Atlas of Crustacean Larvae

26. Mystacocarida

26 general: Mystacocaridans are tiny, interstitial, somewhat worm-like crustaceans. They are a species-poor group (13 species) found interstitially in the intertidal zones of sandy beaches in many parts of the world, such as the Mediterranean area, Africa, the western coast of North America, South America, and Western Australia (Hessler 1992b; Boxshall and Defaye 1996). Despite being so widespread geographically, the taxon is characterized by an extreme conservatism in its morphology. A number of features—some also applicable to the known larvae—are characteristic for all mystacocarids and are probably modifications to a life among sand grains: a pair of very long antennules; a characteristic flexing zone in the cephalon (fig. 26.1F); a rather long tubular and flexible body, with the segments bearing characteristic toothed furrows laterally (fig. 26.1A, D, F); and four pairs of small, setaebearing lobate thoracopods (fig. 26.2K) (Pennak and Zinn 1943; Delamare-Deboutteville 1953; Hessler and Sanders 1966; Olesen 2001). Another characteristic is that, in many respects, adults retain a larval appearance. Mystacocarids crawl among sand grains, and locomotion is brought about by alternating cyclic movements of the biramous antennae and mandibles (Lombardi and Ruppert 1982). The Mystacocarida are divided into only 2 genera, Derocheilocaris (8 species) and Ctenocheilocaris (5 species). The phylogenetic position of the Mystacocarida within the Crustacea is not entirely settled. Traditionally, they have been grouped in the now-abandoned Maxillipoda, along with other crustacean taxa such as the Copepoda and Branchiura (Dahl 1956; Walossek and Müller 1998a, 1998b). A close relationship with the Orsten fossil †Skara and with the Copepoda (Walossek and Müller 1998a, 1998b) was recently supported (J. Haug et al. 2011a). A recent comprehensive molecular phylogeny placed the Mystacocarida closest to the Branchiura and Pentastomida (Regier et al. 2010). The life cycle is assumed to start with freely laid eggs, from which the metanauplii (stage 1) hatch. Stage 1 is followed by seven additional immature stages. McLachlan (1977) found that reproduction for Derocheilocaris algoensis occurs throughout the year and that growth from an egg to a reproductive adult appears to take two months. In D. typicus, about 40 days is required (J. Hall and Hessler 1971). These authors also noted that in D. typicus, some young individuals are present at all times of the year, but they are most abundant in the summer months. larval types: Larval development has been examined in 4 species: Derocheilocaris remanei, D. typicus, D. algoensis, and D. katesae, and a few stages were treated briefly for D. delamarei (Pennak and Zinn 1943; Delamare-Deboutteville 1954; Hessler and Sanders 1966; McLachlan 1977; Cals and Cals-Usciati 1982; Olesen 2001; J. Haug et al. 2011a). Nothing is known about development in Ctenocheilocaris. It was long believed that the 2 well-studied species, Derocheilocaris typicus from the Atlantic coast of the United States and D. remanei from the Mediterranean area, which are quite similar in adult morphology, differed significantly with respect to their larval sequences (summarized by Hessler and Sanders 1966). Reinvestigations of D. remanei, however, have corrected the original description and have shown that larval developments in D. remanei and D. typicus are, in fact, very similar, including the exact same number of stages, with largely the same morphology (Olesen 2001; J. Haug et al. 2011a). This provides further evidence for the general view of the mystacocarids being evolutionarily conservative . The development of D. algoensis follows the same course (McLachlan 1977). The treatment of mystacocarid development given below is based on D. remanei but, as far as is known, it can be considered general for mystacocarids. Metanauplius: The earliest-known stage of Derocheilocaris remanei can be termed a metanauplius, as it has three distinct post-cephalic segments and a pair of undeveloped maxillules (figs. 26.1A; 26.2A, D). Delamare-Deboutteville (1954) originally described the earliest stage as an orthonauplius lacking maxillular limb buds, but further investigations have strongly suggested that such a stage is non-existent (Cals and Cals-Usciati 1982; Olesen 2001; J. Haug et al. 2011a). Based on Jørgen Olesen Joachim T. Haug Jørgen Olesen and Joachim T. Haug Mystacocarida Mystacocarida 139 an analysis of size categories and morphology, J. Haug et al. (2011a) identified nine developmental stages of D. remanei: eight immature stages, and the adult. Since the development from larvae to adults is rather gradual, and since the adults in many respects are larva-like (e.g., larval morphology of the antennae and mandible are retained in adults), there are few distinct landmarks during...