Atlas of Crustacean Larvae

27. Copepoda

27 general: Copepods outnumber every other group of multicellular animals on earth, including the hyperabundant insects and nematode worms (Hardy 1970; Huys and Boxshall 1991). These small crustaceans are found throughout the world’s natural and man-made aquatic environments, spanning the entire salinity range from fresh water to hypersaline water, including the most unusual continental habitats (Reid 2001). Copepods, with more than 12,500 described species, and outnumbering the insects in terms of individuals by up to three orders of magnitude (Schminke 2007), carry a global biological importance that is belied by their generally small size. In the pelagic realm, which encompasses a volume of 1,347 million km3 and is the largest biome on the planet, copepods are the dominant members of the holozooplankton, both numerically and in terms of biomass (Harris et al. 2000). In addition to life strategies that encompass free-living, substrateassociated , and interstitial habits, copepods also have extensive impacts in their role as associates or parasites of the majority of aquatic metazoan phyla, from sponges to chordates, including reptiles and marine mammals. This variety of life strategies has generated an incredible morphological plasticity and disparity in body form and shape that are arguably unrivalled among the Crustacea. Copepods underpin the world’s freshwater and marine ecosystems (Costanza et al. 1997). They are sensitive bioindicators of local and global climate change (Richardson 2008), key ecosystem-service providers (Frangoulis et al. 2005; Falk-Petersen et al. 2007), and parasites of economically important aquatic animals (M. Costello 2009). They sustain the majority of the world’s fisheries (Costanza et al. 1997) and, through their roles as vectors of disease (Huq et al. 1983; Dick et al. 1991), also have a number of direct and indirect effects on human health and our quality of life. Copepods are typically small. In planktonic and benthic forms, total body length is usually between 0.2 and 5.0 mm, although some species of Valdiviella (Calanoida) can reach 28 mm in length. The real giants amongst the copepods are the parasites, with the largest being members of the siphonostomatoid family Pennellidae (Pennella balaenopterae reaches about 250 mm in length and carries egg sacs that may exceed 350 mm). Most classifications (e.g., Huys and Boxshall 1991; J. W. Martin and Davis 2001; Boxshall and Halsey 2004) recognize 2 infraclasses: the Progymnoplea (containing the single order Platycopioida) and the far more speciose Neocopepoda, which is divided into the superorders Gymnoplea (order Calanoida) and Podoplea (orders Misophrioida, Cyclopoida, Mormonilloida, Harpacticoida, Siphonostomatoida, and Gelyelloida). The ecological adaptability displayed by copepods is reflected in their tremendous morphological plasticity, which makes it difficult to formulate a rigorous diagnosis of the subclass Copepoda that is both informative yet sufficiently comprehensive to cover the bizarre parasites as well as the free-living forms. Virtually all copepods have a stage in their life cycle—either the adult or one of the copepodid instars— exhibiting a cephalosome into which the maxilliped-bearing first thoracic somite is incorporated and possessing at least two pairs of swimming legs, the members of which are linked by an intercoxal sclerite. The life cycle typically consists of nauplii (0–6) and copepodids (1–6), the last copepodid stage being equivalent to the adult (fig. 27.1A). larval types: Post-embryonic development of copepods is divided into a naupliar phase and a copepodid phase. Primitively , each phase consists of six stages (fig. 27.1A). Nauplius: Larval development and life cycles are highly variable and can be significantly abbreviated (fig. 27.6A), although most copepods hatch at the (ortho)nauplius stage, a simple larval form described earlier (see chapter 2). Typically, after six naupliar instars (often designated NI, NII, NIII, etc.), the final nauplius stage molts into a copepodid stage. Metanauplius: Occasionally the first stage in the life cycle is a metanauplius, and some parasitic species are known to hatch as a copepodid. Copepodid: Copepodid stages are often referred to as CoI, CoII, and so forth (or CI, CII, etc.). The first copepodid reRony Huys Rony Huys Copepoda Copepoda 145 sembles the adult but has a simple, unsegmented abdomen and only three pairs of thoracic limbs (maxillipeds and legs 1–2). There are significant changes in body size and shape, as well as in the appendages, in the molt from NVI to CoI, collectively known as metamorphosis (e.g., Gurney 1942; Dahms 1992). Intermolt stages are important for tracing the origin and homology of larval structures between naupliar and copepodid stages; examples are provided by Hulsemann (1991) for...