Halobates

Halobates or sea skaters are a genus with over 40 species of water striders. Most Halobates species are coastal and typically found in sheltered marine habitats (a habitat where a few other genera of water striders also live), but five live on the surface of the open ocean and only occur near the coast when storms blow them ashore. These are the only known truly oceanic, offshore insects. They are found in tropical and subtropical marine habitats around the world, with a single species recorded in rivers a few kilometers upstream from the ocean. Halobates are generally very common. They were first collected by Johann Friedrich von Eschscholtz, a doctor who was part of a Russian expedition aboard the Rurik between 1815 and 1818. A fossil species H. ruffoi is known from 45-million-year-old deposits in Verona, Italy. Close relatives of the genus include Austrobates and Asclepios. They are small insects with a body that is up to 6.5 mm (0.26 in) long and 3 mm (0.12 in) broad, and a leg span up to at least 15 mm (0.59 in). They lack wings, have long antennae, short front legs used for catching prey (and, in the male, for holding the female during breeding), long middle legs used for propulsion, and somewhat shorter rear legs used for steering. The nymphs resemble miniature versions of the adult. The sexes are quite similar, except that males are thinner than females and have the rear part of the body modified into genitalia, and when gravid the females may have a notably plump abdomen. The various species closely resemble each other in general appearance. Halobates are found in tropical and subtropical marine habitats around the world. They generally prefer temperatures of 24–28 °C (75–82 °F), are infrequent below 20 °C (68 °F) and only exceptionally recorded in waters less than 15 °C (59 °F). The coastal species are largely restricted to the Indo-Pacific region, with the exception of H. robustus from the Galápagos Islands. Some of these coastal species have very small ranges, often restricted to a single archipelago, while others are more widespread. They primarily occur near mangrove or other marine plants. A single species, H. acherontis, has been recorded in rivers a few kilometers upstream from the ocean. The absence of coastal species in the Atlantic region may in part be explained by Trochopus. That genus of veliid water striders inhabit coastal mangrove areas in the Atlantic region; the same niche inhabited by coastal Halobates in the Indian and Pacific oceans. The five offshore, pelagic species are H. micans, H. germanus, H. sericeus, H. splendens and H. sobrinus, of which the last four are found in the Indian and/or Pacific Oceans. H. micans has a circumglobal range, occurring offshore in warmer seas around the world and it is the only one found in the Atlantic Ocean, including the Caribbean, where it ranges from about 40° north to 40° south. Their occurrences are generally patchy, but where found they can be very common. During scientific surveys with relatively fast-moving surface nets, they are caught in more than 60% of the tows (less in slow-moving tows, likely because of their ability to avoid them). Studies show that densities locally can be as high as 1 individual per 19 m2 (200 sq ft) in the oceanic species, and 120 individuals per m² (11 per sq ft) in breeding aggregations of the coastal species.