THE BIOLOGY OF ASCIDIANS
35
for 0.26% of the fauna by number of individuals and 2349% of the
total dry weight.
One of the few estimates of the standing crop of ascidians on natural
hard substrata was made by Gutikrrez and Lay (1965) who recorded
Pyura chilensis at a density of 320 per m2 with a mean individual weight
of 350 g. Under favourable natural conditions Ciona intestinalis may
attain population densities of 1 500-5 000 per m2 (Dybern, 1963), and
Ascidiella scabra (Miiller) at Bohuslan, Sweden, 1 000 per m2 (Dybern,
1969b). C. Monniot (19654 records Microcosmus vulgaris at a density of
several hundred per m2 on stones dredged from 120 m in the Gulf of
Lions.
It is evident that the group may form an important component in
the fauna of some loose and solid substrates, and may sometimes play
a significant or even dominant part in the economy of coastal waters.
B. Changes in populations
Estimations of population density and biomass should take account
of seasonal and year-to-year variations. Seasonal changes in the
structure of the population depend on reproduction, growth and mortality, and have been referred to in sections 111 and IV, but little is
known regarding the way in which ascidian populations vary over a
period of years. Fouque and Franc (1953) noted that Ascidia mentula,
Ascidiella aspersa and Ciona intestinalis decreased markedly on the
shore near Dinard, France, within the space of one year, whereas
Polysyncraton lacuzei Giard did not. In another part of the same region
Molgula manhattensis declined from a very abundant species to a rare
one. No explanation was found for these events but it is sornetimcs
possible to relate faunistic changes with environmental changes, as for
example when several species of ascidian penetrated to parts of the
Rance estuary in France following dry summers (Fischer-Piette and
Gaillard, 1950). And the appearance of Molgula manhattensis in the
Caen canal was obviously made possible only by the increased salinity
following the influx of sea water (Durchon, 1948). Some instances of
this kind probably represent short-term fluctuations and species are
generally in a state of dynamic equilibrium ; in other cases there may be
a more lasting change. Carlisle (1954a) recorded a continued rise in the
numbers of Trididemnum niveum (Giard) at Plymouth between 1951
and 1954 until in some places it had become the most abundant
ascidian. He suggested that the species may have arrived from the
French coast with the large brown alga Laminuria ochroleuca De La
Pylaie and be increasing along with it. Certainly a few species, having
found their way to new areas either by natural means or through human
35
for 0.26% of the fauna by number of individuals and 2349% of the
total dry weight.
One of the few estimates of the standing crop of ascidians on natural
hard substrata was made by Gutikrrez and Lay (1965) who recorded
Pyura chilensis at a density of 320 per m2 with a mean individual weight
of 350 g. Under favourable natural conditions Ciona intestinalis may
attain population densities of 1 500-5 000 per m2 (Dybern, 1963), and
Ascidiella scabra (Miiller) at Bohuslan, Sweden, 1 000 per m2 (Dybern,
1969b). C. Monniot (19654 records Microcosmus vulgaris at a density of
several hundred per m2 on stones dredged from 120 m in the Gulf of
Lions.
It is evident that the group may form an important component in
the fauna of some loose and solid substrates, and may sometimes play
a significant or even dominant part in the economy of coastal waters.
B. Changes in populations
Estimations of population density and biomass should take account
of seasonal and year-to-year variations. Seasonal changes in the
structure of the population depend on reproduction, growth and mortality, and have been referred to in sections 111 and IV, but little is
known regarding the way in which ascidian populations vary over a
period of years. Fouque and Franc (1953) noted that Ascidia mentula,
Ascidiella aspersa and Ciona intestinalis decreased markedly on the
shore near Dinard, France, within the space of one year, whereas
Polysyncraton lacuzei Giard did not. In another part of the same region
Molgula manhattensis declined from a very abundant species to a rare
one. No explanation was found for these events but it is sornetimcs
possible to relate faunistic changes with environmental changes, as for
example when several species of ascidian penetrated to parts of the
Rance estuary in France following dry summers (Fischer-Piette and
Gaillard, 1950). And the appearance of Molgula manhattensis in the
Caen canal was obviously made possible only by the increased salinity
following the influx of sea water (Durchon, 1948). Some instances of
this kind probably represent short-term fluctuations and species are
generally in a state of dynamic equilibrium ; in other cases there may be
a more lasting change. Carlisle (1954a) recorded a continued rise in the
numbers of Trididemnum niveum (Giard) at Plymouth between 1951
and 1954 until in some places it had become the most abundant
ascidian. He suggested that the species may have arrived from the
French coast with the large brown alga Laminuria ochroleuca De La
Pylaie and be increasing along with it. Certainly a few species, having
found their way to new areas either by natural means or through human