Halobates - Oceanic Insects

Biology and Ecology

Nils Møller Andersen
Zoological Museum, University of Copenhagen

Ó Copyright, N. M. Andersen & Zoological Museum, University of Copenhagen  

Life history

Habitats

Food and feeding

Predators

Literature cited

Life history

The life history of Halobates includes the egg, five juvenile instars (called nymphs), and the adult stage. The eggs are deposited at or slightly above the water-level but some gerrids submerge completely to place their eggs below water. The eggs of Halobates are deposited in an exposed horizontal position glued to the substratum by a gelatinous substance. The egg shell is finely and densely porous with an inner spongy layer. There is one micropyle located at the anterior end of the egg. During the early part of its development the egg content is yellowish or orange. Later, when the embryo becomes visible the egg becomes bright orange in color and the eyes appear as a pair of reddish spots. The appendages are light brown. The long middle and hind legs are neatly folded around the end of the abdomen. During eclosion the shell is split open lengthwise by an embryonic egg burster. This structure remains attached to the embryonic cuticle which is moulted immediately after eclosion.

The eggs of Halobates are large (0.8-1.3 mm) compared to the size of the female (rarely exceeding 5 mm). The number of mature or semi-mature eggs found in the body cavity of a gravid female may range from 10 to 40. To accomodate all these eggs, the abdomen are often expanded to nearly twice its normal length and the thoracic cavity is also packed with eggs. The number of eggs laid by a single female is difficult to estimate since the egg-masses attached to floating objects most certainly are the result of the combined efforts of several females. For instance, the cork stopper depicted above is completely covered by almost 10 layers of eggs on top of each other. The total number of eggs was estimated to be about 100,000! It is most likely that a few thousand H. micans females have been engaged in forming this astonishing concentration of eggs.

Lundbeck (1914) first pointed out that the eggs of Halobates could be divided into several groups on the basis of size and the structure of the shell surface. He found eggs dissected from the ovaries of female H. micans, sericeus, and sobrinus, to be smooth. In a few species examined, e.g., H. germanus, the shell is greatly sculptured.

Coastal species of Halobates probably lay their eggs in blocks of coral on seaweeds near the shore, close to the low-tide mark (Foster & Treherne, 1986). The oceanic species attach their eggs to all kinds of floating material, e.g. seaweed, cork, shells of the cuttlefish Spirula and Sepia, floating timber, tar lumps, bits of coal, birdfeathers or even to the tailfeathers of living seabirds (Lundbeck, 1914; Cheng, 1974; Andersen & J. Polhemus, 1976). Many of these egg-bearing objects have been collected from the ocean surface far from land. Others can be found when they are washed ashore in great numbers as experienced by the author in Gambia, West Africa.

Sea skaters have not so far been successfully reared from egg to adult in the laboratory. Herring (1961) estimated the period required for the total development from egg to adult to be about 56 days for Halobates hawaiiensis Usinger whereas Cheng (1985) estimated that the development time may be as long as 60 to 70 days, strongly depending on temperature. The newly hatched Halobates nymph is very feeble, but after it has stretched out its legs and become tanned (about 30 minutes after hatching) it becomes very active. When the eggs are submerged and hatched under water the nymph may spend 1-2 hours trying to break through the surface film. The nymphs are rather similar to the adult in structure except for size, proportion of body parts, lack of segmental differentiation of the tarsi, etc. The post-embryonic development includes five moults during which the old cuticle splits open along a dorsal Y-shaped line on the head. The newly moulted adult is pale and soft. The so-called teneral development which includes the complete hardening and darkening of the cuticle may last for a few days.  

Habitats

Although the open-ocean species of Halobates have attracted most interest, the majority of the 47 described species of sea skaters prefer near-shore, marine habitats. Our knowledge about the biology and ecology of sea skaters is generally quite sparse. Fortunately, a few coastal species have been studied more intensively during the past two decades.

Halobates robustus is endemic to the Galapagos Islands. This species inhabits protected, rocky coasts with mangroves. Adults tends to aggregate in large "flotillas" very close to mangrove trees or rocks. Nymphs (all instars) are usually found further away from the shore. Mating pairs are very frequently observed and the male (which is smaller than the female) stays with the female for a prolonged period of time (mate-guarding behaviour). Egg-laying has never been observed but oviposition probably takes place on rocks and/or roots of mangrove trees (Birch et al., 1979; Foster & Treherne, 1980, 1982).

