34
R. H. MILLAIt
In sublittoral arcas species of the family Molgulidae are sometimes
strikingly prolific, and Dragovich and Kelly (1964) found molgulids to
be the most numerous organisms in trawlcd samples in Tampa Bay,
Florida. Abbott (1951) recorded a mass of Bostrichobranchus digonas
Abbott cast up on a Florida beach and forming a belt 4-6 in wide and
about 100yd long, indicating the abundance of the species in an adjacent sublittoral arca. And at Point Barrow, Alaska one short haul
brought up a dredge one third filled with thc molgulid Rhizomolgula
globularis (Pallas), and containing little else (MacGinitie, 1955). Large
areas of the bottom in the shallow inshore parts of the Gulf of Naples
are " literally covered " with Ascidiella aspersa, which is there the
dominant animal (Parenzan, 1959), and in the harbour of Genoa
ascidians are abundant with Ciom intestinulis a very important species
in the quantitative composition of the sessile benthos (Relini, 1962).
The frequent descriptions of escidians as " abundant " or " in great
numbers " testify to the significant role which they must play in many
parts of the sea.
Nevertheless, rather few measurements have been madc of their
abundance. Quantitative studies of the sublittoral fauna are more
easily made on soft than on hard substrata, and most of the data have
resulted from the use of grabs applied to sandy and muddy areas. A
benthic survey of Scottish and adjacent waters indicated maximum
ascidian densities in some places of about 100 per m2 for Eugyra arenosa
(Alder and Hancock) and Polycarpa Jibrosa (Stimpson) and of nearly
200 per m2 for Pelonaia corrugata Forbes and Goodsir (Thompson, 1930 ;
1931 ; 1932 ; 1934). During an investigation of the fauna of brackish
ponds in Japan, Kikuchi (1964) estimated the biomass of different
animal groups and the figures show that ascidians could constitute about
an eighth of the total benthic crop. Somewhat similar figures were
obtained by Pdrhs (1967) for a typical coastal detritus fauna in the
Mediterranean, where ascidians accounted for 1 1.5y0, 10.Syo and 0% of
the total dry weight of the bottom fauna at three stations, although in
numbers of specimens they represented only 0.7%) 2.6% and 0%
respectively. At two stations on a muddy detritus bottom 60.3% and
19.3% of the total fauna by dry weight were ascidians and 5.3% and
2.2% of the total number of specimens. The discrepancy between
estimates of importance using dry weight and numbers is, of course,
explained by the relatively large size of the ascidians compared to that
of other animals. Sanders (1960) listed 76 species in a soft-bottom community in Buzzards Bay, U.S.A., and the only ascidian, Bostrichobranchus pilularis (Verrill), ranked sixteenth in order of numerical
abundance but second in order of dry weight. This species accounted
R. H. MILLAIt
In sublittoral arcas species of the family Molgulidae are sometimes
strikingly prolific, and Dragovich and Kelly (1964) found molgulids to
be the most numerous organisms in trawlcd samples in Tampa Bay,
Florida. Abbott (1951) recorded a mass of Bostrichobranchus digonas
Abbott cast up on a Florida beach and forming a belt 4-6 in wide and
about 100yd long, indicating the abundance of the species in an adjacent sublittoral arca. And at Point Barrow, Alaska one short haul
brought up a dredge one third filled with thc molgulid Rhizomolgula
globularis (Pallas), and containing little else (MacGinitie, 1955). Large
areas of the bottom in the shallow inshore parts of the Gulf of Naples
are " literally covered " with Ascidiella aspersa, which is there the
dominant animal (Parenzan, 1959), and in the harbour of Genoa
ascidians are abundant with Ciom intestinulis a very important species
in the quantitative composition of the sessile benthos (Relini, 1962).
The frequent descriptions of escidians as " abundant " or " in great
numbers " testify to the significant role which they must play in many
parts of the sea.
Nevertheless, rather few measurements have been madc of their
abundance. Quantitative studies of the sublittoral fauna are more
easily made on soft than on hard substrata, and most of the data have
resulted from the use of grabs applied to sandy and muddy areas. A
benthic survey of Scottish and adjacent waters indicated maximum
ascidian densities in some places of about 100 per m2 for Eugyra arenosa
(Alder and Hancock) and Polycarpa Jibrosa (Stimpson) and of nearly
200 per m2 for Pelonaia corrugata Forbes and Goodsir (Thompson, 1930 ;
1931 ; 1932 ; 1934). During an investigation of the fauna of brackish
ponds in Japan, Kikuchi (1964) estimated the biomass of different
animal groups and the figures show that ascidians could constitute about
an eighth of the total benthic crop. Somewhat similar figures were
obtained by Pdrhs (1967) for a typical coastal detritus fauna in the
Mediterranean, where ascidians accounted for 1 1.5y0, 10.Syo and 0% of
the total dry weight of the bottom fauna at three stations, although in
numbers of specimens they represented only 0.7%) 2.6% and 0%
respectively. At two stations on a muddy detritus bottom 60.3% and
19.3% of the total fauna by dry weight were ascidians and 5.3% and
2.2% of the total number of specimens. The discrepancy between
estimates of importance using dry weight and numbers is, of course,
explained by the relatively large size of the ascidians compared to that
of other animals. Sanders (1960) listed 76 species in a soft-bottom community in Buzzards Bay, U.S.A., and the only ascidian, Bostrichobranchus pilularis (Verrill), ranked sixteenth in order of numerical
abundance but second in order of dry weight. This species accounted