at the University of Washington in Seattle

some flies lay their eggs in seawater

The ability to hydrate in desiccating environments requires physiological feats accomplished by the coordination of numerous cellular, biochemical, and organ-system processes.

Insects have evolved mechanisms to cope with water scarcity many times over, and some of the most impressive and uncommon examples are found amongst marine (and marine-associated) insects. We study marine flies−–specifically, the saline-tolerant larvae of mosquitoes and midges−–that inhabit coastal rock pools and tidal marshes including one shown in this image from the San Juan Islands in Washington.

The majority of fly species with aquatic larvae occupy freshwater environments, and only 10% of dipteran flies and 5% of mosquito species can survive and develop in brackish or saline waters. The fly species that we study are capable of maintaining almost constant hemolymph (or, blood) osmolarities and ion levels when developing in external salinities that span very dilute freshwater to hypersaline (~300% seawater) levels.

A very salty supratidal saline rock pool with developing Aedes togoi mosquito larvae in the San Juan Islands of Washington State.
Illustration by Elissa Khodikian showing of the posterior end of mosquito larvae showing the hindgut organs that produce and modify urine. Mosquito species that have saline-tolerant larvae have an additional organ segment in their hindgut that is a salt-secreting organ, and this is depicted in yellow in the two right illustrations.
The hindgut of mosquito larvae houses the renal and excretory systems that constantly filters the hemolymph (or blood) and produces and modifies urine. The larvae of saline-tolerant species have these organs and also a specialized salt-secreting gland to concentrate urine. Mosquito larvae hindgut illustrated by Elissa Khodikian

the salt-secreting gland of aquatic larvae

The basic blueprint associated with fly saline-tolerance involves a remodeled hindgut with a ‘new’ hindgut segment—specifically the appearance of a salt-secreting gland shown in yellow in the illustration to the left—that functions to remove salt from the hemolymph against powerful ion and osmotic gradients to produce a very salty urine.

We study this organ across multiple levels of evolutionary divergence in Aedine mosquitoes to uncover common physiological pathways that are required for conferring a robust tolerance to salt. Using a variety of molecular and cellular techniques, electrophysiology, and confocal microscopy, we are describing the transport mechanisms that drive ion flux and attenuate water flux across the salt gland epithelium.

A transverse tissue section of the posterior end of a marine midge larvae that is stained with H&E dyes and shows distinct organs. A tissue section of the posterior end of a marine midge larvae stained with antibodies targeting ion pumps in osmoregulatory organs and other key organs.

Illustration of a mosquito larvae by Dr. Chun Chih Chen, adapted to include the salt gland.

the anal ‘gills’ of aquatic larvae

The four anal gills (or papillae)—finger-like projections at the posterior end of larvae—play critical functions in ion uptake and nitrogenous waste excretion in mosquito species with obligate-freshwater larvae, and also in saline-tolerant (euryhaline) species when larvae develop in freshwater. However, the anal gills are reduced in size in saline-tolerant larvae compared to their freshwater cousins. It is for this reason that the rectal salt gland of salt-tolerant larvae has received most of the physiological attention to date. This research challenges the longstanding premise that the truncated anal gills of saline-tolerant dipteran larvae are functionally redundant in seawater. This is based on our recent incidental findings of very high ion pump expression in these larval gills in both freshwater and seawater conditions. Our current research explores whether the anal papillae support active salt secretion to rid of excess ions when larvae are reared in seawater and whether this occurs via a complete reversal in ion-transporting machinery required for ion uptake in freshwater. If so, this would be very euryhaline teleost fish-like!

The anal gills of mosquito larvae are used to achieve salt balance in freshwater. These gills actively absorb salts directly into the hemolymph from the surrounding water. The anal gills of salt-tolerant (euryhaline) larvae are greatly reduced in size, but they still function to absorb salts when larvae are developing in dilute freshwater pools. We test the hypothesis that the anal gills of euryhaline larvae reverse function in seawater and are capable of secreting ions from the hemolymph to maintain salt balance with high salinity. Graphical illustration by Elissa Khodikian.

Stay tuned for more information and exciting pre-print articles to come soon!