Atlas of Crustacean Larvae

20. Acrothoracica

20 general: The Acrothoracica is a group of small burrowing epibiotic barnacles, found largely in carbonate sediments and the skeletons of marine mollusks. Acrothoracicans are dioecious, with suspension-feeding females bearing dwarf males attached externally to the mantle sac. Females lack calcareous plates and live inside burrows (Tomlinson 1969; Kolbasov 2009). Currently, the acrothoracicans encompass approximately 67 species, assigned to 11 genera and 2 orders , the Lithoglyptida and Cryptophialida (Kolbasov 2009). The acrothoracicans were first discovered at relatively high latitudes, but their greatest diversity is found in the tropical seas of the world. The life cycle consists of at least four freeswimming naupliar stages (when present), a cypris larva, a juvenile , and an adult. Larval stages are free living; juvenile and adults are epibiotic, living in burrows. There are no known morphological differences between male and female larvae, and sex may not be determined until the time of attachment. larval types Nauplius: Free-swimming nauplii of the type typical for cirripedes (fig. 20.1A–D) were found for several genera in the Lithoglyptida: Berndtia, Armatoglyptes, Lithoglyptes, and Trypetesa (Kühnert 1934; Utinomi 1961; Turquier 1967; Tomlinson 1969; plus our own unpublished data). Cypris: Most acrothoracicans, including the Cryptophialida , are supposed to hatch as cyprids, while the naupliar phase is passed in embryonic form within the egg coat (Kolbasov et al. 1999; Kolbasov 2009). The terminal larval stage is always a typical cyprid (figs. 20.1E–G; 20.2). morphology Nauplius: The nauplii have a developed head shield, with long frontolateral horns and a long dorsocaudal spine (fig. 20.1A, B, D). Frontolateral horns are applied to the shield in stage 1 (fig. 20.1A) and then extended from the body in subsequent stages (fig. 20.1B, D). A nauplius eye and frontal filaments are located in the anterior part of the body (fig. 20.1A, B). The thoracoabdominal process terminates with a pair of sharp ventral spines (fig. 20.1A, B). Nauplii have three pairs of typical natatory limbs: uniramous antennules, biramous antennae, and mandibles covered with long setae (fig. 20.1A, B). The labrum is either rudimentary (fig. 20.1A, D) or well developed, long, and triangular (fig. 20.1B, C). There seem to be no clear-cut criteria for separating acrothoracian nauplii from those of other cirripedes (which explains their absence from the key to naupliar larvae in chapter 2). Cypris: Cypris larvae of the Lithoglyptida (figs. 20.1E; 20.2A, B, F) and Cryptophialida (figs. 20.1F, G; 20.2C, D) differ significantly in their morphology. The lithoglyptids have typical spindle-shaped cirripede cypris larvae, with a well-developed carapace that completely encloses the body. As in thoracican cirripedes, they have frontolateral pores (fig. 20.2A, B, H), lattice organs with a conspicuous terminal pore (figs. 20.1E; 20.2G), and the setae on the fourth antennular segment arranged in subterminal and terminal groups (fig. 20.2L). Similarly, they have six pairs of natatory thoracopods and a distinct, although very small, abdomen and telson (figs. 20.1E; 20.2B, E). The cypris larvae of the Cryptophialidae have a much smaller carapace, leaving parts of the body directly exposed (figs. 20.1F, G; 20.2C, D). The carapace is conspicuously ornamented by deep pits and hexagonally arranged ridges. It bears a few very long setae but lacks frontolateral pores (figs. 20.1F). The lattice organs are plate-shaped, with a thick cuticular border, but they lack a terminal pore (fig. 20.2I). The fourth antennular segment has all the setae clustered in one terminal group (fig. 20.2J, K). The thorax and thoracopods are rudimentary and not suitable for swimming (figs. 20.1F, G; 20.2C). morphological diversity: The cyprids of the Lithoglyptida differ in the shape of the carapace (i.e., the ratio between length and height), in details of their antennular setaGregory A. Kolbasov Benny K. K. Chan Jens T. Høeg Gregory A. Kolbasov, Benny K. K. Chan, and Jens T. Høeg Acrothoracica 108 Gregory A. Kolbasov, Benny K. K. Chan, and Jens T. Høeg tion, in the armament of the thoracopods, and in the degree of telson cleavage. The cyprids of the Cryptophialida differ in carapace ornamentation, size, and the number of carapace setae , as well as in the number of setae of the fourth antennular segment (Kolbasov and Høeg 2007; Kolbasov 2009). natural history: Nauplii of Trypetesa are lecithotrophic , with a reduced labrum, mouth, and anus, and they do not increase in size during subsequent molts (Turquier 1967), whereas nauplii...