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The new cymothoid species, Elthusa winstoni sp. n., a branchial parasite of fishes from the family Acanthuridae Bonaparte, 1835 in Hawaii, is described and figured. The female adults can be distinguished by the strongly vaulted body and compacted body shape; rostrum with a small median point; short antennae which are close together (only 6 articles in both antennula and antenna); short and wide uropods extending to half the length of the pleotelson; short dactyli on pereopod 7; and large recurved robust setae on the maxilla. This is the first record of an Elthusa Schioedte & Meinert, 1884 species from the Hawaiian Islands and only the fifth cymothoid described from this region.

The Blueline Surgeonfish, Acanthurus nigroris Valenciennes, formerly considered as wide-ranging in the central and western Pacific, is restricted to the Hawaiian Islands. Acanthurus nigros Gunther, type locality Vanuatu, is available for the sister species from the Pitcairn Islands west to the Great Barrier Reef and Caroline Islands. Although these two species are very similar in color, there are fin-ray and gill-raker differences, and the genetic difference (i.e., 4.12 % mtDNA cytochrome b sequence divergence) alone warrants species recognition.

Understanding genetic connectivity is fundamental to the design of marine protected areas in the service of ecosystem-scale management. Here we evaluate such trends for a Pacific surgeonfish (Acanthurus nigroris;N=544) at two spatial scales: (1) within the Hawaiian archipelago, and (2) across the entire species range from the central to southwest Pacific. The mtDNA cytochromebdata reveal genetic divergence (d=0.041) between Hawaii and the rest of the Pacific range indicating a cryptic species pair, with one taxon endemic to Hawaii. Johnston Atoll, 1400 km SW of Hawaii, also has the Hawaiian species but is distinct from most Hawaiian locations in population genetic comparisons, indicating the limits of gene flow for this widespread reef species. No consistent population genetic differences were observed among Hawaiian sites or among the other Pacific island sites. We also detected a modest bias in gene flow from the southeast towards the northwest islands of the Hawaiian Archipelago, indicating that the Papahānaumokuākea Marine National Monument may be a recipient, rather than a source of propagules to replenish reef resources.

A study of the osteology of the Acanthuridae (surgeonfishes) and a cladistic analysis of the 42 osteological and external characters resulted in a well-corroborated hypothesis (consistency index = 0.96) of the phylogenetic relationship of the six recognized acanthurid genera. Removal of autapomorphies for terminal taxa resulted in a data matrix of 20 characters, consistency index of 0.91, and the same phylogeny. Character polarity was determined using the outgroup method. Zanclus, Luvarus, and the Siganidae were selected as the first, second, and third outgroups, respectively. Evidence is presented to support the following hypotheses: (1) Acanthurids form a monophyletic group; (2) Naso is monophyletic; (3) Prionurus + Zebrasoma + Paracanthurus + Acanthurus + Ctenochaetus form a monophyletic group with Naso as its sister group; (4) Prionurus may or may not be monophyletic; (5) Zebrasoma + Paracanthurus + Acanthurus + Ctenochaetus form a monophyletic group; (6) Zebrasoma is monophyletic; (7) Paracanthurus is monophyletic; (8) Zebrasoma and Paracanthurus are sister groups; (9) Acanthurus and Ctenochaetus form a monophyletic group; (10) Zebrasoma + Paracanthurus form the sister group of Acanthurus + Ctenochaetus; (11) Acanthurus may or may not be monophyletic; (12) Ctenochaetus is monophyletic. The monophyly of Acanthurus is questioned. Evidence is presented that suggests that Acanthurus nigroris may be more closely related to Ctenochaetus than to the other eight species of Acanthurus examined.