Family MICRONECTIDAE Jaczewski, 1924 – pygmy waterboatmen.
P.-p. Chen & N. Nieser
This family consists of small – the largest species have a body length of 5 mm but most are between 2 and 3 mm – aquatic bugs related to the Corixidae. Although already Fieber (1851a) recognized this as an independant taxon as “turma Sigarae” under Corisidea and Douglas & Scott (1865) raised it to family Sigaridae the taxon was usually considered a part of the family Corixidae. Poisson in publications from 1928 to 1938a considered Micronectidae as a separate family but from 1938b on used Micronectinae as a subfamily of Corixidae. Cobben (1968) suggested to raise them to family level. Finally Nieser (2002) formally raised the taxon to family Micronectidae.
The family is divided in two subfamilies: Synaptogobiinae Nieser & Chen, 2006 with three species in two genera restricted to S. America and Micronectinae with 171 species in five genera with a nearly worldwide distribution. However, there are no Micronectidae in Antarctis and no indigenous species in the Nearctis and New Zealand. The Palaearctic Region is relatively poor in species: 23 species, all belonging to the genus Micronecta. The highest species density in the Palaearctis is found in SW China where a number of species associated with the Oriental fauna is found. The Oriental fauna is by far the richest with 66 species. Most Micronectidae live in shallow water, either stagnant ponds or the edges of lakes or quiet bays of streams. Due to their small size they have been less intensively studied than Corixidae s. str. and their biology is consequently less well known.
Micronectidae, at least some species from the temperate zones of the Palaearctic, are able to exist exclusively on dissolved oxygen (Jansson, 1986a). Such species can be used as indicators for water quality (Jansson, 1977a, b). Some species such as M. quadristrigata Breddin can live in fertilized habitats with low oxygen content. It seems that such species, at least partly, breathe atmospheric air. However, experimental support for this hypothesis is so far lacking. Other species prefer more oligotrophic habitats with higher oxygen content. It is unclear what the actual food of Micronectidae is under natural conditions. Stomach contents of some West Malayan Micronecta species showed unicellular algae, rests of filamentous algae, bacteria and some setae of freshwater annelids. M. quadristrigata could be reared on a diet of the unicellular alga Pleurococcus sp. (Fernando & Leong, 1964). However, one of the larger species (Micronecta grisea (Fieber, 144)) is, at least in the laboratory, an avid predator of mosquito larvae (Amrapala et al., 2009). In addition we found in a stream in Vietnam Micronecta sp. exclusively around water buffalo droppings, in the National Museum of Wales there is a sample of Micronecta skutalis Nieser & Chen, 1999 from a stream in Danum Valley, Sabah: “ex elephant (Elephas maximus ) dung”. This points in the direction that some species predate on bacteria and/or protozoa. Popham, Bryant & Savage (1984) have shown that Sigara Fabricius, 1775 (Corixidae s. str.), has a broad scope of diet according to the species. In one species, Sigara falleni (Fieber, 1848), the males are primarily algal feeder, whereas the females are omnivorous. Thus, different species of Micronectidae have probably different diets. In short, more research into the diet of Micronectidae is needed. In most species, perhaps in all, the males can produce sound under water. Palaearctic species do this by rubbing the basal lobe of the right paramere against a ridge on the right part of segment VIII (Jansson, 1989). This in contrast to Corixidae s. str. where sound is usually produced by rubbing the fore femur against the side of the head (Jansson, 1972). Many species have a brachypterous and a macropterous form which in the past led to mutiple descriptions of the same species. This led to numerous synonyms for the West and Central European species. This confusion was sorted out by Wróblewski (1958) which is still the best paper to identify West and Central European specimens. For the Mediterranean one should use Jansson (1986a) and for the NE Palaearctic Kanyukova (2006). For China one has, unfortunately, to consult the scattered basic literature.