Salinity tolerance and osmoregulation in several subtropical decapods
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Date
2012
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University of Zululand
Abstract
This study investigated salinity tolerances and osmoregulatory strategies of several subtropical brachyuran
mangrove crabs and an anomuran prawn, with particular reference to Uca vocans, Uca urvillei, Uca
chlorophthalmus, Uca annulipes, Dotilla fenestrata, Macrophthalmus depressus, Macrophthalmus
grandidieri, Metopograpsus thukuhar, Chiromantes eulimene and Callianassa kraussi. All species
investigated were either directly exposed or acclimated to salinities between 0-75 and their tolerance to these
salinities and osmoregulatory strategies monitored over a 4 day period. Other experiments conducted
included an investigation of the time dependant responses of species following direct transfer to various
salinities, and for one species also the influence of temperature on salinity tolerance and osmoregulatory
strategy. All the species were shown to be euryhaline, as would be expected for species inhabiting an
estuarine environment. However, the degree of euryhalinity varied between species. The general salinity
range they could tolerate was between 0-55, but species like U. annulipes, D. fenestrata, C. eulimene and C.
kraussi tolerated salinity as high as 65. Direct exposure was shown to be more stressful than acclimation,
especially in low and high salinities. Out of seven species that were directly exposed and acclimated, C.
eulimene was the only species able to tolerate freshwater (salinity 0) following direct exposure. All crab
species followed an osmoregulation strategy by hyper-regulating at low salinities and hypo-regulating at
higher salinities. The hyper-regulatory ability of most species was stronger than the hypo-regulatory ability,
as this was shown by the hemolymph osmolality line which was much closer to the isosmotic line at
salinities above the isosmotic point and also by the lower osmotic capacity (OC) at comparable salinity
differences below and above the isosmotic point. Callianass kraussi osmoregulated at salinities lower than
25 and osmoconformed at salinities above 25.
All Uca species investigated were able to tolerate direct transfer to freshwater for up to eight hours without
experiencing any mortality. All specimens of U. vocans, which occurs lowest in the intertidal zone, died
within 24 hours of exposure. All U. urvillei died within two days of exposure. Uca annulipes, which lives in
the highest region of the intertidal zone, was the most tolerant to rapid freshwater exposure, with 70% of
crabs surviving up to 72 hours. Uca annulipes regulated its hemolymph osmolality more efficiently than U.
urvillei and U. vocans, which live on the lower level of the intertidal zone. The ability of Uca crabs to
survive as well as regulate their hemolymph osmolality when directly transferred to freshwater was closely
linked to the level they occupy in the intertidal zone.
The third part of this study looked at the influence of temperature on the salinity tolerance and
osmoregulation of C. eulimene. Temperatures between 14-22oC had no effect on salinity tolerance or
osmoregulatory capability of C. eulimene at salinities between 0-45. Exposure of C. eulimene to lowered
temperatures had no effect on the salinity tolerance and osmoregulation capacity of this species. Lower
temperatures do not inhibit the distribution of this species from South and West Coast of Africa.
Description
Submitted to the Faculty of Science In fulfilment of the requirements for the degree of
Master of Science in the Department of Zoology at the University of Zululand, South Africa, 2012.
Keywords
Osmoregulatory strategies, Subtropical decapods