4
a. E. M&LAR
been measured using various experimental methods, by Hecht (1916),
Jrargensen (1949, 1952), Goldberg et al. (1951), Hoyle (1953) and Cnrlisle
(1966). The results vary considerably. Thus Hecht estimated 80 ml/h
per g wet weight of animal in the case of Ascidia atra Lesueur, and
Jrargensen’s value for Molgula was 540ml/h per g wet weight. It is
probable that performance under favourable natural conditions will be
higher than in experiments, particularly in those involving considerable
interference with the animal, such as Hecht’s method using a tube
inserted into the siphon. Hoyle’s (1953) criticism of Hecht’s work is
partly invalid, since he failed t o realize that Hecht measured particle
velocity in the inhalent, not the exhalent, current. Hoyle believed that
ciliary currents would provide insufficient food and oxygen. He
measured the water exchange resulting from spontaneous rhythmic
contractions in Phallusia mammillata (Cuvier) and concluded that these
introduced much more water than the ciliary current. The process he
visualized consists of water being drawn into the branchial sac during
relaxation of the body, and the water being filtered on the branchial
walls with the aid of ciliary currents. However, Jnrrgensen (1955) did
not accept this idea and calculated that in Ciona at least 30 times as
much water is transported by ciliary action as by rhythmic contractions.
One advantage claimed for feeding by body contractions is the ability
to regulate the rate of feeding by varying the frequency of contraction,
and Hoyle found the frequency to increase at lower food concentrations.
It is evident that the role of spontaneous contraction needs further
investigation, especially in relation to feeding. Indirect evidence for the
adequacy of ciliary currents is based on the available particulate
organic matter in the sea, and Jnrrgensen (1955) concluded that ascidians
can meet their needs from this source.
3. Food
Despite our knowledge of the mechanism of feeding, little is known
of the food itself. Phytoplankton and organic particles in suspension
apparently constitute the bulk of the food of many species. For instance, in Paramolgula gregaria (Lesson), a mainly Subantarctic species
which attains a length of over 20cm, the gut was found to contain
principally unicellular planktonic algae and diatoms, and only a little
sand and animal remains (Millar, 1960). The waters of the Patagonian
Shelf, where the specimens were collected, are rich in phytoplankton
which, not surprisingly, constitutes the food of even such a large-bodied
species. And in Microcosmus sulcatus (Coquebert) the branchial sac has
been found to contain organisms (bacteria, diatoms and radiolarians)
characteristic of the water immediahly above the substratum (Costa,
a. E. M&LAR
been measured using various experimental methods, by Hecht (1916),
Jrargensen (1949, 1952), Goldberg et al. (1951), Hoyle (1953) and Cnrlisle
(1966). The results vary considerably. Thus Hecht estimated 80 ml/h
per g wet weight of animal in the case of Ascidia atra Lesueur, and
Jrargensen’s value for Molgula was 540ml/h per g wet weight. It is
probable that performance under favourable natural conditions will be
higher than in experiments, particularly in those involving considerable
interference with the animal, such as Hecht’s method using a tube
inserted into the siphon. Hoyle’s (1953) criticism of Hecht’s work is
partly invalid, since he failed t o realize that Hecht measured particle
velocity in the inhalent, not the exhalent, current. Hoyle believed that
ciliary currents would provide insufficient food and oxygen. He
measured the water exchange resulting from spontaneous rhythmic
contractions in Phallusia mammillata (Cuvier) and concluded that these
introduced much more water than the ciliary current. The process he
visualized consists of water being drawn into the branchial sac during
relaxation of the body, and the water being filtered on the branchial
walls with the aid of ciliary currents. However, Jnrrgensen (1955) did
not accept this idea and calculated that in Ciona at least 30 times as
much water is transported by ciliary action as by rhythmic contractions.
One advantage claimed for feeding by body contractions is the ability
to regulate the rate of feeding by varying the frequency of contraction,
and Hoyle found the frequency to increase at lower food concentrations.
It is evident that the role of spontaneous contraction needs further
investigation, especially in relation to feeding. Indirect evidence for the
adequacy of ciliary currents is based on the available particulate
organic matter in the sea, and Jnrrgensen (1955) concluded that ascidians
can meet their needs from this source.
3. Food
Despite our knowledge of the mechanism of feeding, little is known
of the food itself. Phytoplankton and organic particles in suspension
apparently constitute the bulk of the food of many species. For instance, in Paramolgula gregaria (Lesson), a mainly Subantarctic species
which attains a length of over 20cm, the gut was found to contain
principally unicellular planktonic algae and diatoms, and only a little
sand and animal remains (Millar, 1960). The waters of the Patagonian
Shelf, where the specimens were collected, are rich in phytoplankton
which, not surprisingly, constitutes the food of even such a large-bodied
species. And in Microcosmus sulcatus (Coquebert) the branchial sac has
been found to contain organisms (bacteria, diatoms and radiolarians)
characteristic of the water immediahly above the substratum (Costa,