Extent of gill pathology in the toadfish Tetractenos hamiltoni caused by Naobranchia variabilis (Copepoda: Naobranchiidae).

Sanguinivorous Naobranchia variabilis prefer the first gill arch, external hemibranch and anterior end of the gill arch. The smallest N. variabilis observed attached to fish by a thin filament which connects fused tips of second maxillae to a 'plug' inserted into the gill tissue. Second maxillae enlarge to encircle and increasingly compress the gill filament, which results in a thin layer of epithelium and connective tissue overlying the cartilaginous supporting bar. Early juveniles cause little tissue proliferation, but the extent of proliferated epithelial and connective tissue (PR) adjacent to the maxillae increases from late juveniles to subadult and adult copepods. Most variation in length of gill filament damage (PL, proliferated and compressed tissue) among age classes is explained by maxilla length (ML, length of compressed gill filament); adult trunk width (TRW) explains an extra, small amount of variation, but not trunk length (TRL) or total fish length (TL). Most variation in ML is explained by TRW of adults, subadults and late juveniles, and TRL of early juveniles. PR is explained by TRW for adults, but by ML for other ages. These patterns are due to elongation of the juvenile trunk during growth and lateral expansion of adult egg pouches during maturation. Up to 38 N. variabilis, average (avg.) = 9.3, infected individual Tetractenos hamiltoni and damaged up to 3.4% (avg. 0.72%) of total filament length and 8.6% (avg. 2.1%) of gill filaments per fish.