Phylum: Arthropoda, Crustacea Americorophium salmonis Class: Multicrustacea, Malacostraca, Eumalacostraca Order: Peracarida, Amphipoda, Senticaudata, Corophiida, Corophiidira Family: Corophioidea, Corophiidae, Corophiinae, Corophiini Taxonomy: Corophium salmonis was urosome), and finally a telson at the animal among the first corophiid amphipods de- posterior (see Plate 254, Chapman 2007). In scribed in North America (Stimpson 1857). members of the genus Americorophium, the It was transferred to the genus Americoro- body is flattened dorso-ventrally and rarely phium in 1997 based on morphological char- exceeds 1 cm in total length (including anten- acters (Bousfield and Hoover 1997) (see nae) in local specimens (see Fig 46, Kozloff Possible Misidentifications). Researchers 1993). have not always followed this transition in Cephalon: other Americorophium species (e.g. A. spini- Rostrum: The male rostrum is straight, corne, Lester and Clark 2002; Sakamaki and slightly convex or with low central projection Richardson 2009), but we follow the nomen- (Fig. 1) (Shoemaker 1949). The female clature used in other current local intertidal rostrum, on the other hand, is a broad and low guides (Chapman 2007). triangle (Fig. 7). Eyes: Description Antenna 1: Reaches to middle of arti- Size: Largest males are 6 mm in length, cle four of second antenna in males. Their from rostrum to end of uropods and the av- flagellum comprises 14–16 articles erage size range is 4–6 mm (Coos Bay) and (occasionally 11–12) and the first article of the 7.5 mm (Siuslaw Estuary). Females are ap- peduncle is flat and greatly expanded laterally proximately 7 mm in length (Shoemaker (Fig. 1) (Shoemaker 1949). First antenna 1949). The illustrated specimen (from Coos about as long as the second in females. The Bay) is 6 mm. female flagellum comprises ten joints Color: Males are transparent, with brown (Shoemaker 1949) and the first article is not mottling, especially on large second antenna expanded. (Fig. 3). Females, like other Antenna 2: Much longer than body in Americorophium species, are clear, with mature male specimens. The fourth article brown mottling, especially on the second has large distal tooth, forming a half-moon, antennae. and small tooth within (Fig. 3). The fifth article General Morphology: The body of amphi- has two teeth below: one at distal end and pod crustaceans can be divided into three one near proximal end (Fig. 3). The proximal major regions. The cephalon (head) in- tooth lies below the flexed half-moon tooth. cludes antennules, antennae, mandibles, The gland cone on second article below, is bi- maxillae and maxillipeds (collectively the lobed and elaborate (Fig. 2) (Shoemaker mouthparts). Posterior to the cephalon is 1949). The second antenna in females is not the pereon (thorax) with seven pairs of as massive as in males. The fourth article is pereopods attached to pereonites followed without a large half-moon tooth and acces- by the pleon (abdomen) with six segments sory, but with two single spines on the lower comprising three pleonites (together the edge and two on the third article (Fig. 4). The pleosome), three urosomites (together the 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. 2015. Americorophium salmonsis. 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, Charles- ton, OR. gland cone of females is simpler than that of most females can be reliably identified to the male and is without lobes (Fig. 8). species as well (Chapman 2007). Five Mouthparts: corophiid genera occur locally: Americoro- Pereon: phium, Corophium, Crassicorophium, Lati- Coxae: Setose lamellae (pairs of corophium and Monocorophium. The three brood plates attached to bases of coxae) are common estuarine species in this guide (A. present in females only and are used for brevis, A. salmonis, and A. spinicorne) were holding eggs and young. Do not confuse previously members of the genus Coro- with fleshy gills, which are also attached to phium (see Shoemaker 1949), but were coxae. transferred to the genus Americorophium in Gnathopod 1: 1997 (Bousfield and Hoover 1997). Gnathopod 2: Filtering type, with fine All Americorophium species have long setae, present in both sexes, morpholo- filtering-type second gnathopods and long gy as in other Americorophium species (see setae on the third uropods. Of the four local Fig. 3, A. brevis in this guide). Americorophium species, sexual Pereopods 3 through 7: dimorphism is strong in the three species A. Pleon: brevis, A. salmonis, and A. stimpsoni. In Pleonites: particular, the second antenna and fourth Urosomites: Urosome with posterior segment differ between males and females margin straight, slightly concave and with a (Shoemaker 1949). This is not the case, spine in each corner as well as two spines however, for the fourth Americorophium