324
P. S. MEADOWS AND J. I. O.UPBELL
Apart from the rather special example of the Teredo larvae, nothing
is known of exactly how or why larvae settle on certain kinds of primary
films, of the relative importance of the different film constituents, or
of the way in which the film is formed. We have no idea whether mobile
adult invertebrates in the sea respnnd to the film, and what is really most
surprising, no evidence at all, apart from somewhat circumstantial
evidence presented by Marzolf (1966), of any freshwater invertebrate
responding to primary films on flat surfaces or detecting the microbial
fauna of muds and sands.
a. Food selection
It goes without saying that almost all animals select the food they
eat, and for an animal that moves around this means searching for food
which will in turn lead animals to aggregate where suitable food is
abundant.
Firstly we will consider the food preferences of marine and freshwater planktonic invertebrates. A number of marine calanoid copepods
and larvae show clear preferences for particular species of diatom
(Harvey, 1937), feed preferentially on larger as opposed to smaller
phyto- and zoo-plankton (Mullin, 1963) or on zoo- rather than phytoplankton (Haq, 1967), or avoid eating their own young (Mullin and
Brooks, 1967). The veliger larvae of the marine gastropod Nassarius
obsoletus, when presented with a mixture of two species of diatoms and
one green alga (Cyclotella, Phaeiductylum and Dunaliella) usually prefer
Cyclotella and always avoid eating Dumliella (Paulson and Scheltema,
1968). These authors say their " experiments do not reveal how the
larvae select their choice of food, whether it be by size, concentration,
or chemotactic sense ". The gymnosomatous pteropod molluscs Clione
limacina (Phipps) and Pneumodermopsis paucidens (Boas), both
carnivorous species, are highly selective in the food they eat, and will
only accept certain species of thecosomatous pteropods when offered a
choice (Sentz-Braconnot, 1965; Lalli, 1970). Their feeding responses
are elicited solely after direct contact with their prey, and so do not
depend on a distance chemical sense. Any gymnosome aggregations
resulting from feeding must therefore be caused not by movement
towards areas where there are thecosomes, but by gymnosomes h d i n g
thecosomes by chance and then remaining in that area. The experimental evidence for food selection by freshwater planktonic animals is
more sparse. Burns (1969) fed two species of Daphnia with a suspension of different sized spherical plastic beads mixed with a yeast; the
size distribution of beads the animals ingested differed significantly
from the size distribution of those in suspension, but since the yeast was
P. S. MEADOWS AND J. I. O.UPBELL
Apart from the rather special example of the Teredo larvae, nothing
is known of exactly how or why larvae settle on certain kinds of primary
films, of the relative importance of the different film constituents, or
of the way in which the film is formed. We have no idea whether mobile
adult invertebrates in the sea respnnd to the film, and what is really most
surprising, no evidence at all, apart from somewhat circumstantial
evidence presented by Marzolf (1966), of any freshwater invertebrate
responding to primary films on flat surfaces or detecting the microbial
fauna of muds and sands.
a. Food selection
It goes without saying that almost all animals select the food they
eat, and for an animal that moves around this means searching for food
which will in turn lead animals to aggregate where suitable food is
abundant.
Firstly we will consider the food preferences of marine and freshwater planktonic invertebrates. A number of marine calanoid copepods
and larvae show clear preferences for particular species of diatom
(Harvey, 1937), feed preferentially on larger as opposed to smaller
phyto- and zoo-plankton (Mullin, 1963) or on zoo- rather than phytoplankton (Haq, 1967), or avoid eating their own young (Mullin and
Brooks, 1967). The veliger larvae of the marine gastropod Nassarius
obsoletus, when presented with a mixture of two species of diatoms and
one green alga (Cyclotella, Phaeiductylum and Dunaliella) usually prefer
Cyclotella and always avoid eating Dumliella (Paulson and Scheltema,
1968). These authors say their " experiments do not reveal how the
larvae select their choice of food, whether it be by size, concentration,
or chemotactic sense ". The gymnosomatous pteropod molluscs Clione
limacina (Phipps) and Pneumodermopsis paucidens (Boas), both
carnivorous species, are highly selective in the food they eat, and will
only accept certain species of thecosomatous pteropods when offered a
choice (Sentz-Braconnot, 1965; Lalli, 1970). Their feeding responses
are elicited solely after direct contact with their prey, and so do not
depend on a distance chemical sense. Any gymnosome aggregations
resulting from feeding must therefore be caused not by movement
towards areas where there are thecosomes, but by gymnosomes h d i n g
thecosomes by chance and then remaining in that area. The experimental evidence for food selection by freshwater planktonic animals is
more sparse. Burns (1969) fed two species of Daphnia with a suspension of different sized spherical plastic beads mixed with a yeast; the
size distribution of beads the animals ingested differed significantly
from the size distribution of those in suspension, but since the yeast was