Meterythrops Smith, 1879

Fukuoka, Kouki & Murano, Masaaki, 2006, Taxonomy of the genus Meterythrops (Crustacea: Mysida: Mysidae), with a redescription of M. microphthalmus and descriptions of two new species, Journal of Natural History 40 (27 - 28), pp. 1641-1674 : 1642-1643

Genus Meterythrops Smith, 1879

Meterythrops Smith 1879, p 93 ; Holt and W. Tattersall 1905, p 116; Zimmer 1909, p 85; Hansen 1910, p 63; Banner 1948, p 376; W. Tattersall and O. Tattersall 1951, p 209; Ii 1964, p 312; Pillai 1965, p 1706; Murano 1977, p 166; Kathman et al. 1986, p 187.

Type species. Meterythrops robusta Smith, 1879 .

Diagnosis

Body robust, with constriction between thorax and abdomen. Anterolateral corner of carapace pointed. Eyes developed. Antennular and antennal peduncles thick. Lateral margin of antennal scale without setae, almost straight or serrated, terminating in spiniform process (except for M. megalops ). Labrum without spiniform process on anterior margin. Second segment of endopod of maxilla longer than broad, without spines. Endopod of third to eighth thoracopods slender; carpopropodus divided into three subsegments. Female with two pairs of developed oostegites. Male pleopods biramous; endopod of first pleopod reduced to unsegmented small lobe; exopod of first pleopod and both rami of second to fifth pleopods developed, multi-segmented; fifth pleopod or both fourth and fifth pleopods furnished with several modified setae, which are slightly thicker than normal setae and furnished with minute setae. All pleopods of female uniramous, reduced to unsegmented lobe. Endopod of uropod armed or unarmed with spines on mesial ventral margin. Telson triangular with narrowly truncate apex; lateral margin smooth or finely serrated; apex with two pairs of spines and median pair of plumose setae. Gender is masculine.

Remarks

The diagnosis of Meterythrops is revised with respect to the marsupium of the female and the pleopods of the male.

Smith (1879) described the brood pouch of Meterythrops as being composed of four lamellae, as in Mysis Latreille, 1802 , but later authors reported the number of lamellae to be three pairs ( Zimmer 1909; Banner 1948; Ii 1964; Kathman et al. 1986) or three or four pairs ( Tattersall and Tattersall 1951). We confirm that females in all Meterythrops species, except M. megalops in which only a young specimen has been described, have two pairs of developed oostegites. In addition to the developed oostegites, females have a reduced oostegite in Meterythrops sp. Murano, 1977 and M. tenuispinis , and a tuft of setae in M. microphtahlmus , M. pictus , and M. intermedius on the sixth thoracopod.

Ii (1964) reported that male Meterythrops have pleopods lacking modified setae. However, we report modified setae on the fifth pleopod in M. microphthalmus and M. robustus , and on the fourth and fifth pleopods in M. pictus . It is unknown whether the other two species, M. japonicus and M. megalops , have these modified setae because adult males have not yet been collected.

Meterythrops resembles Amathimysis Brattegard, 1969 , Katerythrops Holt and W. Tattersall, 1905 , and Parerythrops Sars, 1869 in the shape and armature of the telson. It differs, however, from Amathimysis and Katerythrops by having a median pair of plumose setae on the telson apex. It differs from Parerythrops in that the first pleopod of males has a multi-segmented exopod.

Meterythrops is also similar to Pleurerythrops Ii, 1964 in the shape and armature of the telson. However, there is taxonomic confusion within Pleurerythrops , which currently includes five species: P. inscita Ii, 1964 , P. secunda Murano, 1970 , P. constricta Panampunnayil, 1977 , P. americana Zoppi de Roa and Delgado, 1989 , and P. monospinosa Liu and Wang, 1986 . Pleurerythrops inscita and P. secunda have a long, bare, or slightly barbed seta on the ultimate and penultimate segments of the fifth pleopod in males ( Ii 1964; Murano 1970a; Panampunnayil 1998). Pleurerythrops americana males also have two modified setae on the ultimate segment of the endopod of the fifth pleopod, but unlike all other Pleurerythrops species, do not have a pair of plumose setae on the apex of the telson ( Zoppi de Roa and Delgado 1989). The pleopods of male Pleurerythrops monospinosa lack modified setae ( Liu and Wang 1986). Only female Pleurerythrops constricta are known ( Panampunnayil 1977). Meterythrops is distinct from Pleurerythrops s. str. (comprising P. inscita and P. secunda ) in both body shape and in the characters of the pleopods of males. Meterythrops has a cylindrical body, as opposed to the somewhat depressed body in Pleurerythrops . The pleopods of male Meterythrops are densely covered in minute, modified setae, whereas those of Pleurerythrops are remarkably long and either bare or sparsely barbed distally.