Marine Biology Research Division

The oceans are harsh environments where insects are not expected to thrive, yet a few skaters of the genus Halobates Eschscholtz, 1822 (Hemiptera: Heteroptera: Gerridae) have completely adapted to life on the open seas. There are five oceanic Halobates species that have well-established and distinct distributions, but little is known about their population dynamics and intraspecific genetic variation. Moreover, existing knowledge on most species has been largely based on limited sample sizes. We examined the phylogeographic patterns and inferred past population dynamics of three Halobates species (H. micans Eschscholtz, 1822, H. sobrinus White, 1883 and H. splendens Witlaczil, 1886) based on an unprecedented large number of specimens (73–199 individuals each) collected from the Eastern Tropical Pacific (ETP) Ocean. These species have distinct biogeographies, with H. sobrinus occurring mostly along coastal Mexico, H. micans in the ETP north of the equator, and H. splendens largely south of the equator in the cold tongue of water derived from the Peru (Humboldt) current. We did not find evidence for sub-population structure within each species over distances as far as 6000–7000 km. Populations of all three species were found to deviate from neutrality, with evidence of past population growth. Genetic diversity and haplotype genealogies varied between species, implying distinct evolutionary trajectories.

Coalescent analyses using Bayesian skyline plots suggested that H. splendens underwent a population expansion ~ 1 Ma, whereas H. sobrinus and H. micans experienced demographic growth ~ 120 Ka to 100 Ka, respectively. The period of population expansion of H. splendens roughly corresponds to the establishment of cool, productive waters in the cold tongue starting ~ 1 Ma and reaching modern temperatures ~ 800 Ka. Population expansions of both H. micans and H. sobrinus north of the equator occurred mostly during the last interglacial period, characterized by increased frequency and dominance of El Niño conditions, and a relatively southerly position of the Intertropical Convergence Zone of high rainfall. Intensification of El Niño conditions between ~ 75 and 125 Ka may have produced a warmer, low wind and nutrient-depleted habitat favoring population growth for both H. micans and H. sobrinus. Key insights drawn from the results of this study, alongside future resolution of evolutionary relationships among Halobates species, will complete our understanding of how these remarkable insects conquered the high seas where no other insect could.

Despite the remarkable evolutionary success of insects at colonizing every conceivable terrestrial and aquatic habitat, only five Halobates (Heteroptera: Gerridae) species (~0.0001% of all known insect species) have succeeded at colonizing the open ocean – the largest biome on Earth. This remarkable evolutionary achievement likely required unique adaptations for them to survive and thrive in the challenging oceanic environment. For the first time, we explore the morphology and behavior of an open-ocean Halobates germanus and a related coastal species H. hayanus to understand mechanisms of these adaptations. We provide direct experimental evidence based on high-speed videos which reveal that Halobates exploit their specialized and self-groomed body hair to achieve extreme water repellence, which facilitates rapid skating and plastron respiration under water. Moreover, the grooming behavior and presence of cuticular wax aids in the maintenance of superhydrophobicity. Further, reductions of their body mass and size enable them to achieve impressive accelerations (~400 ms⁻²) and reaction times (~12 ms) to escape approaching predators or environmental threats and are crucial to their survival under harsh marine conditions. These findings might also inspire rational strategies for developing liquid-repellent surfaces for drag reduction, water desalination, and preventing bio-fouling.

We document the discovery of a unique flightless marine midge Pontomyia (Chironomidae) in the Indian Ocean, presenting descriptions and photographs of the insect and its environment at Christmas Island, Australia. The insect is identified as P. natans, with distinctive male genitalia and rudimentary claws on the hind legs. This species is widely distributed in the Pacific Ocean but the only previous record from the Indian Ocean was from the Maldives. It is highly likely that other populations are present along the continental coast of the latter ocean basin.

Five species of the marine insect Halobates share similar ecology but have distinct biogeographic ranges in the eastern tropical Pacific, a region from approximately 75°W–160°W and 10°S–35°N. Between 2001 and 2010, the Sea Education Association collected Halobates from 682 neuston tows (surface net 1 m × 0.5 m, 335-μm mesh) during fifteen cruises between San Diego, USA, Mexico and Tahiti. Total Halobates spp. densities varied substantially from year to year, but our data do not show a sustained change from a data set collected 40 years earlier from 1967 to 1968 (Cheng and Shulenberger in Fish Bull 78(3):579–591, 1980). Halobates are sensitive to sea surface temperature and we observed significant differences in species distributions over time, but these were not due to differences in water temperature or climate change. Our analyses show that the patterns observed are attributable to substantial but previously undescribed seasonal shifts that occur each year in the ranges for both Halobates sobrinus and Halobates micans. There is substantial overlap in ranges during seasonal shifts, but very little co-occurrence of H. sobrinus and H. micans in individual net tows, suggesting biological mechanisms rather than physical factors are restricting distribution and co-occurrence of these two species.

