Archaeomysis Grebnitzkii Subphylum: Crustacea Class: Malacostraca a Mysid Or Opossum Shrimp Order: Mysida Family: Mysidae

Phylum: Arthropoda Archaeomysis grebnitzkii Subphylum: Crustacea Class: Malacostraca Order: Mysida A mysid or opossum shrimp Family: Mysidae Taxonomy: Archaeomysis grebnitzkii was biramous, antennae have a long scale (or described from a specimen collected from cod squama), pleopods are often reduced, gut contents by Czerniavksy in 1882. Later, thoracic legs bear swimming exopodites and Holmes described the same species under a uropods are lamellar and form tail fan. different name, Callomysis maculata, which Mysids are easily distinguished from other was collected from a sandy beach (Holmquist Peracardia by the presence of a statocyst on 1975). In 1932, Tattersall transferred C. the uropod endopods (see Plate 220, Moldin maculata to A. maculata and Holmquist 2007; Vicente et al. 2014; Fig. 1, Meland et al. (1975) synonymized Archaeomysis maculata 2015). and Callomysis maculata as A. grebnitzkii, a Cephalon: (see also Figs. 3–4, Hanamura species which exhibited a wide North Pacific 1997). range (Hanamura 1997; Moldin 2007). These Carapace: Attached to first two or species were previously differentiated by three thoracic segments and free dorsally at subtle variation in morphological characters posterior edge (Banner 1948) (Figs. 1, 1a). that were deemed to be intraspecific (e.g. Posterior margin with rounded lateral lobes rostrum shape, third pleopod exopod (Archaeomysis, Banner 1948) and no fringe segments, telson length, Hamanura 1997). or ornamentation. Carapace pronounced anteriorly into a short rostrum (Fig. 2). Description Rostrum: Rostrum length shorter Size: Male body length ranges from 9–15 than eyestalk (Fig. 2) and can be grooved, mm, and females 13–22 mm (Holmquist bent down slightly, or rounded (Holmquist 1975; Hanamura 1997). The illustrated 1975). ''Shortly produced”, according to specimens (from Columbia River) are 4.5 mm original genus description (Archaeomysis, (male, Fig. 1a) and 17 mm (female, Fig. 1). Holmquist 1975). Color: Transparent, with stellar Eyes: Large, movable, stalked, with melanophores (“maculate”) (see Fig. 265, black corneas and somewhat pear shaped. Kozloff 1993). Some mysids can change Eye and eyestalk less than twice as long as color to blend with their background broad (Fig. 2). Visual pigment absorbances (Tattersall and Tattersall 1951). were measured for A. grebnitzkii at 496 nm, General Morphology: Mysids are shrimp- which phylogenetically placed this species like crustaceans that are occasionally called within a shorter-wavelength spectral opossum shrimp due to the female sensitivities species clade (Porter et al. 2007). marsupium or brood pouch, which is Antennae: Almost as long as body composed of oostegites. Like other and with slender multi-articulate flagellum Peracarida (e.g. cumaceans, isopods, (Fig. 1). The peduncle has three joints and is amphipods, tanaids), their bodies are longer than its antennal scale, but shorter elongated and composed of an anterior than the antennular peduncle (Fig. 2). cephalon and pereon (thorax, covered by a Antennule peduncle with first joint about equal carapace), and a pleon (abdomen). At the in length to remaining joints combined. The posterior end, they have a telson and second joint is with two spines on the outer uropods. Among the Mysidacea specifically, margin (Tattersall 1951) (Fig. 2). the carapace is attached to the thorax by Antennae Scale: (= squama) anterior segments only and the posterior Extends to distal end of second penducular dorsal edge is free (Banner 1948) (Fig. 1). joint and is 3½ times as long as broad. Sclae Mysid eyes are stalked, antennules are with straight outer margin, without setae 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, Charleston, OR. A publication of the University of Oregon Libraries and the Oregon Institute of Marine Biology Individual species: http://hdl.handle.net/1794/12692 and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] (Archaeomysis, Banner 1948), with strong Hanamura 1997). The first pleopod is with an terminal spine, and weak or absent distal elongate protopodite, with basal and distal suture. On anterior and inner margins, scale tufts of long setae, the endopod is longer than is setose only (no spines) (Kozloff 1974) (Fig. the exopod, and more than ½ as long as 2). protopodite (Fig. 6). The second pleopods Mouthparts: For general mouthpart are with short protopodite and exopod, and a morphology for the Mysida see Fig. 3, Meland longer endopod (Fig. 7). The third, fourth and et al. 2015. Labrum longer than broad fifth