Strategies of ionoregulation in the freshwater nymph of the mayfly (Hexagenia rigida)

Abstract

This study investigated ionoregulatory strategies used by freshwater (FW) nymphs of the mayfly Like other FW organisms, nymphs maintain hemolymph ion levels (in mmol l: Na ∼102; Cl ∼84; K ∼6; pH ∼7.35) far in excess of their surroundings. This appears to be accomplished by the combined actions of the alimentary canal, Malpighian tubules (MTs) and tracheal gills. The alimentary canal contributes in a region-specific manner, a view supported by: (1) spatial differences in the activity of basolateral Na/K-ATPase (NKA) and apical V-type H-ATPase (VA) and (2) region-specific Na and K flux rates. Both indicate a prominent role for the hindgut (rectum) in K reabsorption. MTs also exhibit region-specific differences in Na and K flux rates that are coupled with an organized but tortuous architecture. NKA and VA activities were highest in MTs versus all other organs examined. Tracheal gills were found to be sites of Na uptake, but no difference in Na uptake was found between gills taken from different regions of the abdomen or spatially along individual gills. This is likely because each gill exhibited a dense population of NKA and/or VA immunoreactive cells (putative ionocytes). Data provide new insight into how FW mayfly nymphs regulate salt and water balance using the alimentary canal, MTs and tracheal gills as well as the first direct evidence that tracheal gills acquire ions from FW.

Why are mayflies (Ephemeroptera) lost following small increases in salinity? Three conceptual osmophysiological hypotheses

“…(Freshwater animals that start to osmoconform above the isoosmotic point do not necessarily increase their internal concentrations of inorganic ions as they may produce organic osmolytes [23,33]). Osmoregulation in aquatic animals is thought to be one of the best understood processes in biology [19], although it is acknowledged that it has been relatively poorly studied in freshwater insects in general [20] and even more so in Ephemeroptera [21]. The introductory biology text books by Reece et al [34], for example, state: 'The energy cost of osmoregulation depends on how different an animal's osmolality is from its surroundings.'…”

“…Recently, Clements & Kotalik [11] 'seeded' experimental mesocosms with invertebrates from a low salinity site (0.06 -0.07 mS cm 21 ) and then applied various experimental salinity treatments. They observed that salinity of %0.3 mS cm 21 caused declines in the abundance of baetid and heptageniid mayflies and total Ephemeroptera, while Ephemeroptera drift increased. The richness of the insect orders Ephemeroptera, Plecoptera (stoneflies) and Trichoptera (caddisflies), collectively abbreviated to EPT, in southeast Australia [12] and the abundance of Ephemeroptera in Kentucky, USA [13] are reduced monotonically as salinity increases above %0.175-0.2 mS cm 21 .…”

“…They observed that salinity of %0.3 mS cm 21 caused declines in the abundance of baetid and heptageniid mayflies and total Ephemeroptera, while Ephemeroptera drift increased. The richness of the insect orders Ephemeroptera, Plecoptera (stoneflies) and Trichoptera (caddisflies), collectively abbreviated to EPT, in southeast Australia [12] and the abundance of Ephemeroptera in Kentucky, USA [13] are reduced monotonically as salinity increases above %0.175-0.2 mS cm 21 . This is in contrast to total macroinvertebrate species richness, which peaks at intermediate salinities, %0.3-0.5 mS cm 21 , with fewer species at lower and higher salinities [12].…”

Evidence that Rh proteins in the anal papillae of the freshwater mosquitoAedes aegyptiare involved in the regulation of acid base balance in elevated salt and ammonia environments

“…For most air-breathing aquatic insects, such as mosquito larvae, ions and acid-base equivalents are absorbed or excreted directly between the hemolymph and the surroundings through ion-permeable epithelia. This is unlike water-breathing aquatic insects, such as the aquatic nymphs of mayflies, which possess ionocyte-dense tracheal gills for active ion transport (Nowghani et al, 2017).…”

Geographical origin determines responses to salinity of Mediterranean caddisflies

“…Furthermore, there is evidence of rapid evolution of osmoregulatory adaptation in aquatic organisms, which indicates that local adaptation could occur within few generations in the wild [42]. Recent findings showed these adaptation being present in benthic macroinvertebrates facing sublethal salinity levels [43,44]. The mechanisms involve adjustment (i.e.…”