Among the million or so insect species known, only a few thousand are found in marine habitats. The genus Halobates is almost exclusively marine and is unique in having the only known species to live in the open ocean. Of the 46 Halobates species described, only five are completely oceanic in habitat, with the majority of species living in coastal areas associated with mangroves or other marine plants. This review presents a brief historical account of the genus and provides information on various aspects of its life history, ecology, special adaptations, distribution, and biogeography. Distribution maps of the five oceanic species as well as several of the more widely distributed coastal species have been updated. The phylogeny and evolution of Halobates based on morphology and recent molecular data are also discussed. A key to all known species of Halobates and related genera and a checklist of all species and their distributions are included as appendices.

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The marine insect Halobates (Heteroptera: Gerridae): biology, adaptations, distribution, and phylogeny

Conflicts over mating decisions characterize the sexual behaviour of many insects,in particular when males encounter females that already carry enough sperm to fertilize their eggs, since a mating often will inflict greater costs than benefits upon females. Therefore, coevolutionary models predict adaptation and counter-adaptation by the sexes in a battle to control the outcome of sexual encounters. A phylogenetic analysis was performed on patterns of sexual dimorphism and mating systems within water striders (Hemiptera, Gerridae). Phylogenetic effects or «constraints» have significantly shaped patterns of sexual dimorphism in female/male size ratios, legs and genitalia of males, and the structure of the female abdomen. Males of ancestral gerrids were probably slightly smaller than conspecific females, had powerful fore legs adapted to grasp the female's thorax during mating, and had clasping genitalic structures suited to grasp or pinch the female posteriorly. Most gerrids have a female/male size ratio between 1.05 and 1.14, but more pronounced sexual size ratios (above 1.25) have independently evolved several times in the family, usually in association with extended post-copulatory mate guarding. The comparative, phylogenetic analysis suggests coevolution of female anticlasper and male clasping devices for the clade comprising the subfamilies Cylindrostethinae, Ptilomerinae, and Halobatinae while female anticlasper devices have evolved in the absence of male clasping genitalia in the Gerrinae. The ancestral and most common mating system in gerrids is «scramble competition polygyny» from which has evolved «resource defence polygyny» at least four times independently of each other. The phylogenetic effects on patterns of mating behaviour are much less obvious, as exemplified by the large amount of interspecific variation in some genera.

A phylogenetic analysis of the evolution of sexual dimorphism and mating systems in water striders (Hemiptera: Gerridae)

Relatively few insects have invaded the marine environment, and only five species of sea skaters, Halobates Eschscholtz (Hemiptera: Gerridae), have successfully colonized the surface of the open ocean. All five species occur in the Pacific Ocean, H. germanus White also occurs in the Indian Ocean, whereas H. micans Esch- scholtz is the only species found in the Atlantic Ocean. We sequenced a 780 bp long region of the mitochondrial cytochrome oxidase subunit I gene (COI) for a total of 66 specimens of the five oceanic Halobates species. Our purpose was to investigate the genetic variation within species and estimate the amount of gene flow between populations. We defined 27 haplotypes for H. micans and found that haplotype lineages from each of the major oceans occupied by this species are significantly different, having sequences containing five to seven unique base substitutions. We conclude that gene flow between populations of H. micans inhabiting the Atlantic, Pacific, and Indian Ocean is limited and hypothesize that these populations have been separated for 1 to 3 million years. Similarly, there may be limited gene flow between H. germanus populations found in the Pacific and Indian Ocean and between H. sericeus populations inhabiting the northern and southern parts of the Pacific Ocean. Finally, we discuss our findings in relation to recent hypotheses about the influence of oceanic diffusion on the distribution and population structure of oceanic Halobates spp.

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L. Herring

Papers

The genus Halobates (Hemiptera : Gerridae)