318
P. 9. MEADOWS AND J. I. OILI\IPBELL
E. Larva$ ehemoreceptim at settlement
We may now consider how larvae detect the chemicals stimulating
them to settle near adults of their own species or on a particular species
of seaweed. It is possible for larvae to detect specific chemicals at a
distance and swim up a concentration gradient, or to respond only after
contact with the surface from which the chemical is diffusing or to
which the chemical is adsorbed. Certainly, larger animals such as some
polynoid commensals are apparently capable of detecting and of moving
up concentration gradients of host substances in solution (see section
on commensalism and parasitism). Crisp and Meadows (1962, p. 615)
however, have pointed out the theoretical inaccuracies involved in
scaling this situation down to the size of many larvae (1-2 mm in
length). “ Firstly, the distance separating its (the larva’s) sense organs
would be so small in comparison with the scale of chemical gradients
in the fluid that concentration differences are not likely to be detectable.
Secondly, a chemical diffusing from a small surface area into a moving
fluid (sea water) gives rise to a sharp gradient of concentration near the
boundary layer where the movement of fluid is restricted by the solid
surface (Crisp, 1966), but outside this boundary layer the concentration
and its gradient are small. This boundary layer is unlikely to reach a
thickness exceeding the dimensions of the larva. Thus for a very small
animal situated at a distance greater than its own dimensions from a
source of diffusing material both the above factors operate, so that the
chemical gradient could not be detected and a directional response
would be impossible.” Perhaps this last statement is a little dogmatic,
but they were able to prove in a later paper (Crisp and Meadows, 1963)
that cyprids recognize the settling factor responsible for their gregariousness only when it is adsorbed to a surface. They started from the
observation that even when extract treated surfaces were placed very
close to untreated ones there was no spread of settlement from one to
the other, and they then offered cyprids a choice of extract treated and
untreated panels in duplicate dishes. In one dish the cyprids were
resuspended in sea water, and in the other they were resuspended in the
same extract that had previously been used to treat the extract treated
panels (this particular extract had been made up in sea water and the
cyprids behaved entirely normally in it). The cyprids distinguished
equally well between the extract treated and the untreated panels both
in sea water and in extract. The only way that cyprids could have
responded in the latter instance was to the settling factor as an
adsorbed layer on the surface of the panels, since they were surrounded
by exactly the same concentration and there would be therefore no
P. 9. MEADOWS AND J. I. OILI\IPBELL
E. Larva$ ehemoreceptim at settlement
We may now consider how larvae detect the chemicals stimulating
them to settle near adults of their own species or on a particular species
of seaweed. It is possible for larvae to detect specific chemicals at a
distance and swim up a concentration gradient, or to respond only after
contact with the surface from which the chemical is diffusing or to
which the chemical is adsorbed. Certainly, larger animals such as some
polynoid commensals are apparently capable of detecting and of moving
up concentration gradients of host substances in solution (see section
on commensalism and parasitism). Crisp and Meadows (1962, p. 615)
however, have pointed out the theoretical inaccuracies involved in
scaling this situation down to the size of many larvae (1-2 mm in
length). “ Firstly, the distance separating its (the larva’s) sense organs
would be so small in comparison with the scale of chemical gradients
in the fluid that concentration differences are not likely to be detectable.
Secondly, a chemical diffusing from a small surface area into a moving
fluid (sea water) gives rise to a sharp gradient of concentration near the
boundary layer where the movement of fluid is restricted by the solid
surface (Crisp, 1966), but outside this boundary layer the concentration
and its gradient are small. This boundary layer is unlikely to reach a
thickness exceeding the dimensions of the larva. Thus for a very small
animal situated at a distance greater than its own dimensions from a
source of diffusing material both the above factors operate, so that the
chemical gradient could not be detected and a directional response
would be impossible.” Perhaps this last statement is a little dogmatic,
but they were able to prove in a later paper (Crisp and Meadows, 1963)
that cyprids recognize the settling factor responsible for their gregariousness only when it is adsorbed to a surface. They started from the
observation that even when extract treated surfaces were placed very
close to untreated ones there was no spread of settlement from one to
the other, and they then offered cyprids a choice of extract treated and
untreated panels in duplicate dishes. In one dish the cyprids were
resuspended in sea water, and in the other they were resuspended in the
same extract that had previously been used to treat the extract treated
panels (this particular extract had been made up in sea water and the
cyprids behaved entirely normally in it). The cyprids distinguished
equally well between the extract treated and the untreated panels both
in sea water and in extract. The only way that cyprids could have
responded in the latter instance was to the settling factor as an
adsorbed layer on the surface of the panels, since they were surrounded
by exactly the same concentration and there would be therefore no