Atlas of Crustacean Larvae

29. Ostracoda: Podocopa

29 general: The Podocopa is one of the 2 extant subclasses (the other being the Myodocopa) that form the Ostracoda, commonly called seed shrimps. The Podocopa is divided into 3 orders: the Platycopida, Podocopida, and Palaeocopida. Podocopans are small, typically 0.3–5 mm in length, and are characterized by a calcified bivalved carapace that entirely encloses the body. Many taxa have smooth carapaces, while others have ornamented carapaces with ridges, spines, pits, reticulations, and striations. The body typically sports eight pairs of appendages, plus the sexual organs, and has no clear division between the head, thorax, and abdomen. There are approximately 33,000 described species, 8,000 of which are living, and the 3 extant orders consist of 7 suborders and 10 superfamilies (Morin and Cohen 1991; Horne et al. 2002). They inhabit almost all aquatic environments, including the deep sea, coral reefs, brackish estuaries, beach deposits, rivers , lakes, springs, groundwater, and temporary puddles. Most podocopans are benthic, nektobenthic, or interstitial, but a small number of neustonic species, as well as species living semiterrestrially in the water films of wet leaf litter, are also known (Horne et al. 2004). The life cycle consists of seven or eight free-living larval/juvenile instars and the adult instar. larval types Instars (Including Nauplius): There are either eight or nine post-hatching stages, including the adult, that are known as instars (fig. 29.1), the first of which is considered to be the nauplius. All species show direct development, with all preadult instars consisting of a carapace that usually encloses the body, thus somewhat resembling the adult. No distinction is made between larvae and juveniles, and the stages are often referred to as the A–8 instar (i.e., the eighth stage before adult), A–7 instar, A–6 instar, and so forth. morphology: The first instar typically consists of antennules , antennae, mandibles, and incipient caudal rami (the furcae of some authors), with the carapace totally enclosing the body (fig. 29.2A–D, G). The carapace is hinged along the dorsal margin in all instars of most groups, and it functions in a similar way to that of the adult, that is, opening along the anterior, ventral, and posterior margins to allow the limbs to be extended through the gap for locomotion and feeding (fig. 29.1E, H–J). The calcitic carapace is shed during each molt and regrown; because of this, it lacks growth lines. The caudal rami typically have an intermediate form in early instars, resembling a walking leg (figs. 29.1F; 29.2E, F). The fourth through seventh limbs are added in sequence, one pair at a time, during later molts. These limbs first appear as developmental buds and gain functionality during the subsequent molt. The sexual organs develop in the last two or three instars . morphological diversity: While development is similar in many respects, there is some variation in the general morphology of instars among podocopan taxa. Podocopida and Platycopida species have a bivalved carapace in all instars, with the exception of the first instar of the podocopidan Darwinuloidea ; the A-8 instars of the Darwinuloidea have a onepiece uncalcified flexible carapace covering the body (R. Smith and Kamiya 2008). The ontogenetic development of the Palaeocopida is incompletely studied, but the carapaces of the three smallest of the four known instars are remarkably different from those of the Podocopida and Platycopida, consisting of a shallow one-piece dorsal shield under which the body is suspended (fig. 29.2I) (Swanson 1989). The largest Palaeocopida larval instar that has been collected has a developed hinge along the dorsal margin, and the carapace (consisting of two valves) encloses the body, more closely resembling the carapaces of the other 2 orders. The ontogeny of the appendages of the Podocopida is generally similar in all groups that have been studied, with differences restricted to the development of the fifth limbs, which change from walking legs into feeding appendages in some groups; the caudal rami, which appear late or become reduced Robin J. Smith Robin J. Smith Ostracoda: Podocopa 166 Robin J. Smith in some groups; and the maxillules of the commensal entocytherids , which appear later, compared with other groups (R. Smith and Martens 2000; R. Smith and Kamiya 2005, 2008). Compared with the Podocopida, the Platycopida have a different ontogeny of the appendages, with the maxillules, fifth limbs, and incipient sexual organs appearing earlier, and the seventh limbs becoming reduced and eventually lost (Okada et al. 2008). Without a complete range of Palaeocopida...