Phylum: Arthropoda, Crustacea Ligia pallasii Class: Multicrustacea, Malacostraca, Eumalacostraca Order: Peracarida, Isopoda, Oniscidea A rock louse or shore isopod Family: Ligiidae Taxonomy: The genus Ligia was very brief- 2007). The suborder to which L. pallasii be- ly called Ligyda in the early 1900s. Since longs, Oniscidea, is the largest isopod subor- then, the genus has been split into four gen- der and the only fully-terrestrial crustacean era (Geoligia, Megaligia, Nesoligia and group (Brusca et al. 2007). Ligia) based on morphological characters Cephalon: More than twice as wide as long (e.g. antennulae, mouthparts, telson) with rounded anterior margin and without (Jackson 1927). However, Van Name re- lobes (Fig. 1) (family Ligiidae, Brusca et al. duced these genera to subgeneric status 2007). and reinstated the genus Ligia in 1936 Eyes: Large, round, composite, and (Brusca 1980). Currently, these subgeneric close to lateral margin (Fig. 1) (Welton and names are rarely used and, instead, re- Miller 1980). Separated in front by twice the searchers refer to Ligia pallasii (e.g. Brusca length of the eye. et al. 2007). Antenna 1: First antennae are vestigial (Oniscidea, Brusca et al. 2007). Description Antenna 2: Second antennae reach to Size: To 35 mm in length (including uro- middle of fourth thoracic segment (Fig. 1). pods, which are 3 mm long) and approxi- The second antennae are with peduncles of mately 11 mm wide (Brusca and Brusca five articles: the first two are short, the third is 1978). The figured specimen (from Coos twice as long as the second, the fourth is 1½ Bay) is 22 mm long. x longer than the third, and the fifth 1½ x lon- Color: Mottled gray and often brown, with ger than the fourth (Welton and Miller 1980). granular surface. The flagellum has 15 articles (Hatch 1947). General Morphology: Isopod bodies are Mouthparts: In order from outside of dorso-ventrally flattened and can be divided buccal cavity: maxillipeds with palp of five ar- into a compact cephalon, with eyes, two an- ticles (Fig. 8), second maxillae with two plu- tennae and mouthparts, and a pereon mose processes on inner side of lobe (Fig. 5), (thorax) with eight segments, each bearing first maxillae with three plumose processes on similar pereopods (hence the name “iso- the inner lobe (Fig. 4), and the mandible with pod”). Posterior to the pereon is the pleon, large, broad molar surfaces, and no palp (Fig. or abdomen, with six segments, the last of 3). which is fused with the telson (the pleo- Rostrum: telson) (see Plate 231, Brusca et al. 2007). Pereon: First segment fused with head fol- The Isopoda can be divided into two groups: lowed by seven free pereonites. Contains a ancestral (“short-tailed”) groups (i.e. subor- tubular heart and cardiac ganglion consisting ders) that have short telsons and derived of six neurons (see Fig. 1, Sakurai and (“long-tailed”) groups with long telsons, L. Wilkens 2003). pallasii groups among the former (see Fig. Pereonites: First four pereonites are 3, Garthwaite and Lawson 1992; Brandt and subequal, last three are somewhat shorter Poore 2003; Plate 248C, Brusca et al. 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. Ligia pallasii. 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. along medial line and extend downward idae, see Holdrich et al. 1984 in Schmidt laterally. Epimera (flattened lateral 2000, 2002) is a fully-terrestrial group com- extensions to pereonites) form broad plates, posed of 4,000 described species, with 22 especially in males (Figs. 1, 4). known locally (among 10 families, Schmidt Pereopods: Seven pairs of delicate 2002; Brusca et al. 2007). Members are char- walking legs. Carpus and merus of first pair acterized by seven pereonites, the first not is swollen and not grooved (Hatch 1947). fused with the head, seven pairs of pereo- Pleon: Pleon as wide as thorax and with five pods, male penes on the sternum of pereonite free pleonites and a short pleotelson (Fig. seven, a pleotelson that does not curve dor- 1). sally, vestigal (or very small) antennules and Pleonites: First two pleonites narrow a pleon with five free pleonites (Brusca et al. and without downwardly extending lateral 2007). The first and second pleopods are al- edges, which mark last three segments (Fig. so elongated in males for copulation, many 1). species have a water conducting system and Pleopods: Paired breathing append- some have respiratory structures on pleopods ages beneath pleonites have whitish tissue. called pseudotracheae. The Oniscidea can Male genitalia, paired but not fused, is pre- be divided into three major ecological groups: sent on second pleopods (Fig. 7). the runners (slender bodies with long