Atlas of Crustacean Larvae

24. Branchiura

24 general: Branchiurans are commonly known as fish or carp lice. They are a relatively small group of mostly limnic fish ectoparasites, consisting of 4 genera and approximately 230 species. They can also occur in marine habitats. Argulus is widespread in fresh waters, where it feeds on a wide variety of hosts, and it is a significant problem in aquaculture. Branchiuran adults are approximately 5–15 mm long, characteristically dorsoventrally flattened, and always equipped with a carapace (of varied shape, sometimes completely two-lobed) covering four pairs of thoracopods used for swimming (figs. 24.1D, 24.3N). The abdomen is unsegmented and carries two rudimentary furcal rami, which are more clearly visible in larvae and juveniles (figs. 24.1C, 24.2R; 24.3F, O, R). Two large and well-developed compound eyes are always present, as well as a smaller three-cupped nauplius eye (fig. 24.1C, D) (Thiele 1904; Møller 2009). The host attachment always involves the maxillules, which are modified into muscular suction cups (in Argulus, Chonopeltis, and the monotypic Dipteropeltis) (fig. 24.3N, P) or strong hooks (in Dolops) (fig. 24.1E–G) (Møller et al. 2008; Kaji et al. 2011). The attachment is non-permanent, and adults, which are excellent swimmers, often leave the host (Chonopeltis excepted). They feed on their hosts by biting into the integument with their mandibles, situated at the tip of the mouth cone (figs. 24.2A, F; 24.3A, E, F, K, N), and feed on the blood and tissue fluids from the wound. Argulus and Dipteropeltis have a stylet-like pre-oral spine (figs. 24.2A, F, G; 24.3A, B, K). This structure is not connected to the esophagus or alimentary canal, but it presumably is used to puncture and penetrate the host’s integument and thus physically promote hemorrhaging (Debaisieux 1953; Shimura 1983). Glands are associated with this pre-oral spine and are suggested to secrete anticoagulants, but this is not completely clarified (Shimura and Inoue 1984; Swanepoel and Avenant-Oldewage 1992; Gresty et al. 1993). Our knowledge of branchiuran larvae is unfortunately not nearly as complete as that of the adults, and hatching-stage larvae are known from fewer than 20 species thus far, most of them from the genus Argulus. Unfortunately, larvae of some of the more aberrant Argulus species, such as A. paranensis or A. brachypeltis, have not been described. Until recently, the larvae from the single African species of Dolops (D. ranarum) were the only ones known from this genus. Gomes and Malta (2002) mentioned the general shape of the hatching stage of the South American D. carvalhoi, and the larva of this species was recently described using SEM and light microscopy by Møller and Olesen (2012). Chonopeltis larvae are known only from the literature, and nothing is known about the larvae of Dipteropeltis. The life cycle varies among genera. larval types: Thus far, three larval types have been described from the Branchiura (table 2.1). The first type is often referred to as a metanauplius (but is more progressed in development ), such as in Argulus foliaceus (figs. 24.1C; 24.2A). The second type is referred to as juvenile-like and is found in at least 2 species of Dolops (fig. 24.1E, G), as well as in Argulus, where it can occur as either the hatching stage or as stage 2 following a metanauplius-like hatching stage (Fryer 1961; Avenant et al. 1989; Rushton-Mellor and Boxshall 1994; Gomes and Malta 2002; Møller and Olesen 2012). The final type is an odd-looking non-swimming type found in Chonopeltis (fig. 24.1H). Metanauplius-Like Larvae: The metanauplius-like larvae of Argulus foliaceus resemble other crustacean nauplii by hatching with a naupliar swimming apparatus (exopods of the antennae and mandibular palps) (figs. 24.1C; 24.2A, C–E) and by lacking functional thoracopods (figs. 24.1C, 24.2A, K, O, Q) (RushtonMellor and Boxshall 1994; Møller et al. 2007). Having functional post-mandibular appendages and even differentiated first thoracopods, however, by definition disqualifies this kind of larva from being considered a metanauplius, and thus we employ the term metanauplius-like (see chapter 2). Juvenile-Like Larvae: The juvenile-like larvae of Argulus megalops hatch with all four pairs of thoracopods developed and active. They resemble the adults in most aspects, except for a lack of suction discs and a few other details of setal and spinal armature; hence the term juvenile-like (C. Wilson 1902). Ole Sten M...