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Marine Fossils and their Living Relatives


CRABS

Some of the world's best-preserved crab fossils have come from western Washington, but they are relatively rare. Crabs shells tend to fall apart soon after death, so these complete fossil shells must have been buried rapidly before scavengers and bacteria could break them up. True crabs (brachyurans) evolved some 120 million years ago, in the Cretaceous Period. Many of the Washington crab fossils represent the oldest occurrence of either their family or genus.

Crabs are carnivores and scavengers. During their evolution, true crabs developed larger and stronger claws to catch larger and more mobile prey such as snails. In turn, snails evolved thicker and more ornamented shells to resist breakage by crabs. Thus we have had an arms race on our coast over the last 100 million years!

Pulalius crab fossil
Pulalius crab fossil, body length 6 cm.
Photo by Ron Eng

Fossil: Xanthid Crab 
Pulalius vulgaris 

One of the fossil crabs found on the Olympic Peninsula and southernmost Vancouver Island, B.C., Pulalius vulgarisoccurs in marine rocks from 40 to 25 million years old. It was previously called Zanthopsis vulgaris and belongs in the xanthid crab family.

Xanthid crabs are identified by the oval body shell, or "carapace," which is much broader than long. Today, some relatives of Pulalius vulgaris live around the Pacific margin on shallow coral reefs and beaches, while others live in mangrove swamps. They are all nocturnal predators, and a few species in the eastern Pacific and Indian Ocean carry poisonous chemicals in their body. These toxins are extremely poisonous to the human nervous system if eaten, causing paralysis and death.

Northern kelp crab
Northern kelp crab, Pugettia producta, ranges in color from greens to reds, to provide camouflage in its surrounding vegetation.
Photo by Dr. Randy Shuman

Northern Kelp Crab 
Pugettia producta 
 

The Northern kelp crab is often seen clinging to kelp and other algae in rocky habitats around the Olympic Peninsula. It is well camouflaged in this habitat by the smooth texture and cryptic coloration of its exoskeleton, which ranges from olive green to red-brown, depending on the surrounding vegetation. The kelp crab not only lives in kelp, it also feeds directly on the kelp, resorting to animal foods like barnacles, bryozoans, and hydroids only when the algae dies back in the winter months.

Northern kelp crabs can mate year-round. The females carry the eggs for about 30 days before releasing them as planktonic larvae, which pass through several larval stages before settling. Like other arthropods, kelp crabs molt their exoskeleton periodically. Kelp crabs have powerful claws and can inflict a painful pinch, so it is best to handle this species with care.


Pigmy Rock Crab 
Cancer oregonensis

Although small, no more than 4 cm across, the pigmy rock crab is a feisty carnivore that preys on bivalves, limpets, snails, and other crabs. With its powerful pincers it is able to open bivalves much larger than it is itself, and it is even known to tear the arms off small sea stars. Because of its small size the pigmy rock crab is able to slip into oyster cultivation trays used in raising gourmet oysters, opening oysters that foil much larger crab species, and making itself a nuisance to the oyster industry.

This little crab looks a bit like a tiny Red Rock Crab with a slightly more rounded carapace. It's common to see them nestled snugly into the abandoned shells of the giant barnacle, Balanus nubilus, where they find protection from octopus and other predators.

The pigmy rock crab is common in subtidal rocky habitats throughout northern Puget Sound and in waters surrounding the Olympic Peninsula.

Pygmy rock crabs
Pygmy rock crabs are feisty little carnivores—just 4 cm wide.
Photo by Jennifer Vanderhoof

crab fossil
Top view of crab fossil, showing its head and thorax shield (carapace). 4 cm wide.
Photo by Ron Eng
crab fossil
Bottom view of crab fossil, showing its legs and abdomen. 4 cm wide.
Photo by Ron Eng

Fossil: Clallam rock crab 
Cancer starri

This crab fossil is from rocks in western Washington that are around 15 million years old. It is related to the Dungeness crab, which itself has a fossil record in Oregon dating back some 5 million years.

Cancer crabs are carnivorous, hunting and eating snails, clams, and other crabs and shrimps. In order to grow, crabs moult their entire shell (the exoskeleton) many times before they become fully grown. Sometimes parts of this shed shell become fossilized, and sometimes the entire animal is preserved, as seen in this specimen. This fossil species is named after David Starr, one of the Burke Museum's longtime donors.