pereo- Uropods: Terminal and styliform, with bases pods, the clingers (broad bodies with short about as long as wide. No process at inner pereopods) and the rollers (convex bodies distal margin of basal joint (Fig. 6), uropod that roll into balls) (Schmalfuss 1984 in Brus- rami equal and about twice the length of the ca et al. 2007). Ligia pallasii are fast runners 1 peduncle (less than /2 total body length, (Brusca 1980). Ligia, Hatch 1947). The Ligiidae are usually littoral, they Pleotelson: Rounded on middle of posterior can swim, but in our area are restricted to the edge and postero-lateral projections not upper littoral (spray) zone (Hatch 1947). They quite as long as middle (Fig. 1). have terminal uropods that are conspicuous Sexual Dimorphism: Males with penial pro- dorsally, flagellum antenna with more than 10 cesses on second pleopods, and with wide articles and eyes with more than 50 ommatid- epimera (Fig. 2). Females, when ovigerous, ia each (Brusca et al. 2007). The Ligiidae can with oöstegites. Mature males are larger further be distinguished from the other onis- and broader than females (Carefoot 1973a, cidean families by having more than four arti- but see Kozloff 1993). cles in the flagellum of the second antennae, and by their lack of anterolateral head lobes. Possible Misidentifications This family is represented by four local spe- The order Isopoda contains 10,000 cies, two in the genus Ligia, including L. occi- species, 1/2 of which are marine and com- dentalis and L. pallasii and two in the genus prise 10 suborders, with eight present from Ligidium, including L. gracile and L. lactum. central California to Oregon (see Brusca et The former genus is semi-terrestrial and oc- al. 2007). Among isopods with small, short curs in higher intertidal marine habitats, while telsons, there are several groups (i.e. subor- the two latter species occur in riparian habi- ders) including Phreatoicidea, Asellota, Mi- tats (Brusca 1980; Brusca et al. 2007). Ligidi- crocerberidea, Calabozoidea and Oniscidea. um species have uropods with processes at The monophyletic Oniscidea the inner distal margin, to articulate the endo- (previously part of the paraphyletic Scyphac- pod (Ligia species do not). The genus Ligia is 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] characterized by a pleotelson that bears cool, moist conditions (Wilson 1970), prefer- posterolateral projections (see Fig. 1) and a ring shaded rocky cliffs and caves (Ricketts uropod that has endo- and exopod inser- and Calvin 1952; Eberl 2012) along the open tions at the same level (Ligidium species coast (Garthwaite and Lawson 1992). Young lack these projections) (Brusca et al. 2007). individuals prefer moist macroalgae, particu- The species closest to L. pallasii on larly Enteromorpha (Carefoot 1979). Phylo- the northeastern Pacific shore is Ligia occi- genetically, isopods in the genus Ligia are dentalis, an inhabitant mostly of rocky outer thought to be in evolutionary transition from shores, which, like L. pallasii, is often found sea to land environments. Thus, they have a near fresh-water seeps (Wilson 1970). It variety of unique physiological traits asso- can tolerate greater extremes of dryness ciated with each habitat (Wilson 1970; Zim- than L. pallasii. The two species can be dis- mer 2002). The composition of respiratory tinguished morphologically. Ligia proteins, hemocyanin present in L. pallasii occidentalis is narrower than L. pallasii, have been described (see Terwilliger 1982). being over twice as long as wide and its Salinity: Found in full salt water habitats, but eyes that are closer together, about one near fresh water seeps. Individuals can os- eye's length apart (Garthwaite and moregulate well and are found in areas of var- Lawson1992). Furthermore, its uropod iable salinity (Wilson 1970). Ligia pallasii has bases are several times longer than broad been the subject of some toxicity research (L. pallasii's are almost square) (Brusca et (e.g. Carefoot 1990a, b). al. 2007). Its second antennal flagella are Temperature: Does not tolerate extended longer, extending to the sixth thoracic heat or drying, lives permanently in cool moist segment, and contain 29 articles, not 15. habitats (Wilson 1970). This species occurs on rocky shores, from Tidal Level: Individuals reported living on Oregon south (Brusca et al. 2007). cliffs that are 1.5–6 meters above tide level in Moss Beach, California. In estuarine beaches Ecological Information of South Slough in Coos Bay, individuals are Range: Known range includes western found at about 1.5 meters. Aleutians south to Santa Cruz, California Associates: Beach wrack and wood debris (Welton and Miller 1980).