TEE BIOLOGY OF IWCIDIANS
39
tuft of basal filaments which anchor the animal (Fig. 16c and d). It
may be more accurate to say that species with a stalk are able to take
advantage of soft deposits, and that stalked rock-dwelling forms were
pre-adapted for this habitat. Amongst the Pyuridae the short-stalked
Pyura legumen Lesson living on hard inshore substrata may indicate
the ancestral form from which P . bouvetemis (Michaelsen) evolved. In
P . bouvetemis the development of filaments appears to be a response to
the kind of substratum, for some specimens instead have a small basal
plate (Millar, 1960), more suitable for attachment to solid objects. It
haa been suggested that the stalk of P . bouvetemis may be too thin and
flexible to support the body off the substratum (Millar, 1960), but Kott
(1969) contests this view, and cites underwater photographs taken by
the USNS " Eltanin " showing some specimens of the related P .
georgianu (Michaelsen) with the body well above the sea-bed. Further
striking examples of stalked species are the molgulid Eugyra aernbaeckae Millar, 1960, an Antarctic ascidian living in depths of 55-400 m,
and the abyssal genus Culeolw.
The third way in which ascidians have become adapted to life on
soft bottoms is the more radical one of becoming interstitial animals.
The discovery of interstitial ascidians was made by Weinstein (1961),
who described Psammostyela delamurei, a small sand-dwelling styelid
from shallow water in the Mediterranean. Since then a number of other
species have been found in European waters (see F. Monniot, 1965,1966
for references) and one known species (Heterostigma separ drnbackChristie-Linde, 1924) has also been added to this ecological group
(F. Monniot, 1966). Despite the systematic diversity-interstitial
species are now known from the families Ascidiidae, Corellidae, Styelidae, Pyuridae and Molgulidae-certain features are held in common.
F. Monniot (1966) has listed these as : small body size ; shape of body
(flattened or, more usually, spindle-shaped), lack of pigmentation,
mobility, incubation of embryos and a kind of neoteny. Mobility is perhaps the most noteworthy character. It results from a modification of
the muscular activity which in attached ascidians merely achieves contraction of the body, but enables these specialized forms to creep
amongst sand grains (Fig. 16e and f).
Many species living on a loose substratum are neither interstitial nor
stalked, but nevertheless are intimately affected by the nature of the
sediment. Glkmarec and Monniot (1966) found a close relationship between the distribution of ascidian species and the granulometric composition of the soft sea-bed off Brittany, France, and concluded that
ascidians are good ecological indicators of the nature of the sediment.
The ascidian fauna of the Patagonian Shelf affords further evidence of
39
tuft of basal filaments which anchor the animal (Fig. 16c and d). It
may be more accurate to say that species with a stalk are able to take
advantage of soft deposits, and that stalked rock-dwelling forms were
pre-adapted for this habitat. Amongst the Pyuridae the short-stalked
Pyura legumen Lesson living on hard inshore substrata may indicate
the ancestral form from which P . bouvetemis (Michaelsen) evolved. In
P . bouvetemis the development of filaments appears to be a response to
the kind of substratum, for some specimens instead have a small basal
plate (Millar, 1960), more suitable for attachment to solid objects. It
haa been suggested that the stalk of P . bouvetemis may be too thin and
flexible to support the body off the substratum (Millar, 1960), but Kott
(1969) contests this view, and cites underwater photographs taken by
the USNS " Eltanin " showing some specimens of the related P .
georgianu (Michaelsen) with the body well above the sea-bed. Further
striking examples of stalked species are the molgulid Eugyra aernbaeckae Millar, 1960, an Antarctic ascidian living in depths of 55-400 m,
and the abyssal genus Culeolw.
The third way in which ascidians have become adapted to life on
soft bottoms is the more radical one of becoming interstitial animals.
The discovery of interstitial ascidians was made by Weinstein (1961),
who described Psammostyela delamurei, a small sand-dwelling styelid
from shallow water in the Mediterranean. Since then a number of other
species have been found in European waters (see F. Monniot, 1965,1966
for references) and one known species (Heterostigma separ drnbackChristie-Linde, 1924) has also been added to this ecological group
(F. Monniot, 1966). Despite the systematic diversity-interstitial
species are now known from the families Ascidiidae, Corellidae, Styelidae, Pyuridae and Molgulidae-certain features are held in common.
F. Monniot (1966) has listed these as : small body size ; shape of body
(flattened or, more usually, spindle-shaped), lack of pigmentation,
mobility, incubation of embryos and a kind of neoteny. Mobility is perhaps the most noteworthy character. It results from a modification of
the muscular activity which in attached ascidians merely achieves contraction of the body, but enables these specialized forms to creep
amongst sand grains (Fig. 16e and f).
Many species living on a loose substratum are neither interstitial nor
stalked, but nevertheless are intimately affected by the nature of the
sediment. Glkmarec and Monniot (1966) found a close relationship between the distribution of ascidian species and the granulometric composition of the soft sea-bed off Brittany, France, and concluded that
ascidians are good ecological indicators of the nature of the sediment.
The ascidian fauna of the Patagonian Shelf affords further evidence of