Pontomyia (Diptera: Chironomidae) is an exclusively marine and flightless insect genus with four described species from the Indo-Pacific and one undescribed taxon known only by its larvae, pupal skins and females from the western Atlantic. A previous study of relationships among three of the Indo-Pacific species reported each of them to be monophyletic, with high genetic diversity within P. natans Edwards, the type species, and P. pacifica Tokunaga.  The evolutionary affinities of the Australian endemic P. cottoni Womersley that resembles P. natans, as well as the putative Atlantic species are hitherto undetermined. A complete molecular phylogeny of the genus based on two nuclear and two mitochondrial DNA markers indicates that P. cottoni and a Puerto Rican (Atlantic) larval population are nested within the P. natans clade. Furthermore, P. natans and P. cottoni are inseparable in all morphological characters used previously to distinguish them. Therefore, we synonymize P. cottoni with P. natans, syn. nov., whose known range now encompasses all three ocean basins after including the Puerto Rican population. This distribution warrants further investigation into the life history of Pontomyia, a midge with one of the shortest known adult lifespans among insects.

The question of how many marine species exist is important because it provides a metric for how much we do and do not know about life in the oceans. We have compiled the first register of the marine species of the world and used this baseline to estimate how many more species, partitioned among all major eukaryotic groups, may be discovered.

Plastic pollution in the form of small particles (diameter less than 5 mm)—termed ‘microplastic’— has been observed in many parts of the world ocean. They are known to interact with biota on the individual level, e.g. through ingestion, but their population-level impacts are largely unknown. One potential mechanism for microplastic-induced alteration of pelagic ecosystems is through the introduction of hard-substrate habitat to ecosystems where it is naturally rare. Here, we show that microplastic concentrations in the North Pacific Subtropical Gyre (NPSG) have increased by two orders of magnitude in the past four decades, and that this increase has released the pelagic insect Halobates sericeus from substrate limitation for oviposition. High concentrations of microplastic in the NPSG resulted in a positive correlation between H. sericeus and microplastic, and an overall increase in H. sericeus egg densities. Predation on H. sericeus eggs and recent hatchlings may facilitate the transfer of energy between pelagic and substrate-associated assemblages. The dynamics of hard-substrate-associated organisms may be important to understanding the ecological impacts of oceanic microplastic pollution.

Among millions of insect species known in the world only five species of Halobates (Heteroptera: Gerridae) are able to live in the high seas. The general public is probably not aware that there are insects living on the open ocean and even most marine scientists have never seen a live or preserved specimen with their own eyes. The genus Halobates was first collected during a Russian oceanographic expedition around the world between 1815 and 1818.

We analyzed the foraging ecology of seabirds in the eastern tropical Pacific Ocean during 1983–1991 on a series of oceanographic cruises during spring and fall of each year. We report details about the consumption of sea skaters Halobates spp., marine insects that are small, can hide well within sea foam, and can be very fast moving. One abundant sea skater of the four species present in the study area, H. sobrinus, is not taken by sea birds, and the reason is unknown. Among the predators, it appears that frigate storm-petrels, White-faced Storm-Petrel Pelagodroma marina and White-bellied Storm-Petrel Fregetta grallaria (likely also White-throated Storm-Petrel Nesofregetta fuliginosa), make directed efforts to consume sea skaters, a fact that may explain their unique flight and foraging behavior: slow, with extensive “kicksplashing” against the sea surface, to incite movement in Halobates. The few other seabirds for which sea skaters constitute more than an incidental component of the diet (Herald Petrel Pterodroma heraldica, Bulwer’s Petrel Bulweria bulwerii) also move slowly across and close to the sea surface. In the case of the White Tern Gygis alba, it readily hovers, an ability that would be advantageous to taking these insects. Among the avian species that rarely take a sea skater, almost all are included in the guild of seabirds that associate with tuna, and as a result they must fly quickly to keep pace.