282
P. 9. MEADOWS AND J. I. UAMPBELL
selection. Bohn (1912), for instance, observed how lobster larvae,
which were normally photonegative, became photopositive when the
pressure was increased and how, as they aged, the effect waned.
Knight-Jones and Morgan (1966, p. 268) quote other examples.
In general, they visualize the response of planktonic animals to pressure
and its relation to depth regulationand vertical migration “ as involving
accommodation to gradual changes of limited range, until the pressure
builds up sufficiently to evoke a long-sustained compensatory swimming.
This would provide an oscillatory feedback mechanism which probably
helps, not only in setting bounds to the vertical migrations of planktonic
animals, but also in maintaining their cyclical activity’’ (loc. cit. p. 278).
As they point out, pressure is unlikely to be the only variable involved,
since cyclic behaviour continues in shallow laboratory tanks (Harris,
1963), and we have already drawn attention to the probable interactions between pressure and light. We are not aware of any investigations of the influence of pressure on benthic marine invertebrates
although intertidal invertebrates are known to respond to pressure.
We have seen that planktonic larvae of benthic animals respond to
light and pressure so as to maintain themselves well above the bottom,
and in this way they are dispersed from place to place by water
currents. Their responses to gravity effect the same end, as during
most of their planktonic life they are geonegative as well as photopositive (Loeb, 1893; Bayne, 1964) or geonegative only if they have
no light receptors (Lyon, 1906 ; Grave, 1926). As settlement approaches
their light and gravity responses reverse and they become geopositive
and photonegative (Bayne, 1964). Amongst adult planktonic invertebrates, the copepod Centropages typicus K r ~ y e r is geonegative as
well as being photopositive (Johnson and Raymont, 1939). We are not
aware of other studies on adult planktonic invertebrates. The few
benthic invertebrates investigated appear to be geonegative (a nudibranch, Crozier and Arey, 1919; various starfish, Crozier, 1935 for references) but only Crozier and Arey have attempted to link the behaviour with the species’ distribntion.
Various species of planktonic animals and larvae stop swimming
upwards (Lance, 1962 ; Lyster, 1965) or become photonegative (Loeb,
1893; Rose, 1925, p. 465) when they meet layers of less saline water
and the adaptive advantage of this at the mouths of estuaries is obvious.
There are no comparative studies on bottom-dwelling animals although
simple methods have been described (Jansson, 1962 ; McLusky, 1970)
which offer animals a horizontal rather than a vertical salinity gradient,
and these could be modified to accommodate animals of differing sizes
and to allow animals contact with a suitable substrate as they made
their choice. An interesting variant of the vertical salinity gradient
P. 9. MEADOWS AND J. I. UAMPBELL
selection. Bohn (1912), for instance, observed how lobster larvae,
which were normally photonegative, became photopositive when the
pressure was increased and how, as they aged, the effect waned.
Knight-Jones and Morgan (1966, p. 268) quote other examples.
In general, they visualize the response of planktonic animals to pressure
and its relation to depth regulationand vertical migration “ as involving
accommodation to gradual changes of limited range, until the pressure
builds up sufficiently to evoke a long-sustained compensatory swimming.
This would provide an oscillatory feedback mechanism which probably
helps, not only in setting bounds to the vertical migrations of planktonic
animals, but also in maintaining their cyclical activity’’ (loc. cit. p. 278).
As they point out, pressure is unlikely to be the only variable involved,
since cyclic behaviour continues in shallow laboratory tanks (Harris,
1963), and we have already drawn attention to the probable interactions between pressure and light. We are not aware of any investigations of the influence of pressure on benthic marine invertebrates
although intertidal invertebrates are known to respond to pressure.
We have seen that planktonic larvae of benthic animals respond to
light and pressure so as to maintain themselves well above the bottom,
and in this way they are dispersed from place to place by water
currents. Their responses to gravity effect the same end, as during
most of their planktonic life they are geonegative as well as photopositive (Loeb, 1893; Bayne, 1964) or geonegative only if they have
no light receptors (Lyon, 1906 ; Grave, 1926). As settlement approaches
their light and gravity responses reverse and they become geopositive
and photonegative (Bayne, 1964). Amongst adult planktonic invertebrates, the copepod Centropages typicus K r ~ y e r is geonegative as
well as being photopositive (Johnson and Raymont, 1939). We are not
aware of other studies on adult planktonic invertebrates. The few
benthic invertebrates investigated appear to be geonegative (a nudibranch, Crozier and Arey, 1919; various starfish, Crozier, 1935 for references) but only Crozier and Arey have attempted to link the behaviour with the species’ distribntion.
Various species of planktonic animals and larvae stop swimming
upwards (Lance, 1962 ; Lyster, 1965) or become photonegative (Loeb,
1893; Rose, 1925, p. 465) when they meet layers of less saline water
and the adaptive advantage of this at the mouths of estuaries is obvious.
There are no comparative studies on bottom-dwelling animals although
simple methods have been described (Jansson, 1962 ; McLusky, 1970)
which offer animals a horizontal rather than a vertical salinity gradient,
and these could be modified to accommodate animals of differing sizes
and to allow animals contact with a suitable substrate as they made
their choice. An interesting variant of the vertical salinity gradient