Salinity tolerance, and several aspects of osmoregulation,
ionic regulation and permeability were measured for Palaemon
longirostris at three temperatures (4, 12 & 20°C). Influence
of ontogenetic stage on salinity tolerance and
osmoregulation was investigated by testing separately
individuals sorted, using carapace length, into 'small'
(10-18mm), 'medium' (18-24mm), 'large' (>24mm) and
'ovigerous' (>24mm) size groupings. Effect of seasonal
acclimatization on salinity tolerance and osmoregulation was
taken into account by comparing responses of summer- with
winter-collected prawns.
Irrespective of temperature and size, summer Palaemon
longirostris were extremely euryhaline and had >90% survival
in various salinities from 0.5-34 º/oo . For summer prawns,
survival in 43 º/oo was reduced, particularly at 4°C. Salinity
tolerance of winter prawns was generally less than that of
summer individuals, this difference being marked at salinity
extremes in combination with low temperature.
Over the salinity range 0.5-34 º/oo , prawns were very
efficient hyper-hypo-osmoregulators at each temperature. At
43 º/oo, blood osmolalities tended towards the isosmotic,
indicating that osmoregulation was breaking down. There was
no clear effect of prawn size or season on osmoregulation,
however, low temperature appeared to be disruptive. Transfer
of prawns from 14 º/oo to either 5 º/oo or 34 º/oo, and from 1 º/oo to
34 º/oo, resulted in a new steady blood osmolality within
6-12h. Transfer from 34 º/oo to 1 º/oo , caused blood osmolality to
drop significantly within 12h, and a new equilibrium was
not reached until 72h. The inorganic ions sodium, chloride,
potassium, calcium and magnesium accounted for >94% of total
blood osmolality over the salinity range 0.5-34 º/oo . There was
no consistent effect of temperature on the regulation of
these ions