Opifex fuscus

Opifex fuscus, known commonly as the saltpool mosquito, is a species of mosquito that is endemic to New Zealand. This species was first discovered in 1902 by Frederick Hutton and has received much attention from entomologists. The mosquitoes occur on the coast, where the larvae live in rock pools within the spray zone. To cope with their habitat, the larvae are able to tolerate a wide range of water salinities. They are also known by the Māori name "naeroa", which is generally applied to mosquitoes as a whole.

Opifex fuscus
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Class:	Insecta
Order:	Diptera
Family:	Culicidae
Genus:	Opifex
Species:	O. fuscus
Binomial name
Opifex fuscus Hutton, 1902

Taxonomy

This species was described in 1902 by Frederick Hutton from a specimen collected in Wellington by George Vernon Hudson. It was the first species to be described from the Opifex genus, of which it is the type species. It was also incorrectly categorized as a crane fly.^[1] The type specimen is stored in Canterbury Museum.^[2]

In 1921, David Miller stated that Hudson had informed him O. fuscus was in fact a mosquito, rather than a crane fly.^[3] In the same year, Frederick Edwards examined specimens presented to the Natural History Museum of London by Hudson and confirmed that the species was in fact a mosquito.^[4] In 1922, Miller followed up with a revision of the species and designated it its own subfamily, the Opificinae.^[5] However just two years later, this subfamily was later synonymized by Edwards.^[6] The species last had a major revision in 1968, in which the mosquito was described in greater detail and it was assigned to the Aedini tribe on the basis of its more primitive characteristics.^[2] It is commonly referred to as the "salt pool mosquito".^[7]

Description

Adult male

Larva

As adults, they can be distinguished from the rest of New Zealand's mosquitoes by the presence and absence of certain bristles on the abdomen, the shape of the antennae and the absence of scales on the vertex of the head.^[2][8]

The adults are stocky, generally darkish in colour and are roughly 5 mm (0.19 in) in length. The antennae are blackish and have three long bristles near the base of it. The vertex of the head doesn't have scales that stick out. Like many other mosquitoes, they have a long proboscis (straw-like mouthpart for sucking blood). The legs are brownish black, with the first pair of legs in the male having very long claws. The thorax is blackish and is coated with gold and black hairs on top. The abdomen is a black brown colour and is covered in blackish bristles and whitish scales.^[5] Due to the body hairs, the mosquito is water resistant. In flight, they make a slight buzzing sound.^[9]

Larva

The different instars of the larvae can be distinguished by the size of the head capsule.^[9] In their final instar before pupating, the larvae are about 12.5 mm (0.49 in) in length. They are coloured blackish brown but are sometimes also greenish. The siphon (a straw like organ used for breathing air) has a pair of hair tufts just above its middle. The head has brownish maxillae (mouth parts).^[5] There are also mouth brushes that come in two forms, with each larvae only having one type. In the first form, the brushes have simple hairs whereas in the second, the hairs are pectinate (having small branches protruding off them). The reason for this is not yet known.^[9] The thorax is somewhat wider than the head and has a distinctive pattern of small hairs. There are tufts of hair on the segments of the abdomen, with longer tufts at the beginning of the abdomen.^[5]

Egg

The eggs dimensions are 0.3 mm and 0.5 mm (0.011 and 0.019 in). They are brownish to blackish in colour but are somewhat transparent. The underside of the egg is flattened. Sections of the egg have protuberances that stick to surfaces.^[10]

Distribution and habitat

This species is endemic to New Zealand and is found throughout the rocky coastline of the entire North Island and most of the South Island.^[2] In the south east of the South Island, they co-occur with Aedes australis, an exotic species which also utilizes rock pools. It has been suggested that A. australis competes with O. fuscus and has displaced it in this region.^[11] The distribution of O. fuscus also includes nearby small islands such as the Three Kings Islands and the Mokohinau Islands.^[2] In addition, populations have been recorded in the much further away Kermadec Islands and the subantarctic Snares Islands.^[12][13]

Habitat

Typical rockpool habitat of Opifex fuscus.

