reabsorption (Aladin and Potts 1996). Hypoosmotic regulation in both adults and
juveniles may be achieved by drinking the medium and by the excretion of salts by
cells in the inner, non-calcified shell layer. The cuticle of this zone is characterised
by high permeability to ions, and salt excretion seems to be under strict control of
‘caplike structures’, which are most likely salt glands (Aladin 1983, 1984, 1993;
Aladin and Potts 1996).
Tolerant, euryhaline species are capable of adapting to changing salinity conditions. In the Australian euryhaline species Mytilocypris praenuncia Aladin and
Potts (1996) observed changes in external morphology depending on salinity:
below 4 ‰ cells had clear borders and there were numerous depressions in the
cuticle but between 8 and 12 ‰ the cell borders and holes in the cuticle vanished.
Finally, when M. praenuncia was raised in salinities from 20–34 to 44–48 ‰ the
cells regained clear borders, and salt glands appeared. It is worth noting that all
these changes in morphology occurred only during moulting, so the physiological
adaptations from one level of osmoregulation to the next can be completed only
during a moult. Aladin and Potts (1996) also noticed that after moulting haemolymph concentration quickly returns to its previous state, and the time taken to
reach equilibrium varied from 2 to 26 h.
There are few studies on the effect of salinity on foraminiferal physiology under
experimental conditions. Murray (1963) observed that the benthic species Elphidium crispum thrived in water at 30–35 ‰, survived in 25 ‰ and died in salinities
below 25 ‰. Specimens which had been exposed to low salinity for several days
quickly resumed normal feeding rates when returned to normal sea water and had
apparently not been permanently affected by the unfavourable conditions. Murray
(1963) also noticed that E. crispum was capable of tolerating lower salinity for a
few weeks (38 days), and survival was better in lower temperature. Most probably
the main effect of lowering temperature is the slowing of the rate of metabolic
processes. In elevated salinity (50 ‰) Murray observed retarded growth and
inhibited reproduction, but exposure did not result in death. This observation was
confirmed by Bradshaw (1955) in Rotaliella heterocaryotica, who observed active
growth when salinity reached 23.5 ‰ and a cessation in too high (37 ‰) and in too
low (16.8–20.1 ‰) salinities.
Osmoregulation in ostracods seems to be more specialised than in Foraminifera.
Ostracods possess specialised cells, tissues and glands which can provide effective
osmoregulation over wide range of salinity (0–70 ‰). Additionally, crustaceans in
general are known for their ability to reduce membrane permeability (Lee and Bell
1999). Foraminifera also seem to be adaptable in regard to changeable salinity
conditions. Typically they are equipped with numerous vacuoles, which are used in
various physiological processes. It seems to be probable that some of them are not
strictly specialised, and can change their role with changing environmental conditions. Undoubtedly, another important factor in osmoregulation is the permeability of the test and body membrane, but this should be confirmed by further
investigations.
The Adaptations of the Foraminifera and Ostracoda …
99
juveniles may be achieved by drinking the medium and by the excretion of salts by
cells in the inner, non-calcified shell layer. The cuticle of this zone is characterised
by high permeability to ions, and salt excretion seems to be under strict control of
‘caplike structures’, which are most likely salt glands (Aladin 1983, 1984, 1993;
Aladin and Potts 1996).
Tolerant, euryhaline species are capable of adapting to changing salinity conditions. In the Australian euryhaline species Mytilocypris praenuncia Aladin and
Potts (1996) observed changes in external morphology depending on salinity:
below 4 ‰ cells had clear borders and there were numerous depressions in the
cuticle but between 8 and 12 ‰ the cell borders and holes in the cuticle vanished.
Finally, when M. praenuncia was raised in salinities from 20–34 to 44–48 ‰ the
cells regained clear borders, and salt glands appeared. It is worth noting that all
these changes in morphology occurred only during moulting, so the physiological
adaptations from one level of osmoregulation to the next can be completed only
during a moult. Aladin and Potts (1996) also noticed that after moulting haemolymph concentration quickly returns to its previous state, and the time taken to
reach equilibrium varied from 2 to 26 h.
There are few studies on the effect of salinity on foraminiferal physiology under
experimental conditions. Murray (1963) observed that the benthic species Elphidium crispum thrived in water at 30–35 ‰, survived in 25 ‰ and died in salinities
below 25 ‰. Specimens which had been exposed to low salinity for several days
quickly resumed normal feeding rates when returned to normal sea water and had
apparently not been permanently affected by the unfavourable conditions. Murray
(1963) also noticed that E. crispum was capable of tolerating lower salinity for a
few weeks (38 days), and survival was better in lower temperature. Most probably
the main effect of lowering temperature is the slowing of the rate of metabolic
processes. In elevated salinity (50 ‰) Murray observed retarded growth and
inhibited reproduction, but exposure did not result in death. This observation was
confirmed by Bradshaw (1955) in Rotaliella heterocaryotica, who observed active
growth when salinity reached 23.5 ‰ and a cessation in too high (37 ‰) and in too
low (16.8–20.1 ‰) salinities.
Osmoregulation in ostracods seems to be more specialised than in Foraminifera.
Ostracods possess specialised cells, tissues and glands which can provide effective
osmoregulation over wide range of salinity (0–70 ‰). Additionally, crustaceans in
general are known for their ability to reduce membrane permeability (Lee and Bell
1999). Foraminifera also seem to be adaptable in regard to changeable salinity
conditions. Typically they are equipped with numerous vacuoles, which are used in
various physiological processes. It seems to be probable that some of them are not
strictly specialised, and can change their role with changing environmental conditions. Undoubtedly, another important factor in osmoregulation is the permeability of the test and body membrane, but this should be confirmed by further
investigations.
The Adaptations of the Foraminifera and Ostracoda …
99