Why monitor mole crabs? In short, Pacific mole crabs are one of the most important herbivores on the beach, and they are a vital link in the sandy beach food web. They are prey for birds, mammals, and fishes, including endangered species such as the western snowy plover.
Mole crabs are hosts to a parasitic worm in the Phylum Acanthocephala. When birds and sea otters eat infected crabs, they too can become infected. We monitor these worms to learn more about the life cycle, seasonal cycles, and prevalence of the parasites in the populations of mole crabs along our coast. Learn more about these species below.
Pacific Mole Crab (Emerita analoga)
- Description
Egg-shaped and small in size; carapace of females grows up to 35 mm, males up to 22 mm; gray or sand colored. Unlike many other crabs, mole crabs do not have claws or spines; have two pairs of antennae to gather food and oxygen
- Distribution
- Along the Pacific Coast from Alaska to Baja California in the northern hemisphere and from Ecuador to Argentina in the southern hemisphere.
- Habitat
- Live on exposed sandy beaches; spend most of their time buried in the shifting sand of the swash zone. The swash zone is the area between the highest and lowest reach of the waves.
- Diet
- Suspension feeder; uses large, feathery secondary antennae to sieve food from the water; feeds primarily on small, drifting plant plankton (phytoplankton).
- Fun Fact
These are fast moving, rapidly burrowing crabs with excellent orientation and swimming abilities. They dig into the sand backwards and can bury themselves completely in less than 1.5 seconds.
- Reason for Monitoring
One of the most important herbivores on the beach, and a vital link in the sandy beach food web. Prey for birds, mammals, and fishes. Humans collect mole crabs for bait in recreational fishing. In southern California, approximately 2 million crabs were taken for bait in one year. Significant changes in mole crab populations can be an indicator of a larger problem in the ecosystem. Has shown potential as an indicator species for monitoring levels of contaminants and domoic acid in the waters off of California. Domoic acid is a neurotoxin produced by diatoms, a type of plankton. When mole crabs eat toxic plankton or contaminants, they become toxic to the birds, otters, and fish that eat them.
- References
Dugan, JE et al. 2005. Final Report for the Central Coast Regional Water Quality Control Board: Monitoring of Coastal Contaminants Using Sand Crabs (pdf).
Ferdin, ME et al. 2002. Emerita analoga (Stimpson) possible new indicator species for the phycotoxin domoic acid in California coastal waters. Toxicon 40(9):1259-65.
The Acanthocephelan Parasite (Profilicollis altmani and others)
- Description
Pacific mole crabs are a primary intermediate host for several species of parasitic worm in the Phylum Acanthocephala. These worms have an evertable proboscis, armed with spines, which it uses to attach itself to the intestinal walls of its final host.
- Life Cycle
Requires two hosts to survive: they live in crustaceans (e.g., mole crabs) and insects as juveniles, and in the digestive tracts of vertebrates as adults. Some coastal birds, like surf scoters, are a definitive host for these parasites, meaning that the worms can reproduce in them. Birds carry adult parasites in their small intestine. The female parasites are able to produce eggs inside the intestine, which are then passed in the bird’s feces. Intermediate hosts such as mole crabs ingest the eggs, now drifting in the ocean, birds eat the infected crabs, and the cycle continues. The parasites also affect sea otters, but they are dead-end hosts for the parasites, which are unable to reproduce in otters.
- Diet
- Lack a mouth, anus, and a digestive tract; therefore take up nutrients digested from the host directly through the worm’s body surface.
- Effects on Organisms
The parasites are not lethal to mole crabs, although they could affect their behavior, perhaps making them easier prey. In both definitive and dead-end hosts, Acanthocephalans induce peritonitis, which is inflammation around the abdominal cavity. Infection has caused death in both Surf Scoters and sea otters in California. In 1995, an estimated 1000-4000 Surf Scoters died due to infection from these parasites. Infections with parasitic worms are a common and significant cause of death in sea otters, especially in the Monterey Bay region.
- Reason for Monitoring
To understand better the life cycle, seasonal cycles, and prevalence of the parasites in the populations of mole crabs along our coast.
- References
Mayer JA, Dailey MD, Miller MA (2003) Helminth parasites of the southern sea otter Enhydra lutris nereis in central California: Abundance, distribution and pathology. Diseases of Aquatic Organisms53:77-88.
SiMON Sanctuary Integrated Monitoring Network: Southern (California) Sea Otter