Holmes, 1900 
Parapinnixa affinis, the California Bay pea crab, is a species of pinnotherid crab endemic to Southern California. It is a small crab that lives commensally in the tube of a tube-dwelling worm. It was one of the first marine crustaceans to be included on the IUCN Red List in 1996.
Parapinnixa affinis is similar to other pea crabs of the genus Parapinnixa, especially P. nitida. It differs from P. nitida in the proportions of its carapace, which is more than twice as wide as long in P. nitida but less than twice as wide as long in P. affinis. The carapace is approximately 4.5 mm × 2.5 mm (0.177 in × 0.098 in) wide, with a maximum of 6.0 mm × 3.6 mm (0.24 in × 0.14 in). The carapace is usually "light amber mottled with dark ochre", but 5% of crabs are albino.
Parapinnixa affinis appears to be endemic to Southern California, having been recorded from San Pedro, Anaheim Landing, Newport Bay and San Diego. It was once recorded from Siberia, but this record is doubtful.
Conservation and ecology
Parapinnixa affinis was first described by Samuel Jackson Holmes in 1900, in his Synopsis of California stalk-eyed Crustacea. The type specimen was a single female, collected in July 1895 from Dead Man's Island, San Pedro, California (now part of the Port of Los Angeles); it was donated to the University of California. P. affinis no longer lives at the type locality, which has been made unsuitable by the accumulation of "harbor refuse and oil". No further specimens were collected for over thirty years, until one was collected in 1930, and Steve Glassell collected specimens including an allotype in 1932.
- "Parapinnixa affinis Holmes, 1900". Integrated Taxonomic Information System. Retrieved February 23, 2011.
- Inland Water Crustacean Specialist Group (1996). "Parapinnixa affinis". IUCN Red List of Threatened Species. Version 2009.2. International Union for Conservation of Nature. Retrieved August 10, 2007.
- Samuel J. Holmes (1900). "Synopsis of California stalk-eyed Crustacea". Occasional Papers of the California Academy of Sciences. 7 (27): 1–260.
- Steve A. Glassell (1933). "Notes on Parapinnixa affinis Holmes and its allies". Transactions of the San Diego Natural History Society. 7: 319–330.
- Brent P. Thoma; Richard W. Heard & Rita Vargas (2005). "A new species of Parapinnixa (Decapoda: Brachyura: Pinnotheridae) from Isla del Coco, Costa Rica" (PDF). Proceedings of the Biological Society of Washington. 118 (3): 543–550. doi:10.2988/0006-324x(2005)118[543:ansopd]2.0.co;2.
- Jonathan Baillie (1996). "Analysis". 1996 IUCN Red List of Threatened Animals. International Union for Conservation of Nature. Archived from the original on 2007-12-14.
- Armand M. Kurs & Patricia S. Sadeghan (2007). "Biology". In Sol Felty Light & James T. Carlton. The Light and Smith Manual: Intertidal Invertebrates from Central California to Oregon (4th ed.). University of California Press. pp. 632–656. ISBN 978-0-520-23939-5.