44
R. R. MILLAR
3. Temperature
Water temperature is a major factor controlling the occurrence of
ascidians but has been considered more often in relation to geographical
than to ecological distribution ; in pwticular the ways in which it sets
thc limits of distribution by influencing reproduction, have been
examined (see Section 111, p. I)). Indeed, temperature is less likely to
be a critical factor affecting local distribution than is either salinity or
the nature of the substratum, but a few cases of local effects have been
recorded. Low winter temperature killed Ciona intestinalis, Styela
plicata and Molgula manhattensis but not Botryllus schlosseri in the
lagoon of Venice, which has a great annual temperature range but is
free from the complicating factor of fluctuating salinity (Sabbadin,
1958). In sheltered semi-enclosed areas and in docks, where great
numbers of a few species may be present, it is often difficult to separate
the effects of high temperature from those of other factors. But Naylor
(1965s) observed marked changes in the numbers and the relative
abundance of Ciona iatestinalis and AscidieEla aspersa in the docks at
Swansea, Wales, following changes in the water temperature produced
by the discharge of heated eMuent from a power station, over a period
when other factors were comparatively steady. The response of the two
species was complex and resulted in A. aspersa being dominant in
winter and spring and Ciona intestinalis in summer and autumn. Effects
of this kind depend less on lethal temperatures than on alterations in
growth rates and the period and success of breeding (see Sections I11
and IV), although temperature limits for normal embryonic development (Runnstrom, 1929; Knaben, 1952) may determine whether a
species succeeds locally. When temperature and salinity both vary,
each may modify the effect of the other, but often the extremes do not
coincide, and in that situation temperature may be less important than
salinity (Dybern, 1969a).
4. Turbidity
Ascidians, being filter-feeding animals, are affected by the amount
and nature of suspended matter in the surrounding water, but there are
few observations to indicate the importance of this factor in nature.
C. Monniot (1965a) discovered that at a depth of 90 m near Banyulssur-Mer a current of 0.5-1 knot raised clouds of sediment from the seabed on which pyurid ascidians were living. A moderate amount of
suspended matter appeared to be beneficial, but too much led to clogging of the branchial feeding mechanism and subsequent death. In
species bearing a rich epifauna, the death of attached organisms can
damage the ascidians by encouraging pollution. Monniot also records
R. R. MILLAR
3. Temperature
Water temperature is a major factor controlling the occurrence of
ascidians but has been considered more often in relation to geographical
than to ecological distribution ; in pwticular the ways in which it sets
thc limits of distribution by influencing reproduction, have been
examined (see Section 111, p. I)). Indeed, temperature is less likely to
be a critical factor affecting local distribution than is either salinity or
the nature of the substratum, but a few cases of local effects have been
recorded. Low winter temperature killed Ciona intestinalis, Styela
plicata and Molgula manhattensis but not Botryllus schlosseri in the
lagoon of Venice, which has a great annual temperature range but is
free from the complicating factor of fluctuating salinity (Sabbadin,
1958). In sheltered semi-enclosed areas and in docks, where great
numbers of a few species may be present, it is often difficult to separate
the effects of high temperature from those of other factors. But Naylor
(1965s) observed marked changes in the numbers and the relative
abundance of Ciona iatestinalis and AscidieEla aspersa in the docks at
Swansea, Wales, following changes in the water temperature produced
by the discharge of heated eMuent from a power station, over a period
when other factors were comparatively steady. The response of the two
species was complex and resulted in A. aspersa being dominant in
winter and spring and Ciona intestinalis in summer and autumn. Effects
of this kind depend less on lethal temperatures than on alterations in
growth rates and the period and success of breeding (see Sections I11
and IV), although temperature limits for normal embryonic development (Runnstrom, 1929; Knaben, 1952) may determine whether a
species succeeds locally. When temperature and salinity both vary,
each may modify the effect of the other, but often the extremes do not
coincide, and in that situation temperature may be less important than
salinity (Dybern, 1969a).
4. Turbidity
Ascidians, being filter-feeding animals, are affected by the amount
and nature of suspended matter in the surrounding water, but there are
few observations to indicate the importance of this factor in nature.
C. Monniot (1965a) discovered that at a depth of 90 m near Banyulssur-Mer a current of 0.5-1 knot raised clouds of sediment from the seabed on which pyurid ascidians were living. A moderate amount of
suspended matter appeared to be beneficial, but too much led to clogging of the branchial feeding mechanism and subsequent death. In
species bearing a rich epifauna, the death of attached organisms can
damage the ascidians by encouraging pollution. Monniot also records