female pleopods are like the second, but (Tattersall and Tattersall 1951) and with with shorter endopods. strong frontal spiniform process Telson: Telson with distinct apical cleft (Archaeomysis, Banner 1948). (Mauchline 1980) and margins of cleft are Pereon: denticulate (Banner 1948) (Fig. 8). Total Pereopods: Pereopods without well- length is 2½ times as long as broad (at base). developed gills (Mysida, Banner 1948). First Eight to nine spines are present on each leg with exopodite, second leg without a lobe- margin, the last two spines are long, strong, like process on merus. Pereopod 3–8 with and close together (Fig. 8) (See Fig. 7, carpopropodus (carpus and propodus fused) Meland et al. 2015; see Fig. 5, Hanamura of endopod that is divided into many 1997). subjoints: 9–11 in females (Fig. 3), 7–9 in Uropods: Uropods biramous, with neither males (not figured). Exopods in both male branch articulate (Banner 1948). Both rami and female legs 3–8 has a basal joint with an with setae on the distal margin acute outer distal corner (Fig. 3). No (Archaeomysis, Banner 1948). The endopod branchiae are present on thoracic legs is longer than the exopod, with statocyst near (Mauchline 1980). base, and two basal spines (Fig. 8). Six Pleon: Fifth pleonite with a small medial spines are present on the inner edge in projection and sixth with corresponding ridge males, seven in females (Banner 1948). The (Fig. 5). In females, the lateral pleura on the exopod is truncate, without suture (Mauchline first somites help form the brood pouch (Fig. 1980), with 14 (male) to 17 (female) lateral 1). Inconspicuous pleura are present on spines on outer margin (more than 10, somites 3–5, none on six (Banner 1948). Archaeomysis). No setae on the outer Pleopods: Male pleopods variable exopod margin (Banner 1948). (Mysida, Mauchline 1980) and all are Statocyst: Light and balance organ on biramous. The first is with a uniarticulate endopod of uropod (Fig. 8). It is found in all endopod and multi-articulate exopod (Banner neritic and in common oceanic mysids 1948) of 7–9 articles (Holmquist 1975) and (Banner 1948) and distinguishes mysids from the third is with elongate exopod larval decapods (Green 1968). (Archaeomysis, Banner 1948) (Fig. 1a, 4). Sexual Dimorphism: Pleopod morphology The second pleopod with endopod of 4–7 varies between males and females. Mature articles, exopod of 8–9 articles, the third is females are also recognizable by the with elongate copulatory exopod of 8–10 presence of oostegites that form a thoracic articles, endopod of 5 articles (Holmquist brood pouch with two pairs of lamellae 1975) (Fig. 4), and the fourth is with exopod (Mauchline 1980). These oostegites arise of 5–9 articles (Holmquist 1975). The from the seventh and eighth pereopods illustrated fourth male pleopod has 7 articles (Mauchline 1980) to form marsupium (Fig. 1). (Fig. 5), endopod a simple plate, the fifth pleopod is like the fourth, but shorter, and Possible Misidentifications with 4–8 exopod articles (Holmquist 1975) Mysidacea and Euphausiacea, being (see also Fig. 4, Hanamura 1997). superficially similar in appearance, are often Female pleopods are ''usually treated together (e.g. Banner 1948; degenerate"' (i.e. reduced) (Mysida, Mauchline 1980). (They were formerly Mauchline 1980) and all are biramous combined as the Schizopoda.) Both are (Archaeomysis, Mauchline 1980) with each orders of the class Malacostraca, but ramus of one small article (see also Fig. 3, euphausiids are in the group (i.e. 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, Charleston, OR. superorder) Eucarida with the Decapoda Archaeomysis (each with a single species (Martin 2007). Like the mysids, euphausiids represented locally) (Moldin 2007). differ from decapods in having biramous Alienacanthomysis macropsis is thoracic legs (Kasaoka 1974). Unlike the recognizable by elongated eyestalks and mysids, euphausiids have a carapace that is Deltamysis holmquistae has spines on the fused dorsally with all the thoracic lateral telson margins, but only distally, segments. The mysid carapace is attached where other species have spines all over only to the first two or three thoracic the telson margins or in proximal groups. segments. Furthermore, mysid females Alienacanthomysis macropsis, has a have oostegites, but euphausiids do not. broadly triangular rostrum with long acute Other orders of Peracarida include lateral carapace spines and its telson has a lsopoda, Tanaidacea, and Amphipoda, fringe of small spines. It is abundant in San which are all fairly easily distinguished from Francisco

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