The salt pool mosquitoes occur in rocky coastal habitat. The larvae are found in rock pools that occur within the spray zone above the high tide of the coast. The larvae occur in both permanent and temporary rock pools which are refreshed by rain and spray from the ocean.^[14][15] They have also been recorded from pools in freshwater streams and from a horse water trough.^[16][2] The larvae can tolerate a range of water salinity levels. In one study, the highest salinity larvae were reported surviving at was 90% (much higher than sea water) whereas the lowest was 0.4%.^[14] The adults are also frequently observed on or near rock pools occupied by larvae.^[14][15]

Diet

Females of this species feed on the blood of birds and humans.^[17] They will feed by day and night but are most active during the day. Unlike other mosquito species, the female generally does not require a blood meal to produce their first batch of eggs due to their large energy reserves from their larval stage. After producing their first eggs, they will begin feeding on blood.^[10] However, one study contradicted this, where females were only able to lay eggs after feeding on sugar water. This may indicate that if they have insufficient energy reserves (such as when they receive little food as larvae), they may need to feed before laying eggs.^[18] In lab conditions, the females would spend 5 to 12 minutes feeding on human blood.^[10]

Based on gut contents, the larvae feed primarily on diatoms, various algae and detritus. As the larvae get bigger, they feed on larger food particles. However, they are also reported to occasionally cannibalize injured mosquito larvae when food is scarce.^[19][5] By the time they emerge as adults, they have unusually high levels of fat when compared to other mosquitoes.^[10]

Life history

Females lay their eggs on the damp edges of rock pools , which can extend up to 5 cm from the waters surface. Typically, the eggs are laid inside cracks and crevices, but other coastal debris may also be utilized when there are limited egg laying spots available. When at a suitable site, the female tucks her abdomen under her thorax and lays six to ten eggs. Because the rock pools fluctuate in water levels (and may even be temporary), it is risky for larvae to hatch at the wrong time. To prevent this, they hatch when the water level is high (detected by changes in oxygen levels) and can potentially delay hatching for months if the conditions are not suitable.^[14] Upon hatching, the larvae leave the egg shell head first.^[10]

The larvae have four instar stages to pass through before pupating.^[9] The rate at which the larvae grow can be quite variable. In one study (in lab conditions), it was found that it took larvae 10 to 30 days to reach the pupa stage.^[10] The growth rate depends heavily on environmental factors such as temperature and light exposure. In laboratory conditions, larvae grown at 25°C matured faster than larvae at 13°C. The final instar before pupating takes the longest amount of time (and is also the largest increase in biomass).^[14]

Once in the pupae stage they will not feed. Few larvae pupate during winter, with September being the time of year when pupa were most abundant in one study.^[20] As in the larval stage, the pupation stage varies in length and appears to be dependent on the environment. It has been recorded taking as little as 72 hours and as long as 12 days.^[10]

Mating

The males hatch first and use their long legs to sit on the water's surface.^[21] The males wait on the surface of salt water pools that contain larvae of their species. Once a larva pupates it will float to the surface. A male at the surface will detect this, possibly visually and by detecting vibrations in the water, and orientate itself to catch it. Males will attempt to intercept these pupa and grab them with their legs. Once the pupa begins to emerge, the male will attempt to begin genital contact with the individual. If the adult emerging from the pupa is male, then it will disengage and attempt to find another pupa. If the emerging adult is female, it will begin copulating with it before it has even fully left its pupa case. Because of this, all females mate very early in their adult lives.^[15]

There is evidence that larger males tend to be able to mate with more females than smaller males do, which suggests that mating for this species is non-random. This appears to be especially true when there are high levels of reproductive competition. ^[22]

Parasites

The larvae are known to be infected by Coelomomyces opifexi, a parasitic fungus that uses copepods as intermediate hosts. It was first discovered infecting Opifex fuscus larvae in Otago.^[23] The fungus also infects Aedes australis, which co-occurs with O. fuscus in some parts of New Zealand. The fungus is far more effective at infecting the former. In O. fuscus, the natural infection rates have been found to be as high as 47.3%.^[24]

Disease transmission

Opifex fuscus has not been recorded transmitting diseases in the wild. However, in laboratory conditions it has successfully acted as a host of the Whataroa virus, New Zealand's only mosquito-borne virus.^[25][26] In another study, it was found that O. fuscus could act as a vector for some other alphaviruses.^[27]

In Māori culture

The Māori word "naeroa" refers to mosquitoes, which includes Opifex fuscus.^[28]