on each lateral edge and two on the inside species, A. spinicorne, where male and edge (Fig. 6). First uropods with three to six female morphologies are similar. Additional slender spines along outside edge of characteristics that differ between species peduncle. Two to three small, blunt spines (particularly A. brevis and A. salmonis) present at distal corner (Fig. 6). Third include first antenna, telson, first uropods uropods have many slender setae on all and third uropods. edges (Fig. 6). Americorophium stimpsoni, principal- Epimera: ly a northern California species, does not Telson: seem to occur in Oregon. Its chief key char- Sexual Dimorphism: Males and females acteristic is a prominent male rostrum, al- exhibit differing morphology in characters of most as long as the ocular lobes. The fe- the rostrum, Antenna 1 and 2, as well as males are much like those of A. salmonis. overall body size and color. Americorophium spinicorne, another prominent northwest species, has less Possible Misidentifications sexual dimorphism than other The gammarid family Corophiidae Americorophium species. Both males and is characterized by individuals that build U- females have a half-moon tooth on the shaped tubes in both soft sediments and fourth article of the second antenna, but on hard surfaces, sometimes forming without the small accessory tooth. dense aggregations. Species can be dra- Americorophium spinicorne is also strongly matically sexually dimorphic. Although euryhaline and often found in fresh-water males may be easier to identify with taxo- habitats. Segments of urosome are nomically relevant characters including the separate and not fused in A. spinicorne and rostrum and peduncle of second antennae, males and females can be distinguished by 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] the second antennal features and by the cone, instead of having two single spines on presence of lamellae and/or eggs in the underside of the fourth article of the females. second antenna. The first antenna has Males: Of the Americorophium spe- eight joints in the flagellum, while that of A. cies in which males have urosome seg- salmonis has ten. ments dissimilar to females, A. stimpsoni, Ecological Information A. brevis, and A. salmonis all have a half- Range: Type locality is Puget Sound, Wash- moon and accessory tooth on the fourth ington (Bousfield and Hoover 1997). Known article of the second antenna. Americo- range along the west coast of North America rophium brevis and A. salmonis often have includes Coos Bay to Puget Sound and Alas- similar rostrums, but that of A. stimpsoni ka (Barnard 1954). has a prominent central lobe nearly as Local Distribution: Local distribution in mud- long as the ocular lobes. In A. salmonis flats of South Slough as well as Cox Island the first antenna reaches only to the midd- (Siuslaw Estuary), Tillamook Bay, Sixes le of the fourth article. Americorophium River, Ten Mile Creek and Columbia River brevis does not have flat expanded first (Forsberg et al. 1977). articles of the first antenna and A. Habitat: Members of the Corophiidae inhabit salmonis usually has 14–16 articles in the small U-shaped tubes in soft sediments, or on flagellum, (though occasional specimens hard surfaces (Chapman 2007). Occurs in will have 11–12). In A. brevis, the males muddy habitats and sometimes with algae have about 11 articles in the flagellum of (e.g. Ulva). Especially abundant in brackish the first antenna. The uropods of A. estuaries with a high degree of silt and mud salmonis and A. brevis are quite dissimilar. (Raymond et al. 1985; Kozloff 1993). Com- In A. salmonis, the peduncle of the first parisons of macrofaunal communities within uropod is armed on the outside edge with and outside of Dendraster excentricus beds three to six long, slender spines and at the found Americorophium species to be more distal edge with two to three short, blunt prevalent where sand dollars were not pre- spines. Americorophium brevis has instead sent (Smith 1981). Corophiid amphipods are only eight short, blunt spines. The third frequently used in tests of sediment toxicity uropods of A. salmonis have many more and/or water quality (e.g. fluoranthene, Swartz and longer setae than those of A. brevis. et al. 1990; ivermectin, Davies et al. 1998; The telson shape and spination of the two sewage outfall, Arvai et al. 2002; and species are also quite different (compare nonylphenol, Hecht et al. 2004). Figs. 4, A. brevis, and Fig. 5, A. salmonis Salinity: in this guide). Temperature: Females: A. salmonis and A. stimp- Tidal Level: soni females are very much alike, with no Associates: strong distinguishing characteristics, so the Abundance: Populations often very dense species should not be differentiated solely and easily observed or collected in the field.