Phylum: Arthropoda, Crustacea Archaeomysis grebnitzkii Class: Multicrustacea, Malacostraca, Eumalacostraca Order: Peracarida, Mysida A mysid or opossum shrimp Family: Mysidae, Gastrosaccinae, Archaeomysini Taxonomy: Archaeomysis grebnitzkii was covered by a carapace), and a pleon described from a specimen collected from (abdomen). At the posterior end, they have a cod gut contents by Czerniavksy in 1882. telson and uropods. Among the Mysidacea Later, Holmes described the same species specifically, the carapace is attached to the under a different name, Callomysis macula- thorax by anterior segments only and the pos- ta, which was collected from a sandy beach terior dorsal edge is free (Banner 1948) (Fig. (Holmquist 1975). In 1932, Tattersall trans- 1). Mysid eyes are stalked, antennules are ferred C. maculata to A. maculata and biramous, antennae have a long scale (or Holmquist (1975) synonymized Archaeomy- squama), pleopods are often reduced, thorac- sis maculata and Callomysis maculata as A. ic legs bear swimming exopodites and uro- grebnitzkii, a species which exhibited a wide pods are lamellar and form tail fan. Mysids North Pacific range (Hanamura 1997; Moldin are easily distinguished from other Peracardia 2007). These species were previously dif- by the presence of a statocyst on the uropod ferentiated by subtle variation in morphologi- endopods (see Plate 220, Moldin 2007; cal characters that were deemed to be intra- Vicente et al. 2014; Fig. 1, Meland et al. specific (e.g. rostrum shape, third pleopod 2015). exopod segments, telson length, Hamanura Cephalon: (see also Figs. 3–4, Hanamura 1997). 1997). Eyes: Large, movable, stalked, with Description black corneas and somewhat pear shaped. Size: Male body length ranges from 9–15 Eye and eyestalk less than twice as long as mm, and females 13–22 mm (Holmquist broad (Fig. 2). Visual pigment absorbances 1975; Hanamura 1997). The illustrated were measured for A. grebnitzkii at 496 nm, specimens (from Columbia River) are 4.5 which phylogenetically placed this species mm (male, Fig. 1a) and 17 mm (female, Fig. within a shorter-wavelength spectral sensitivi- 1). ties species clade (Porter et al. 2007). Color: Transparent, with stellar melano- Antennae: Almost as long as body and phores (“maculate”) (see Fig. 265, Kozloff with slender multi-articulate flagellum (Fig. 1). 1993). Some mysids can change color to The peduncle has three joints and is longer blend with their background (Tattersall and than its antennal scale, but shorter than the Tattersall 1951). antennular peduncle (Fig. 2). Antennule General Morphology: Mysids are shrimp- peduncle with first joint about equal in length like crustaceans that are occasionally called to remaining joints combined. The second opossum shrimp due to the female marsupi- joint is with two spines on the outer margin um or brood pouch, which is composed of (Tattersall 1951) (Fig. 2). oostegites. Like other Peracarida (e.g. cu- Antennae Scale: (= squama) Extends maceans, isopods, amphipods, tanaids), to distal end of second penducular joint and is their bodies are elongated and composed of 3½ times as long as broad. Sclae with an anterior cephalon and pereon (thorax, 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. Archaeomysis grebnitzkii. 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. straight outer margin, without setae (Mysida, Mauchline 1980) and all are (Archaeomysis, Banner 1948), with strong biramous. The first is with a uniarticulate terminal spine, and weak or absent distal endopod and multi-articulate exopod (Banner suture. On anterior and inner margins, scale 1948) of 7–9 articles (Holmquist 1975) and is setose only (no spines) (Kozloff 1974) the third is with elongate exopod (Fig. 2). (Archaeomysis, Banner 1948) (Fig. 1a, 4). Mouthparts: For general mouthpart The second pleopod with endopod of 4–7 ar- morphology for the Mysida see Fig. 3, ticles, exopod of 8–9 articles, the third is with Meland et al. 2015. Labrum longer than elongate copulatory exopod of 8–10 articles, broad (Tattersall and Tattersall 1951) and endopod of 5 articles (Holmquist 1975) (Fig. with strong frontal spiniform process 4), and the fourth is with exopod of 5–9 ar- (Archaeomysis, Banner 1948). ticles (Holmquist 1975). The illustrated fourth Carapace: Attached to first two or male pleopod has 7 articles (Fig. 5), endopod three thoracic segments and free dorsally at a simple plate, the fifth pleopod is like the posterior edge (Banner 1948) (Figs. 1, 1a). fourth, but shorter, and with 4–8 exopod ar- Posterior margin with rounded lateral lobes ticles (Holmquist 1975) (see also Fig. 4, Ha- (Archaeomysis, Banner 1948) and no fringe namura 1997). or ornamentation. Carapace pronounced Female pleopods are ''usually degener- anteriorly into a short rostrum (Fig. 2). ate"' (i.e. reduced) (Mysida, Mauchline 1980) Rostrum: Rostrum length shorter and all are biramous (Archaeomysis, than eyestalk (Fig. 2) and can be grooved, Mauchline 1980) with each ramus of one bent down slightly, or rounded (Holmquist small article (see also Fig. 3, Hanamura 1975). ''Shortly produced”, according to ori- 1997). The first pleopod is with an elongate ginal genus description (Archaeomysis, protopodite, with basal and distal tufts of long Holmquist 1975). setae, the endopod is longer than the exopod, Pereon: and more than ½ as long as protopodite (Fig. Pereopods: Pereopods without well- 6). The second pleopods are with short developed gills (Mysida, Banner 1948). First protopodite and exopod, and a longer leg with exopodite, second leg without a endopod (Fig. 7). The third, fourth and fifth lobe-like process on merus. Pereopod 3–8 female pleopods are like the second, but with with carpopropodus (carpus and propodus shorter endopods. fused) of endopod that is divided into many Telson: Telson with distinct apical cleft subjoints: 9–11 in females (Fig. 3), 7–9 in (Mauchline 1980) and margins of cleft are males (not figured). Exopods in both male denticulate (Banner 1948) (Fig. 8). Total and female legs 3–8 has a basal joint with length is 2½ times as long as broad (at base). an acute outer distal corner (Fig. 3). No Eight to nine spines are present on each branchiae are present on thoracic legs margin, the last two spines are long, strong, (Mauchline 1980). and close together (Fig. 8) (See Fig. 7, Pleon: Fifth pleonite with a small medial Meland et al. 2015; see Fig. 5, Hanamura projection and sixth with corresponding ridge 1997). (Fig. 5). In females, the lateral pleura on the Uropods: Uropods biramous, with neither first somites help form the brood pouch (Fig. branch articulate (Banner 1948). Both rami 1). Inconspicuous pleura are present on so- with setae on the distal margin mites 3–5, none on six (Banner 1948). (Archaeomysis, Banner 1948). The endopod Pleopods: Male pleopods variable is longer than the exopod, with statocyst near 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] base, and two basal spines (Fig. 8). Six of up to ½ inch long, with an inflated, spines are present on the inner edge in shrimp-like carapace (see Cumella vulgaris, males, seven in females (Banner 1948). this guide), a single compound recessed The exopod is truncate, without suture eye (except for some eyeless females of (Mauchline 1980), with 14 (male) to 17 some species), and a flexible, tubular abdo- (female) lateral spines on outer margin men. Mysids characteristically have large, (more than 10, Archaeomysis). No setae on stalked, movable eyes, and well developed the outer exopod margin (Banner 1948). exopodites on their thoracic legs. Mature Statocyst: Light and balance organ on en- females have oostegites forming a dopod of uropod (Fig. 8). It is found in all marsupium. Additionally, northeast Pacific neritic and in common oceanic mysids mysids lack thoracic gills, have reduced (Banner 1948) and distinguishes mysids pleopods in females (and sometimes in from larval decapods (Green 1968). males). They also have a statocyst on the Sexual Dimorphism: Pleopod morphology inner ramus of the uropod. varies between males and females. Mature Mysicadea is divided into two subor- females are also recognizable by the pres- ders, the Mysida and Lophogastrida. The ence of oostegites that form a thoracic brood former suborder comprises coastal and in- pouch with two pairs of lamellae (Mauchline tertidal species while the latter includes 1980). These oostegites arise from the mostly large, pelagic and deep sea mysids. seventh and eighth pereopods (Mauchline These suborders are easily differentiable by 1980) to form marsupium (Fig. 1). the presence of branchial gills, biramous pleopods and the lack of statocysts in Possible Misidentifications Lophogastrida (branchia are absent, pleo- Mysidacea and Euphausiacea, be- pods are reduced and statocysts are con- ing superficially similar in appearance, are spicuous in the Mysida) (Moldin 2007). Ar- often treated together (e.g. Banner 1948; chaeomysis