Atlas of Crustacean Larvae

23. Tantulocarida

23 general: The Tantulocarida is a small group of microcrustaceans that exhibits a unique protelean life cycle, composed of obligatory ectoparasitic larvae and free-swimming non-feeding adults. Larval tantulocaridans utilize other marine crustaceans as hosts, including copepods, amphipods, tanaidaceans, isopods, cumaceans, and ostracods (Huys 1991; Boxshall and Vader 1993). The Tantulocarida was established as a new class of Crustacea by Boxshall and Lincoln (1983), but the roots of its taxonomic history reach back to the beginning of the twentieth century. The first genera to be formally described were Cumoniscus and Microdajus, both of which were originally classified as epicaridean isopods (Bonnier 1903; Greve 1965). Currently, the group accommodates 34 described species assigned to 22 genera (Knudsen et al. 2009; Kolbasov and Savchenko 2009; Savchenko and Kolbasov 2009; Mohrbeck et al. 2010). The most widely used classification (Huys 1990a) recognizes 5 families, but recent contributions suggest that the Deoterthridae and Basipodellidae are possibly paraphyletic or polyphyletic (Kolbasov et al. 2008a; Savchenko and Kolbasov 2009; Petrunina and Kolbasov 2012). Tantulocaridans have been recorded from abyssal depths to the intertidal zone and from polar to tropical waters, including unusual habitats like anchialine cave pools (Boxshall and Huys 1989), coral reefs (Huys 1990b) and hydrothermal vents (Huys and Conroy-Dalton 1997). Recent contributions (Gutzmann et al. 2004; Mohrbeck et al. 2010) suggest that the current number of described species greatly underestimates their actual diversity. Tantulocaridans exhibit no recognizable cephalic limbs at any stage of their intricate life cycle (except for the antennules in the sexual female), but the shared position of the female copulatory pore (ventrally on the cephalothorax, at about the level of the incorporated first thoracic segment) and the male genital apertures (on the seventh post-cephalic trunk segment) is interpreted as evidence that their affinities lie with the Thecostraca (the Facetotecta, Ascothoracida, and Cirripedia) (Huys et al. 1993; also see Petrunina et al. 2013). The known life-history stages appear to form a complex dual life cycle, combining a sexual phase (with free-swimming adults) and a presumed parthenogenetic multiplicative phase that takes place on the host (fig. 23.1) (Huys et al. 1993). larval types Tantulus Larva: The infective stage, or tantulus larva (figs 23.2C; 23.3A, B), is the only stage that occurs in both cycles. Prior to mate location, the larva goes through a benthic nonfeeding phase—probably at the sediment-water interface—as part of the temporary meiofauna (Huys 1991). Attached larvae undergo development on the host without conventional molting. In the parthenogenetic pathway, the tantulus forms a large dorsal trunk sac immediately behind the cephalon, causing the ventral deflection of the larval trunk, which is subsequently sloughed (fig. 23.2A, B). The contents of the sac differentiate into eggs that are later released as fully developed tantulus larvae (fig. 23.3I). The sexual cycle involves a unique type of metamorphosis in which a free-swimming adult is formed within the expanded trunk sac of the preceding tantulus larva. In the female pathway the trunk sac forms immediately posterior to the cephalic shield, and the larval trunk is sloughed (fig. 23.3F–H, K). In the male pathway the site of trunk sac formation is at or near the back of the larval thorax, causing the ventral deflection of the urosome and thoracopods (figs. 23.2A, D, E; 23.3J). Throughout metamorphosis the developing sexual females and males are supplied with nutrients via an umbilical cord (fig. 23.3F, G). Nauplius: A benthic non-feeding stage that can be recognized as a nauplius has been reported recently but not formally described (Martínez Arbizu 2005; Mohrbeck et al. 2010), suggesting that eggs produced by the sexual female hatch as nauplii rather than as tantulus larvae. morphology: The tantulus larva (body size 75–195 μm) (figs. 23.2A–C; 23.3A, B) is composed of an anterior tagma Rony Huys Jørgen Olesen Alexandra S. Petrunina Joel W. Martin Rony Huys, Jørgen Olesen, Alexandra S. Petrunina, and Joel W. Martin Tantulocarida Tantulocarida 123 (prosome), consisting of the cephalon and a thorax of six pedigerous segments, and a limbless tagma (urosome), consisting of the last thoracic segment and a one-segmented abdomen that bears paired caudal rami (fig. 23.2F, J). The cephalon is covered with a dorsal shield bearing pores, sensilla, and (usually) epicuticular lamellae (figs. 23.2C; 23.3A, B). The tantulus has no cephalic appendages and is permanently attached to the host by a frontal oral disc (figs. 23.2C; 23.3A); in...