Atlas of Crustacean Larvae

30. Ostracoda: Myodocopa

30 general: Myodocopan ostracods consist of 2 orders and 3 suborders—the Myodocopida, containing the Myodocopina ; and the Halocyprida, containing the Halocypridina and Cladocopina (Horne et al. 2002)—with more than 600 extant species. Most Halocypridina species are holopelagic, and most Cladocopina species are benthic, with both groups living in shallow to abyssal waters. Most Myodocopida species also live from shallow to abyssal depths, mostly as benthos, although some are planktonic. Since information on the larval stages of the Myodocopina and Halocypridina is very limited (Tseng 1975; Kornicker and Sohn 1976; Kornicker and Iliffe 1989, 1995; Ikeda 1992), in this chapter we focus mostly on the postembryonic development of the Myodocopina, a group that contains more than 400 extant species. The life cycle consists of four to seven larval stages, followed by the adult. All stages are free living. larval types Instars (Including Nauplius): Newly hatched myodocopid ostracods have a similar morphology to that of the adults, except for their smaller size, fewer appendages, and smaller number of bristles (figs. 30.1C1; 30.2A1, L1, R1, S1, T1). The free-living first larval stage, sometimes called the first instar and considered to be a nauplius, has a bivalved carapace that, when shut, encloses the entire soft body. The carapace is shed and regrown during every molt, whereas the body develops gradually during each molt (fig. 30.1A). Post-adult molting has not been confirmed (see Kornicker and Harrison-Nelson 1999). The total number of larval stages in the suborder Myodocopina is four to six: the Rutidermatidae and Sarsiellidae have four larval stages (figs. 30.2S, T); the Philomedidae has four or five (fig. 30.2L); the Cypridinidae has five or six (fig. 30.1C); and the Cylindroleberididae has four to six (figs. 30.2A, R). The suborder Halocypridina has six or seven larval stages (Thaumatoconcha radiata has six; Conchoecia pseudodiscophora and C. elongata have seven), and the Cladocopina (Metapolycope duplex) has six. Ostracod larval stages (instars) are designated either as first instar, second instar, third instar, and so forth (in ascending order of size) or as A (= Adult), A–1 (= adult minus one), A-2, and so forth, in descending order. morphology: Members of the Myodocopina hatch in almost the same developmental stage: the first instar has six developed pairs of appendages (antennules, antennae, mandibles , maxillules, fifth limbs, and furcae). Instar Characters: In the early instars, members of the Myodocopina show almost the same development in the three characters listed below (fig. 30.1R). Although there are some exceptions, the process is remarkably consistent. Occurrence of bristles on the fourth segment of the antennule : In the first larval stage, the fourth segment is without bristles. In the second stage, one dorsodistal bristle appears. In the next, one ventrodistal bristle is added. Although the third and fourth segments are fused in members of the Sarsiellidae and in Rutiderma species (Rutidermatidae), the bristles grow in the same way as described above (figs. 30.1E–G; 30.2B–D, U–W). Sixth limb: In the first larval stage, the sixth limb is hirsute and without bristles. In the second stage, at least one bristle appears. Usually the limb has one anterior bristle, but some species have two anterior bristles, one anterior and one posterior bristle, or many anterior and ventral bristles. In the third stage, the limb develops into the adult form and is usually furnished with many bristles (figs. 30.1H–L; 30.2E–G, X–Z). Seventh limb: In the first stage, the seventh limb either is not seen or is found as a very small bud-like process. In the second stage, all species exhibit this limb as a thumb-like process. In the third stage, the limb is elongate, but still bare. Bristles appear first at the fourth instar, so that a specimen with bristles on the limb is not younger than the fourth larval stage. In Sarsiella species, however, the seventh limb in males continues to show characteristics found in the first or second larval stages, even in the late larval and adult stages (figs. 30.1M–Q; 30.2H–K). Shin-ichi Hiruta Shimpei Hiruta Shin-ichi Hiruta and Shimpei Hiruta Ostracoda: Myodocopa 170 Shin-ichi Hiruta and Shimpei Hiruta Key: Above is a key to early myodocopid instars. A few species vary from the key, such as Gigantocypris muelleri (development of the antennule), Paradoloria angulata (antennule), Amboleberis americana (seventh limb), and Cycloleberis christiei (seventh limb) (Kornicker 1981; A. Cohen 1983; Hiruta 1983; Kornicker and Harrison-Nelson...