Some of the sea creatures I’ve described for California Diving News are more than a little aggressive. Many crabs, however, prefer hiding to fighting even when equipped with those seemingly formidable defensive tools called claws.

Like 75 percent of all animals, crabs belong to the phylum Arthropoda, which means jointed leg and includes not only aquatic creatures, but also many terrestrial ones, such as spiders, scorpions, centipedes and millipedes. The subphylum Crustacea includes crabs, lobsters, shrimps, barnacles, amphipods and isopods. Some isopods and a few species of crabs are the only crustaceans that live on land. 
Most crustaceans have four or more pairs of limbs. Since crabs have ten, they are classified as decapod crustaceans. They belong to one of two infraorders: Brachyura (Greek for short tail) or Anomura (differently-tailed —  the members can have any of many different tails). 
The ten limbs of decapods are called pereopods. The last pair is often smaller and the tips are equipped with tiny grooming tools, used to clean the gills and the dorsal surface of the carapace. Females also use these two limbs to groom and clean their eggs. 
All crustaceans must molt in order to grow. Even though the pre-molt animal may be missing parts such as legs or antennae, they regenerate and a complete, soft new exoskeleton forms under the old one. The animal extracts itself from the old exoskeleton through a fissure at the rear edge of the carapace, leaving a molt that many divers mistake for a live animal. (When in doubt, look at the eyes because those of a molt will be clear.) 
To avoid predators, crustaceans seek seclusion during molting and remain there while their new bodies swell with water and their exoskeletons harden. That being said, however, the females of many decapods are only capable of mating just after molting. In Pacific Crabs and Shrimps, author Gregory C. Jensen comments: “When a male encounters a receptive female he grasps her in a pre-copulatory embrace that can last from hours to even weeks depending on how close the female is to mating.” Brachyuran eggs are usually fertilized internally and females may store sperm before using it. Anomuran eggs are fertilized externally. 
Female decapods carry their eggs on pleopods (swimmerets) on their abdomens. When the eggs hatch, the larvae are expelled into the water. They look nothing like their parents and drift with the currents, molting several times as they slowly morph into adults.
Three of the crabs described here — the masking, sharpnose and Pacific rock — are Brachyura (true crabs). The odd crustacean out is the pelagic red crab, which is an Anomuran.
Masking Crab
Moss or masking crab (Loxorhynchus crispatus): This species does an excellent job of obscuring its identity and edibility by decorating itself with algae, bryozoans, sponges and other invertebrates. I wouldn’t have seen this one if it hadn’t moved and it only did so when I placed two fingers on a large boulder to steady myself in the surge. The surface of the boulder moved and I realized I was seeing several crabs that had long legs with white claws. The decorations conceal the masking crab’s resemblance to its close cousin, the larger sheep crab. The carapace of males can be 4.8 inches wide, that of females 3.1 inches. If the long thin legs were included in the measurements, however, the crabs would be much larger. Masking crabs are found from British Columbia to Baja and as deep as 600 feet. The one shown here was photographed off Big Sur. 
Once they grow too large to be eaten by their predators, masking crabs cease decorating.
Sharpnose Crab
Sharpnose crab (Scyra acutifrons): The sharpnose and masking crabs are members of the Epialtidae (spider crab) family. If you could rid the sharpnose of its camouflage without harming it, you would see how much these two crabs look alike. As depicted here, this sharpnose is so artfully covered only an expert could identify it. The sponges or tunicates growing on its body help prevent it from becoming a nice hors d’euvre for a sharp-eyed predator. The maximum width of a sharpnose crab carapace is 1.7 inches. The one shown here is hiding among aggregating anemones at Wilson Rock (off San Miguel Island). Sharpnoses range from Alaska to Mexico and are said to be uncommon south of Pacific Grove, CA. They are found as deep as 720 feet and eat detritus and immobile invertebrates. Dr. Greg Jensen, the expert who confirmed the identity of the crabs depicted here, says the crab doesn’t choose most of what grows on it, but simply allows “organisms to colonize its roughened carapace.”
Pacific Rock Crab
Pacific rock crab (Cancer antennarius): This crab has the shape you probably think of when you hear or see the word “crab.” The carapace is wider than it is long. The only member of the Cancridae family described here, the one shown is a juvenile. While the young are covered with hair-like structures called setae, the adults aren’t. The carapaces of males may be seven inches wide; those of the females measure less than six. The Pacific rock crab has large, black-tipped claws and ranges from British Columbia to Cabo San Lucas. If you pick up this crab and look at its underside, you will find red spots, which are a key to its identification. Pacific rock crabs are usually found shallower than 150 feet. They are nocturnal predators that eat small snails and hermit crabs. The latter have their shells gradually broken away until the hapless hermits are homeless. 
Pelagic Red Crab
Pelagic red crab (Pleuroncodes planipes): The carapace of this member of the Munididae (squat lobster) family can be two inches long. Although its home is Baja’s west coast, during an El Nino event, currents have been reported to carry masses of them as far north as San Francisco. When that happens, the surface of the ocean offers large red patches of the crabs, on which sea birds, fish and marine mammals happily gorge. Boat and marina owners are not so happy because the birds leave red guano everywhere. Masses of the crabs may also wash up on beaches. When not swimming in midwater, pelagic red crabs live on sand and mud bottoms as deep as 295 feet. They prey on copepods and other small organisms. 
Phylum: Arthropoda
Subphylum: Crustacea
Class: Malacostraca
Order: Decapoda
Infraorder: Brachyura
Family: Cancridae: 
Cancer antennarius

Epialtidae: Loxorynchus crispatus, Scyra acutifrons
Infraorder: Anomura
Munididae: Pleuroncodes planipes
Reproduction: Sexual