Atlas of Crustacean Larvae

22. Thoracica

22 general: Thoracican cirripedes include the well-known crustaceans referred to as barnacles. The group contains more than 1,000 species worldwide and exhibits an extreme diversity in body forms, including stalked and acorn barnacles, cup-shaped sponge barnacles, and the asymmetrical Verrucomorpha (D. Anderson 1994). Thoracican cirripedes inhabit almost all marine environments, including rocky intertidal zones, mangroves, subtidal reefs, corals, sponges, deep-sea hydrothermal vents, and cold seeps, and they can be epibiotic on a wide range of other marine organisms. Despite this range of life forms and habitats, all thoracicans share a common life-history pattern, consisting of planktonic larval stages that eventually settle and metamorphose into sessile adults (Moyse 1987; Walker et al. 1987; Høeg and Møller 2006). The adults are suspension feeders, except for a handful of parasitic species . The life cycle consists of 4–6 naupliar instars, a cyprid, and metamorphosis into sessile juveniles and adults. larval types Nauplius: Naupliar larvae (figs. 22.1; 22.2) can be planktotrophic or lecithotrophic. Lecithotrophic larvae are larger in size when compared with planktotrophic larvae, and their bodies contain many lipid reserves (fig. 22.1E, F). They also have a rudimentary labrum (fig. 22.1E), a structure otherwise essential for naupliar suspension feeding (Walossek 1993). The setae on the swimming limbs in lecithotrophic larvae are often simple (fig. 22.1 D, G), instead of the plumose setae seen in planktotrophic larvae (fig. 22.2A, B). Cyprid: In some species, the embryos can hatch directly as cyprids (most scalpellids), or the naupliar phase can be spent as brooded larvae inside the mantle cavity until the cyprid stage is reached (intertidal Tetraclitella). Such brooded nauplii also have greatly reduced limbs and a rudimentary labrum (fig. 22.1F). As in all other cirripedes, cyprids of thoracican barnacles (figs. 22.2C; 22.3) are non-feeding. The body is composed of a carapace, which develops from the naupliar head shield and flexes down to cover both sides of the larva (Høeg et al. 2004). morphology Nauplius: Thoracican naupliar larvae are often pear-shaped and have, like all other cirripede nauplii, a pair of frontolateral horns. Instar I larvae are globular, with the limbs and labrum poorly developed (fig. 22.1B). These larvae have short frontolateral horns pointed posteriorly. The frontolateral horns have a horizontal array of pores on the top surfaces, in addition to a large terminal gland pore. All naupliar instars carry a pair of frontal filaments, and most species (except deep-sea hydrothermal vent species) also have a single medial naupliar eye from instars I to VI (figs. 22.1A; 22.2A). Paired compound eyes are also visible late in instar VI, where the larvae also have a much more pronounced globular shape (fig. 22.1D), but the compound eyes are not fully developed until the succeeding cypris instar. The nauplii have three pairs of limbs (figs. 22.1A, C; 22.2A): uniramous antennules, and biramous antennae and mandibles. The antennules, antennae, and mandibles are equipped with a variety of setae (figs. 22.1C; 22.2A, B). On the ventral side, the naupliar labrum covers the pre-oral chamber and the mouth (fig. 22.1C). The hindbody of the nauplius consists of a trunk (figs. 22.1A, C; 22.2A). It represents the developing thorax, which only becomes segmented in the cyprid, and it accordingly changes in shape through the naupliar instars. Cyprid: The cyprid has an elongated torpedo-shaped body that is covered by the carapace, which is drawn down on both sides (fig. 22.3), but it only rarely has a middorsal ridge (fig. 22.3C) and therefore is not truly bivalved. All cyprid appendages can be completely withdrawn into the mantle cavity formed by the carapace (fig. 22.3A). On its surface the carapace bears setae, lattice organs, and various types of gland pores. As in all other thecostracans, there are five pairs of lattice organs, two situated anteriorly and three situated posteriorly (fig. Benny K. K. Chan Jens T. Høeg Ryusuke Kado Benny K. K. Chan, Jens T. Høeg, and Ryusuke Kado Thoracica Thoracica 117 22.3B, C). They are elongated chemosensory organs, perforated by pores (fig. 22.3B), and develop from setae in the head shield of the nauplius (P. Jensen et al. 1994a; Rybakov et al. 2003). Anteroventrally, the carapace has a pair of large pores on each side, which are the exits for the frontolateral glands (fig. 22.3B). Cyprids of a few species (e.g., Lepas) can...