Global diversity of fishlice (Crustacea: Branchiura: Argulidae) in freshwater
The Branchiura of freshwater habitats, consisting of the valid genera Argulus, Chonopeltis, Dipteropeltis, and Dolops, presently contains 113 valid species and 12 undescribed species. The entire group is composed of ectoparasitic species that usually live on fishes. The highest diversity of genera and species occurs in the Afrotropical and Neotropical regions. The diversity of the freshwater species surpasses that of species in marine and brackish waters, but this could be due to inadequate study of the fauna of the latter habitats. One species, Argulus japonicus, has been introduced from east/southeast Asia to all other continents, except Antarctica. Studies of higher level relationships place the Branchiura with either Pentastomida or Ostracoda. Hypotheses about phylogenetic relationships of either the genera or species in this group have not been proposed.
The subclass Branchiura contains a single family, the Argulidae, and four valid genera: Argulus Müller, 1785, Chonopeltis Thiele, 1900, Dipteropeltis Calman, 1912, and Dolops Audouin, 1837. Yamaguti (1963) erected subfamilies within the Argulidae as well as the family Dipteropeltidae; however, neither subfamily designations nor the family group name Dipteropeltidae are used herein. Branchiurans are ectoparasites of fishes primarily but occasionally live on amphibians or invertebrates, and they can move about freely on their hosts (Stuhlmann, 1891; Yamaguti, 1963; Cressey, 1978; Jackson & Marcogliese, 1995; Poly, 2003). Species of Branchiura are known by the common name, fishlouse (plural: fishlice).
Mating takes place on the host fish, and later, the female leaves the host to lay eggs, which are attached with an adhesive substance to objects, such as rocks, plants, or sticks. Sperm are stored in the spermathecae of females in Argulus and presumably in Chonopeltis and Dipteropeltis, whereas males of the genus Dolops deposit a spermatophore on the females (Carvalho, 1941; Fryer, 1958, 1960). Further information on the ecology and morphology of argulids can be found in Jurine (1806), Clark (1902), Wilson (1902), Calman (1912), Tokioka (1936), Loro (1964), van Niekerk & Kok (1989), Rushton-Mellor & Boxshall (1994), and Van As & Van As (1999).
Number of species per genus of Branchiura occurring in freshwater habitats in each of the biogeographic regions of the world
Phylogeny and historical processes
Phylogenetic studies concerning the Branchiura pertained to higher level relationships only. Results of several studies supported a sister group relationship between the Branchiura and the Pentastomida or the Ostracoda (discussed in Martin & Davis, 2001; Wingstrand, 1972; Lavrov et al., 2004; Regier et al., 2005 and references therein). Phylogenetic studies of the generic and specific relationships have not been attempted yet. Further detailed taxonomic study of the group will provide much of the data needed for more meaningful and complete phylogenetic analyses.
Present distribution and main areas of endemicity
Biogeographic analyses of the Branchiura were made by Fryer (1969) with regard to Dolops, which displays a Gondwanan distribution (South America, Africa, and Tasmania), and the biogeography of African species of Argulus, Chonopeltis, and Dolops was discussed by Fryer (1968). Chonopeltis and Dipteropeltis are endemic to Africa and South America, respectively. Little else has ever been published on biogeography of this group. Argulus japonicus Thiele, 1900 has been introduced from east/southeast Asia to all other continents, except Antarctica (Table 1). Native species do not occur across great distances in most cases, except for some species in the Afrotropical region, such as Dolops ranarum (Stuhlmann, 1891) and Argulus africanus Thiele, 1900 (Fryer, 1968), and for Argulus foliaceus (Linné, 1758) in the Palearctic region. Definitive discussions about species’ distributions cannot be made until additional species are described and further collecting and taxonomic study reveal true diversity more accurately.
Human related issues
Branchiurans occasionally attain high densities on their hosts, resulting in fish kills in natural bodies of water (rarely; usually lentic waters) or fish hatchery/aquaculture operations (more commonly). Fishlice also appear at times in large public aquaria, home aquaria, or small outdoor ponds and can contribute to mortality of ornamental fishes.
The author expresses much appreciation to the organizers of the meeting on Freshwater Animal Diversity Assessment held in Mechelen, Belgium for their careful planning, logistical support, and financial assistance, resulting in an immensely productive and enjoyable session, and for all their efforts to assemble this volume. The author also thanks Geoff Boxshall and an anonymous reviewer for their comments on the manuscript.