Water and ion balance and nitrogenous excretion as limitations to terrestrial excursion in the Christmas Island blue crab,Cardisoma hirtipes (Dana)
✍ Scribed by Dela-Cruz, Jocelyn; Morris, Stephen
- Publisher
- John Wiley and Sons
- Year
- 1997
- Tongue
- English
- Weight
- 174 KB
- Volume
- 279
- Category
- Article
- ISSN
- 0022-104X
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✦ Synopsis
Cardisoma hirtipes appears from the species distribution to be dependent on fresh water. The ability of C. hirtipes to maintain ion and water balance and nitrogenous excretion in the absence of free water (water in which the crab can immerse itself) was assessed in the laboratory and in the field during both the wet and dry seasons. During enforced exposure to air for 9 days (only drinking water provided), the osmotic pressure and composition of both the haemolymph and urine remained constant, although the urine differed in composition to the haemolymph. The antennal gland reabsorbed Ca and Mg from the urine. The final excretory product (P) was dilute, containing only 10% of the concentration of ions measured in the haemolymph. Water loss to air was minimised by reduced P flow, but drinking was insufficient to prevent dehydration. Therefore, C. hirtipes might require periodic immersion to maintain long-term water balance. Nitrogen excretion to air was negligible compared to excretion to water, and after 9 days in air C. hirtipes excreted N at an elevated rate within the first 2 min of submersion. In the field, crabs restricted from access to water had urine and haemolymph osmotic pressure and composition similar to unrestricted, free-ranging crabs. The osmotic pressure was highest in crabs sampled in the wet season, although nitrogenous excretion rates were similar during the wet and dry season for both free-ranging and restricted crabs. The distribution of C. hirtipes is dependent on water availability, but under humid conditions water from drinking and food might balance short-term losses. The extent of terrestrial excursions by C. hirtipes is limited by dependency on water for nitrogen excretion.