Dive Dry with Dr. Bill

#235: Striped Shore Crabs

Nearly 39 years ago I arrived on Catalina Island to teach students at the former Catalina Island School for Boys in Toyon Bay. As a young graduate from Harvard, I was supposed to teach my students marine biology using SCUBA. I was new to teaching and, rather than use some textbook that taught using examples from other parts of the world, I wanted to create my own courses and materials that utilized Santa Catalina Island as a natural laboratory. Its unique setting required a unique approach to teaching. I wanted my students to get out of biology labs reeking with formaldehyde and preserved specimens, and into the environment offered by the hills of the island and waters surrounding it.

My very first "lab" exercise asked each of my students to select a species that interested them, and "describe" it fully. My intention was to see if they would go beyond a strictly physical description of the critter, and look into its behavior, its interactions with other species, what it ate, what might eat it, etc., ... in essence its ecological role. Would they look at another critter creatively? It was very important to me that they "saw" beyond the obvious.

One student, Tad Lacey, chose to describe the striped shore crab. It's scientific name Pachygrapsus crassipes reminded me of one of Tad's classmates and close friends... Pachy, er Packy Offield. To this day I still "equate" the two with a smile on my face... but don't tell him that! Like most of my students, Tad felt the scientific name was a bit too formal and decided to call the crab by its real name, which he somehow determined to be Henry (despite the fact it was a female if I remember correctly). Now Tad studied that crab for two weeks, often carrying it around with him to classes and keeping it in his dorm room. That wasn't part of the assignment... Tad just got attached to Henry. Shortly before writing his assignment, Tad asked me if he had to kill and dissect Henry. I never required a dissection in any of my classes since I was teaching about life. However, I told Tad if he felt it was necessary in order to fully describe Henry, it was up to him. Well, Tad decided he had to, but was emotionally affected by the experience. I can't remember the grade I gave him on that report, but I sure remember how intently he studied old Henry!

Many island residents see the ubiquitous striped shore crab among the rocks here at low tide. They may even encounter some of Henry's offspring... if she had the chance to mate before she was "dispatched" in the interests of science. These crabs have bodies up to about 2" wide which have distinct stripes across them. Good thing you don't have to read the scientific guides, because they describe having simple stripes on the body as being "transversely striated anteriorly." No wonder few people understand what we biologists are talking about! The males are slightly larger than the females, but a much easier way to tell them apart is to look at the telson on their underside. The males have a sharply pointed telson while the females have a broad, oval-shaped one under which they carry their eggs. The body is usually a dark green with shades of red and purple evident.

Striped shore crabs are frequently seen during the day scurrying around on the rocks of the upper and middle intertidal zones. In fact these crabs are probably the best adapted for a semi-terrestrial life than any other crab in our temperate waters. They do frequently take a dip in tidepools to wet their gills, but have been observed staying dry for nearly three days in shaded habitats. They tolerate a wide range of temperature, and their tissue biochemistry appears to be intermediate between strictly marine crustaceans like lobsters and those adapted to nearly terrestrial conditions like the common rock louse, also seen on the rocks in our area.

These crabs may often be seen tearing small pieces of seaweed off with their claws, and transferring them to their mouths. A major part of their diet is the thin film of microscopic algae and diatoms that covers the rocks. They are also known to munch on dead critters, serving as scavengers; and may kill and eat prey including limpets, other snails, and hermit crabs. Dr. Gregory Jensen has written that their quick reflexes even allow them to snatch flies out of the air or off the rocks! Their predators often approach them out of the sky and include a variety of shore birds. These must be fairly sneaky to surprise the crabs, which almost always scurry for shelter when I approach. Striped shore crabs have very good eyesight which helps them avoid these predators... and find food. The eyes adapt differently to day and night thanks to shifts in their retinal pigments.

These crustaceans can be very aggressive towards one another. They will defend particular food items, or hiding places, but do not defend actual territories. When two crabs face off in a "boxing match," the winner usually is the larger one... or the better bluffer. I wonder if they are any better at poker than I am (I hope so for their bank accounts). Rarely do these crabs actually injure one another.

Mating (you thought I'd never get there, didn't you) is interesting. When the female molts (sheds its hard outer exoskeleton), it emits a chemical pheromone that attracts the males. The boys engage in a courtship dance which involves turning over on their backs (the equivalent of break dancing?). Actual mating occurs while the female is still soft bodied. Eggs numbering about 50,000 may be found anytime between late February and October here in southern California. The female may brood twice each year. The eggs hatch into larval forms which drift in the plankton before settling. It may take an individual up to three years to reach maturity during which they may molt up to 21 times.

The striped shore crab is known from Oregon to the Gulf of California (Sea of Cortez). I was surprised to learn that the striped shore crab was also accidental;ly introduced into Japanese waters in the 1890's. It is believed their larvae may have been transported in the ballast water of vessels coming from the West Coast after Admiral Perry rather questionably used military force to open Japan to western trade in 1853. Perhaps that is why Japanese vessels have "retaliated" by transporting several invasive species of algae and marine critters here in more recent years.

© 2007 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays. Please help me climb out of self-imposed poverty... buy my "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles," "Gentle Giants: Giant Sea Bass" or "Common Fish and Invertebrates of the Sea of Cortez" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Striped shore crabs on the intertidal rocks of Avalon Bay.

This document maintained by Dr. Bill Bushing.
Material and images © 2006 Star Thrower Educational Multimedia