

• Only sessile (fixed in place) crustaceans• Two types: acorn-style (look like a volcano)

and goose-neck• Use feathery “legs” to filter plankton out of

the water

Acorn barnacles

• Be careful of the sharp edges of these barnacles’ shells!