Another coastal species, H. fijiensis, inhabits bays and lagoons fringed with mangroves around Fiji Islands. Younger nymphs are always found in sheltered waters amongst mangroves. Older nymphs and adults are found in more open water, sometimes several hundred meters from the mangroves. Mating pairs are infrequently observed, and the encounter between male and female (male slightly larger than female) is brief. Egg-laying was observed to take place on stands of sea-grass near the low-water mark, at extreme low spring tides. The newly hatched nymphs then have to make their way for several hundred meters to the protecting mangroves (Foster & Treherne, 1986).

Most near-shore species of Halobates seem to prefer habitats that are sheltered from winds and wave action. Very few species of Halobates tolerate more exposed conditions. Observations by Cheng (1981) of the distribution of H. flaviventris in Palau, West Caroline Islands by D. Polhemus (1990) along Aldabra Atoll, Indian Ocean suggest that this species aggregates along the outer margins of fringing coral reefs.

Five species of sea skaters have successfully colonized the open ocean: H. germanus (Indian and West Pacific Ocean), H. sericeus (Pacific Ocean), H. sobrinus (tropical eastern Pacific), H. micans (Atlantic, Indian, and Pacific Ocean), and H. splendens (tropical southeastern Pacific). Both adults and nymphs of these species live permanently upon the sea surface, always at some distance from nearest land. Eggs are deposited on various floating objects.  

Although observations of preferred habitats are limited, the species of sea skaters found along the coasts of Australia are probably distributed among different marine habitats as shown on an idealized transect (see above). Halobates (Hilliella) robinsoni was collected about 50 km above the mouth of Robinson River, Western Australia and H. (s.str.) acherontis more than 100 km above the mouth of Daly River, Northern Territory. Most Australian species inhabit mangroves in river estuaries, tidal creeks, or protected bays. Species like H. mjobergi, lannae, zephyrus, whiteleggei, darwini, and herringi probably close to H. robustus in their way of living (see above). Species inhabiting tidal pools and lagoons along coral coasts like H. regalis, hayanus, and princeps may be ecologically close to H. fijiensis (see above). A transitional stage between the near-shore and oceanic way of life in sea skaters is represented by H. germanus which usually is found closer to land than the truly open-ocean species, H. micans and sericeus (Andersen & Weir, 1994).

Food and feeding

Sea skaters are predacious fluid-feeders. Their mouthparts are of the piercing and sucking type found in other hemipterous insects. The short rostrum consists of a 4-segment, sheath-like labium enclosing two pairs of long stylets, the outer mandibular and the inner maxillary pair. The tip of the labium is equipped with sensory hairs which aid in the localization of suitable spots for penetration of the prey. The mandibular stylets with their serrated apices are used for piercing the integument of the prey and also serve to anchor the mouthparts. The tissue of the prey is then liquified by salivary enzymes and sucked up by the food tube formed by the highly extensible maxillary stylets held together by hairs (Cheng, 1974; Andersen, 1982).

Coastal Halobates probably feed largely on land insects that have been blown or washed out to sea or have fallen to the water surface from overhanging vegetation. The oceanic species of Halobates have been found to feed on pelagic siphonophorans like the Portuguese man-o'-war (Physalia), the by-the-wind-sailor (Velella), and the jellyfish Porpita (Andersen & J. Polhemus, 1976; Cheng, 1985).  They have also been observed feeding on various planktonic crustaceans and fish larvae trapped in the surface film (Cheng, 1974). Possibly they could also feed on floating fish eggs (Lee & Cheng, 1974).

Halobates belong to the marine pleuston community. Although animals in this two- dimensional environment are confined to the sea surface, they interact with organisms that generally live above or below them, being part of a complicated food-web  (see above). Certain organic compounds and heavy metals are known to concentrate at the sea-surafce film and Halobates species have been found to accumulate some of the heavy metals, in particular cadmium (Cd) (Schulz-Baldes, 1989; Schulz-Baldes & Cheng, 1979, 1980, 1981).

Predators

Surface-feeding seabirds are the most important predators of pelagic Halobates spp.  Sea skaters may be important in the diet of the blue-grey noddy (Procelsterna cerulea), the Broin petrel (Pterodroma hypoleuca), the grey-backed tern (Sterna lunata), and Bulwer's petrel (Pterodroma alba). Cheng & Harrison (1983) found Halobates sericeus remains in 24.4% of 627 regurgitates from the four major bird predators listed above. For the most important of these, Procelsterna cerulea, some 81% of the sampled birds had fed on Halobates.

