Opifex fuscus, known commonly as the saltpool mosquito, is a species of mosquito that is endemic to New Zealand. This species was first formally described in 1902 by Frederick Hutton. 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. The larvae have mouthparts that specialize towards either filter feeding or grazing, depending on what food source is available. They are widespread throughout the rocky coasts of New Zealand, but have been displaced from the Otago region by the introducedAedes australis. As adults they are known to feed on blood whereas the larvae feed on algae and detritus. The males wait on the surface of the rock pools and mate with female pupae before they mature into adults. This species is also known to be parasitised by the fungus Coelomomyces psorophorae, which uses copepods as intermediate hosts. In laboratory studies, O. fuscus can be a competent vector of the Whataroa virus. They are also known by the Māori name naeroa, which is generally applied to mosquitoes as a whole.
In 1921, David Miller stated that Hudson had informed him O. fuscus was in fact a mosquito, rather than a crane fly.[4] 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 and that it belonged in the subfamily Culicinae.[5] In 1922, Miller followed up with a revision of the species and designated it its own subfamily, the Opificinae.[6] However just two years later, this subfamily was later synonymised with Culicinae by Edwards.[7] 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.[3] It is commonly referred to as the "saltpool mosquito".[8] The Māori word naeroa refers to mosquitoes, which includes Opifex fuscus.[9]
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.[3][10]
The adults are stocky, generally greyish in colour and are roughly 5mm (0.2in) in length.[6][3] The antennae are blackish and have three long bristles near the base of it. The vertex of the head does not 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.[6] Due to the hydrophobic body hairs, the mosquito is water repellent. In flight, they make a slight buzzing sound.[11]
Pupa
The thorax of the pupa has a keel-like ridge that is divided by a groove-like depression (which is used in mating). The appendages used in respiration are near the middle of the thorax and have spines on the upper surface. The segments of the abdomen are visible and well defined. The abdominal segments have spines on the upper surface from segments one to six.[6]
Larva
The different instars of the larvae can be distinguished by the size of the head capsule.[11] In their final instar before pupating, the larvae are about 12.5mm (0.49in) 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 (mouthparts).[6] The head also has mouth brushes that come in two forms, with each larva 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).[11] A scanning electron microscope study found all brushes bear pectinate hairs to some degree, indicating a continuum rather than a strict dichotomy.[12] 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.[6]
Egg
The eggs' dimensions are 0.3mm by 0.5mm (0.01 by 0.02in) and are ovoid in shape. 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.[13]
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.[18][19] They have also been recorded from pools in freshwater streams and from a horse water trough, although these are not typical examples.[20][3]
The larvae tolerate a wide range of salinities. In the field, pools containing larvae have been recorded up to ~90‰ (≈2.6× normal seawater of ~35‰) and down to ~0.4‰. In laboratory work, third and fourth-instar larvae withstood step increases to 70‰, with occasional individuals to 105‰, and in an evaporation trial moulting and pupation ceased by ~130‰. The remaining larvae died by ~165‰.[18] Another study reported a single larva surviving in 1.25× seawater.[21] In one study, the permeability of the cuticle was much less than that of mosquitoes that live in freshwater. It was suggested that this is an adaptation for surviving the high salinity of salt pools.[22] The adults are also frequently observed on or near rock pools occupied by larvae, with the males commonly seen floating on the surface of the pools.[18][19] Within salt pools, they frequently cohabit with the copepod Tigriopus fulvus.[23]
Larvae grazing on the bottom of a salt pool
Diet
Females of this species feed on the blood of birds and humans.[24] 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.[13] 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.[25] In lab conditions, the females would spend 5 to 12 minutes feeding on human blood.[13]
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.[26][6] By the time they emerge as adults, they have unusually high levels of fat when compared to other mosquitoes.[13]
The type of mouth brush that the larvae possess appears to be determined by the type of food available. In one study, larvae reared on fish food developed pectinate type brushes, whereas those reared on blood serum only had simple brushes. It appears that simple brush type is best suited for filter feeding, whereas the pectinate type is best suited for grazing.[27]
Life history
Females use their ovipositor to lay their eggs on the damp edges of rock pools, which can extend up to 5cm (2 in) from the water's 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.[18] Upon hatching, the larvae leave the egg shell head first.[13] In lab conditions, the females have been recorded laying up to 100 eggs, but around 30 eggs laid was more typical. Egg laying begins in just under two weeks from emergence.[25]
The larvae have four instar stages to pass through before pupating.[11] 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 pupal stage.[13] The growth rate depends heavily on environmental factors such as temperature and light exposure. In laboratory conditions, larvae grown at 25°C (77°F) matured faster than larvae at 13°C (55°F). The final instar before pupating takes the longest amount of time (and is also the largest increase in biomass).[18]
Once in the pupae stage they do not feed. Few larvae pupate during winter, with September being the time of year when pupae were most abundant in one study.[28] 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.[13] The females are recorded emerging into adults within five to thirty minutes after capture by the male.[18] In the North Island, adults can be found all year. However, in the South Island, the adults are apparently absent during winter.[3]
Mating
Opifex fuscus has an unusual mating system when compared to other mosquitoes. Before newly emerged males can begin mating, they must rotate their terminalia (the terminal segments of the abdomen, which are modified into external genitalia) to at least 135 degrees, which may take up to five hours (which is unusually quick for mosquitoes). The males typically begin looking for females around 6 to 24 hours after emerging.[29] The males wait on the surface of saltwater pools that contain larvae of their species. Once a larva pupates it will float to the surface. The males will then grab the pupae using their front legs with claws that are modified to be extremely long. The males can detect the pupae by eyesight and possibly vibrations caused by the pupae reaching the surface. When the male is in contact with a pupa, he submerges his head underwater (the head is covered in hydrophobic hairs to protect it). The male then bends his abdomen to the pupa and inserts his terminalia into a groove in the pupae. Once the terminalia is locked into place with the groove, the male lets go of his front legs. Around ten to twenty minutes later, the pupae begin to emerge into an adult. The male will slot his abdomen into the pupal case to assist with emergence, which takes up to five minutes. If the emerging mosquito is female, then the male will come into contact with its terminalia and transfer spermatozoa, often before it has even left its pupal case.[19] If the adult emerging from the pupa is male, then it will disengage and attempt to find another pupa (often within a few seconds of capture). It is very rare for these mosquitoes to emerge without the assistance of males, although it is possible. [18]
One potential cause for this mating system is that the females apparently only need to mate once, whilst males will mate many times.[19] Because of this, there is strong competition among males for mating. The selective pressure from this competition may have caused the evolution of the early mating system in this species. 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. [30]
Parasites
The larvae are known to be infected by Coelomomyces psorophorae, a parasitic fungus that uses copepods as intermediate hosts. It was first discovered infecting Opifex fuscus larvae in Otago.[31] 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%.[32]
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.[33][34] In another study, it was found that O. fuscus could act as a vector for some other alphaviruses.[35]
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