Phylum: Arthropoda, Crustacea Pinnixa faba Class: Multicrustacea, Malacostraca, Eumalacostraca Order: Eucarida, Decapoda, Pleocyemata, Brachyura, The pea crab Eubrachyura, Thoracotremata Family: Pinnotheroidea, Pinnotheridae, Pinnothereliinae Description articulates at the inner proximal end of the Size: Female P. faba are much larger than merus. Exognath is with several joints and is males. Females are about 20 mm in width hidden (Rathbun 1918). while males are 10 mm wide (Fig. 1). Aver- Carapace: Carapace is smooth, round- age first true crab size is 1.54 mm (Pearce ed, swollen and oblong with no strong post- or 1966). anterolateral ridges. Carapace is 1.6–1.9 Color: Grayish tan with orange or rust col- times wider than long and sides are truncate, ored markings. Immature crabs are white, slope steeply and meet at an angle (Fig. 1) eggs orange and female cheliped tips are (Zmarzly 1992). Male carapaces sometimes white. Individuals are bright orange just have a vertical, compressed lobe at the anter- after molting (Pearce 1966). olateral angle (Fig. 4). General Morphology: The body of decapod Frontal Area: Narrow, slightly ad- crustaceans can be divided into the cepha- vanced in males and strong medial groove in lothorax (fused head and thorax) and abdo- females (Figs. 4, 1). men. They have a large plate-like carapace Teeth: No anterolateral teeth. dorsally, beneath which are five pairs of tho- Pereopods: The merus of males third racic appendages (see chelipeds and pere- walking leg is more than twice as long as wide opods) and three pairs of maxillipeds (see (Fig. 4). Dactyli of both sexes are short and mouthparts). The abdomen and associated strongly curved (less so in fourth dactyl) appendages are reduced and folded ventral- (Zmarzly 1992). The third walking legs are ly (Decapoda, Kuris et al. 2007). longest and all legs are similar in shape, ex- Cephalothorax: cept the merus of the first leg in males, which Eyes: Orbits oval and eyestalks very is concave above, not convex as are others. short. In males, the eyes fill orbits (Fig. 4) Female legs more alike than in males. (Rathbun 1918). Chelipeds: Chelae are large, smooth Antenna: and about 2/3 width of carapace (Zmarzly Mouthparts: The mouth of decapod 1992). Pollex (thumb or fixed finger) straight crustaceans comprises six pairs of append- and a little shorter than movable dactyl, which ages including one pair of mandibles (on ei- is curved (Fig. 3). Dactyls of female are white ther side of the mouth), two pairs of maxillae -tipped and not gaping (Rathbun 1918) (Fig. and three pairs of maxillipeds. The maxillae 3a). Male chelae manus (palm) are almost and maxillipeds attach posterior to the oblong, widening at tip, pollex shorter than mouth and extend to cover the mandibles dactyl, which is curved, and has a tooth at its (Ruppert et al. 2004). In P. faba, external base (Fig. 3b). The male dactyl is hairy within maxillipeds have a large, separate merus (Fig. 3b) (Zmarzly 1992). (the arm) and ischium (the first large article Abdomen (Pleon): Consisting of seven free of the maxilliped). The carpus articulates at somites in both sexes (Zmarzly 1992). Male the outer angle of the merus and a palp abdomen is narrow with last segment rounded A publication of the University of Oregon Libraries and the Oregon Institute of Marine Biology Individual species: https://oimb.uoregon.edu/oregon-estuarine-invertebrates and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] Hiebert, T.C. and L. Rasmuson. 2015. Pinnixa faba. In: Oregon Estuarine Invertebrates: Rudys' Illustrated Guide to Common Species, 3rd ed. T.C. Hiebert, B.A. Butler and A.L. Shanks (eds.). University of Oregon Libraries and Oregon Institute of Marine Biology, Charleston, OR. and the next to last segment constricted in third walking leg is twice as long as wide (in middle (Fig. 5b). Female abdomen is very males), but not longer as in P. faba. The broad (Fig. 5a). female fingers gape, her walking legs are Telson & Uropods: rather alike and the male pollex is deflexed Sexual Dimorphism: Male and female (bent down) and the movable finger (dactyl) brachyuran crabs are easily differentiable. has no tooth at its base. The two species The most conspicuous feature, the abdo- also differ in color: P. littoralis females are men, is narrow and triangular in males while greenish-yellow. Both these species are it is wide and flap-like in females (Brachyura, found in pairs, not singly as with most pea Kuris et al. 2007). Female P. faba are larger crabs (Pearce 1966). than males, have slightly different general Other Pinnixa species are P. longipes, body shape and chelae morphology (see with exceptionally large third walking legs, above). commensal with tube worms; P. barnharti, which is commensal with a holothurian; P. Possible Misidentifications occidentalis, with cylindrical fourth and fifth All members of the Pinnotheridae