Literature cited

Andersen NM. 1982. The Semiaquatic Bugs (Hemiptera, Gerromorpha). Phylogeny, adaptations, biogeography, and classification. Entomonograph 3: 1-455.

Andersen NM, Polhemus JT. 1976. 8. Water-striders (Hemiptera: Gerridae, Veliidae, etc.). In Cheng L. (ed.), Marine Insects. North Hollan Publ. Co., Amsterdam: 187-224.

Andersen NM, Foster WA. 1992. Sea skaters of India, Sri Lanka, and the Maldives, with a new species and a revised key to Indian Ocean species of Halobates and Asclepios (Hemiptera, Gerridae). J. Nat. Hist. 26: 533-553.

Andersen NM, Weir TA. 1994. The sea skaters, genus Halobates Eschscholtz (Hemiptera: Gerridae), of Australia: taxonomy, phylogeny and zoogeography. Invertebrate Taxonomy 8: 861-909.

Birch MC, Cheng L, Treherne JE. 1979. Distribution and environmental synchronization of the marine insect, Halobates robustus, in the Galapagos Islands. Proc. R. Soc. Lond. (B) 206: 33-52.

Cheng L. 1973. Halobates. Ann. Rev. Oceanogr. Mar. Biol. 11: 223-235.

Cheng L. 1974. Notes on the ecology of the oceanic insect Halobates. Mar. Fish. Rev. 36: 1-7.

Cheng L. 1981. Halobates (Heteroptera: Gerridae) from Micronesia with notes on a laboratory population of H. mariannarum. Micronesica 17: 97-106.

Cheng L. 1985. Biology of Halobates (Heteroptera: Gerridae). Ann. Rev. Ent. 30: 111-135.

Cheng L, Harrison CS. 1983. Seabird predation on the sea-skater Halobates sericeus (Heteroptera: Gerridae). Marine Biology (Berlin) 72: 303-310.

Foster WA, Treherne JE. 1980. Feeding, predation and aggregation behaviour in a marine insect, Halobates robustus Barber (Hemiptera: Gerridae), in the Galapagos Islands. Proc. R. Ent. Soc. Lond. (B) 209: 539-553.

Foster WA, Treherne JE. 1982. Reproductive behaviour of the ocean skater Halobates robustus (Hemiptera: Gerridae) in the Galapagos Islands. Oecologia 55: 202-207.

Foster WA, Treherne JE. 1986. The ecology and behaviour of a marine insect, Halobates fijiensis (Hemiptera: Gerridae). Zool. J. Linn. Soc. 86: 391-412.

Herring JL. 1961. The genus Halobates (Hemiptera: Gerridae). Pacific Insects 3(2-3): 223-305.

Lee RF, Cheng L. 1974. A comparative study of the lipids of water-striders from marine, estuarine, and freshwater environments: Halobates, Rheumatobates, Gerris (Heteroptera: Gerridae). Limnol. Oceanogr. 19: 958-965.

Lundbeck W. 1914. Some remarks on the eggs and egg-deposition of Halobates. Mindeskrift for Japetus Steenstrup 2(27): 1-13.

Miyamoto S, Senta T. 1960. Distribution, marine condition and other biological notes of marine water-striders, Halobates spp., in the south-western sea area of Kyushu and western area of Japan Sea. Sieboldia 2: 171-186.

Polhemus DA. 1990. Heteroptera of Aldabra Atoll and nearby islands, western Indian Ocean, Part I. Marine Heteroptera (Insecta); Gerridae, Veliidae, Hermatobatidae, Saldidae and Omaniidae, with notes on ecology and insular zoogeography. Atoll Research Bulletin 345: 1-16.

Schulz-Baldes M. 1989. The sea-skater Halobates micans, an open ocean bioindicator for cadmium distribution in Atlantic surface waters. Marine Biology 102: 211-215.

Schulz-Baldes M, Cheng L. 1979. Uptake and loss of radioactive cadmium by the sea-skater Halobates robustus (Heteroptera, Gerridae). Marine Biology (Berlin) 52: 253-258.

Schulz-Baldes M, Cheng L. 1980. Cadmium in Halobates micans from the central and south Atlantic Ocean. Marine Biology (Berlin) 59: 163-168.

Schulz-Baldes M, Cheng L. 1981. Flux of radioactive cadmium through the sea-skater Halobates (Heteroptera: Gerridae). Marine Biology (Berlin) 62: 173-178.