Swimming and cleaning in the free-swimming phase of Argulus larvae (crustacea, branchiura)--appendage adaptation and functional morphology.

The free-swimming early larval stages of Argulus foliaceus (Linneaus) (Branchiura) are studied using digital video, light microscopy, and SEM. We analyze and document the mode of swimming in the hatching stage of A. foliaceus and the subsequent juvenile stages with fully developed thoracopods. We present new observations and an analysis of the functional morphology of a cleaning behavior in the first stage. This stage swims very efficiently using the large exopods of the second antennae in concert with the mandibular palp (naupliar limbs), while the subsequent stages use the now developed thoracopods for propulsion. This posterior shift in propulsion is similar to--but independent from--what is seen in other crustaceans. The hatching stage has previously been referred as a "metanauplius" but as the first and second maxillae are developed and active, and buds of all four thoracopods are present, it is too advanced to be included in the naupliar phase. The hooks of the first antennae and the distal hooks of the maxillae are demonstrated to function not only as attachment organs (to the host), but also to play a significant role in the cleaning of the naupliar swimming appendages. A digital video-based analysis of the swimming mode is provided. The larval swimming pattern is generally similar to that of other crustaceans such as Branchiopoda and Cirripedia, but autapomorphies of the Branchiura include the following: 1) While actively swimming, the naupliar appendages are almost straight during the recovery stroke and 2) they have a relatively small deflection during movement ( approximately 25 degrees or approximately 35 degrees for mandible and second antenna respectively), 3) the larval mandible has a uniramous palp which is the retained exopod. The morphological implications of the transition from the possibly nonfeeding pelagic, or free-swimming, first larval stage to the feeding, parasitic second stage are discussed and compared with other crustaceans.