are walking legs, found in echiuroid worm burrows small, have a wide, rounded carapace, small and associated with P. franciscana, P. eyes and short eyestalks. Pea crabs are tubicola, and P. schmitti, species also found in very particular to a specific habitat and/or worm burrows and tubes. The carapace has host. There are 15 pinnotherid species re- a granular cardiac ridge, curved teeth along ported from central California to Oregon and the anterolateral margin and a conspicuous most of them are in the genus Pinnixa (Kuris subhepatic tooth in P. scamit. Little is known et al. 2007). A thorough key to local mem- about the final species, P. weymouthi (Kuris bers of the genus Pinnixa was published by et al. 2007). Zmarzly (1992). The genus is characterized The other local pinnotherid genera in- by a carapace wider than long, frontal mar- clude Pinnotheres (symbiotic with oysters), gin with median groove, short eyestalks with Parapinnixa (symbiotic with polychaetes Tere- orbits ovate and filled by eyes, third maxilli- bella californica and Loimia), Fabia (symbiotic peds with small ischium, large merus and with bivalves, especially Mytilus) Opisthopus large palp, third walking leg longer and more (symbiotic with various molluscs including robust than others and abdomen with seven Tresus, and some holothurians). Scleroplax free somites (Zmarzley 1992). There are 11 granulata, found usually with mud and ghost local species (Kuris et al. 2007). Pinnixa fa- shrimp, has a wide carapace like P. faba, but ba can be differentiated by chelae morpholo- its antero- and posterolateral margins curve gy where the pollex in males is straight with gradually, not forming an angle. an inner dactyl margin bearing a single trian- gular tooth (Fig. 3b). Female chela morphol- Ecological Information ogy also has straight pollex and opposing Range: Type locality is Puget Sound, Wash- fingers that meet tightly, with no gape ington. Known range includes Alaska to Mex- (Zmarzly 1992). ico (Fig. 1, Zmarzly 1992). The closely related Pinnixa littoralis, Local Distribution: In clams found in bay is often found in the clam Tresus capax, as mud, or mud and sand. is P. faba. Pinnixa littoralis is Habitat: Heavily infests Tresus capax, the ga- distinguishable by its carapace, which is per clam, (nearly 100% in Puget Sound indivi- pointed at the sides and the merus of its duals). Adult Pinnixa are rarely found in Tre- A publication of the University of Oregon Libraries and the Oregon Institute of Marine Biology Individual species: https://oimb.uoregon.edu/oregon-estuarine-invertebrates and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] sus nuttalli when T. capax and T. nuttalli co- Fossils from Cape Blanco, Oregon suggest occur, however, south of T. capax’s range P. that P. faba and T. capax have had a symbio- faba occupies T. nuttalli. Tresus capax is tic relationship for at least 33,000 years (Zullo likely preferred because female P. faba at- and Chivers 1969). tach to the visceral fold of their host which is Abundance: Can be very prevalent in certain present in T. capax and not in T. nuttalli clam populations (almost 100% infestation) (Zmarzly 1992). Pinixia faba inhabits Tresus but prevalence varies with season (Pearce in pairs. The large female clings to the 1966). visceral fold in the mantle cavity of the clam Life-History Information and remains there, immobile, and Reproduction: All decapod crustacean fe- permanently close to the food supply. males attach recently laid gelatinous egg Smaller males and immature crabs are masses to their pleopods. The outer embryo found throughout the mantle cavity and membrane thickens and a strand develops around the incurrent siphon, although they that attaches each embryo to pleopod setae are often close to the female. The young (Decapoda, Kuris et al. 2007). Pinnixa faba crabs seem to be free-living. The clam, can have two broods per year (occurring one Tresus, is found in mud or sandy mud, 25– month later than in P. littoralis), the first in late 60 cm below the surface. Pinnixa faba spring to summer and the second in winter to individuals are also found in Saxidomus, early spring. The period between the two Mya, Tapes, Macoma, and as immature broods may or may not be punctuated by a crabs, in Clinocardium (Rathbun 1918). It is molt (Pearce 1966; Jaffe et al. 1987). Copu- found in the invasive manila clam lation occurs within the clam, as the female is (Venerupis philippinarum) and invasive var- sessile. Males are usually found on or next to nish clam (Nuttalia obscurata) (Marshall et females. One to five immature crabs of both al. 2003). Individuals also reported in non- sexes have been found resident in thae clam bivalve hosts such as abalone, sea (particularly in summer and falls), presumably cucumbers, limpets, sea hares, and waiting to assume adult roles at the death of tunicates (Schmitt 1921; Hart 1982).