Lepeophtheirus salmonis (Krøyer, 1837): second nauplius and copepodid locomotor appendages, surface areas and possible appendage functions

Abstract

Locomotor appendage-body relationships were used to examine whether swimming or reduction in sinking rate is the more important function in the second nauplius and copepodid stages of Lepeophtheirus salmonis (Krøyer, 1837). Except for the similarity in swimming appendage surface areas without setae, the appendages of the two stages are morphologically distinct. Although the nauplius is smaller than the copepodid it has long slender appendages that, with setae, provide greater total surface area than the paddle-shaped copepodid thoracic legs. Copepodid thoracic legs are more similar to those used for swimming by planktonic copepods although with more limited propulsion capability. Naupliar appendages project from the body while copepodid appendages can be folded against the ventral surface, improving hydrodynamic flow as well as body position after attachment to a host. Both copepodid and naupliar appendages are of sufficient size that they should provide escape velocities of more than 100 mm ⋅ s−1. The nature and display of the naupliar appendages suggest they could be used to reduce sinking rate by as much as 64%, reducing the need to swim to maintain a suitable location in the water. Although copepodid thoracic legs could reduce sinking rate by over 40%, their position on the ventral surface and the nature of other appendages suggests a more important use, for orientation and attachment once a host is located.