Halobates - Oceanic Insects
Biology and Ecology
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
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.
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.
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.
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).
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,
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.
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).
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).
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).
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.
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of Halobates and Asclepios (Hemiptera, Gerridae).
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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.
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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.
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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.
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Last update: 04 september 2002