T H E BIOLOGY OF ASCIDIANS
45
the destruction of an abundant population of Ascidia sp. following disturbance of the sea-bed and increased turbidity.
The importance of the silt-content is indicated by the presence of
fewer ascidians in silty than clear water, on Scottish shores (Lewis and
Powell, 1960). Moreover, the effect was more marked in the small
compound forms, than in the large Ascidia mentula which withstood
turbid conditions.
Sediments rich in organic matter may favour certain species by
ensuring adequate food in the overlying water, and the presence of
Microcosmus sulcatus is said to indicate such conditions (Carpine, 1964).
5. Light
The influence of light on ascidian larvae-r
rather the spacial
variation in light intensity-is well known (see Section III), and is one
of the principal factors controlling micro-distribution. C. Monniot
(1965a) reports that the distribution of Microcosmus sabatieri in the
Gulf of Lions is strictly related t o the amount of illumination received,
so that on the open sea-bed the upper limit is 15 m, on vertical surfaces
facing south 7-8 m, and facing north 3-4 m, and on under surfaces
50cm. A survey of the Gullmar Fjord, Sweden, showed that Ciom
intestimlis occurs sparingly on shallow horizontal surfaces, and about
75% of all specimens live on steeply sloping rock walls and overhanging
ledges and in caves (Dybern, 1963). It is the larval choice of shaded
places for settlement which determines this pattern of distribution, but
although little is known of the effect of light on the adults, the development of strong pigmentation in individuals living in shallow unshaded
areas (Dybern, 1963) suggests a need for protection from strong light.
This is likely to be more important in species with a thin transparent
test than in those having a thick opaque test, and could have been one
of the factors favouring the evolution of the ocellus and the associated
shadow reflex of the larva which facilitates the choice of shaded areas.
The direction of illumination may affect the orientation of the adult
body. Ciona intestinalis has been observed to grow in aquaria so that
the long axis of the body and more especially the siphons, are aligned
with the direction of the prevailing lighting (Millar, 1953a). I n nature
the resulting posture will generally present the oral siphon to falling
food particles and may also be useful in ensuring that the body grows
up from the substratum.
6. Biotic factors
The gregarious settlement of marine invertebrate larvae has recently
received some attention (see Thorson, 1964 for references), and a few
45
the destruction of an abundant population of Ascidia sp. following disturbance of the sea-bed and increased turbidity.
The importance of the silt-content is indicated by the presence of
fewer ascidians in silty than clear water, on Scottish shores (Lewis and
Powell, 1960). Moreover, the effect was more marked in the small
compound forms, than in the large Ascidia mentula which withstood
turbid conditions.
Sediments rich in organic matter may favour certain species by
ensuring adequate food in the overlying water, and the presence of
Microcosmus sulcatus is said to indicate such conditions (Carpine, 1964).
5. Light
The influence of light on ascidian larvae-r
rather the spacial
variation in light intensity-is well known (see Section III), and is one
of the principal factors controlling micro-distribution. C. Monniot
(1965a) reports that the distribution of Microcosmus sabatieri in the
Gulf of Lions is strictly related t o the amount of illumination received,
so that on the open sea-bed the upper limit is 15 m, on vertical surfaces
facing south 7-8 m, and facing north 3-4 m, and on under surfaces
50cm. A survey of the Gullmar Fjord, Sweden, showed that Ciom
intestimlis occurs sparingly on shallow horizontal surfaces, and about
75% of all specimens live on steeply sloping rock walls and overhanging
ledges and in caves (Dybern, 1963). It is the larval choice of shaded
places for settlement which determines this pattern of distribution, but
although little is known of the effect of light on the adults, the development of strong pigmentation in individuals living in shallow unshaded
areas (Dybern, 1963) suggests a need for protection from strong light.
This is likely to be more important in species with a thin transparent
test than in those having a thick opaque test, and could have been one
of the factors favouring the evolution of the ocellus and the associated
shadow reflex of the larva which facilitates the choice of shaded areas.
The direction of illumination may affect the orientation of the adult
body. Ciona intestinalis has been observed to grow in aquaria so that
the long axis of the body and more especially the siphons, are aligned
with the direction of the prevailing lighting (Millar, 1953a). I n nature
the resulting posture will generally present the oral siphon to falling
food particles and may also be useful in ensuring that the body grows
up from the substratum.
6. Biotic factors
The gregarious settlement of marine invertebrate larvae has recently
received some attention (see Thorson, 1964 for references), and a few