82
E. NAYLOR
temperate marine waters are better documented. As in freshwater localities (van W e t , 1957; Coutant, 1962; Mann, 1965) there is evidence that
some organisms show an increase in numbers and their seasonal cycles of
abundance are shifted or extended. In Britain, Crisp and Molesworth
(1951), for instance, recorded breeding and settlement of Balanus
amphitrite from March to September in artificially heated areas, and the
absence of breeding for the same species within the normal range of sea
temperatures (see also Fig. 5 ) . Another barnacle, Elminius modestus,
also showed a more extended breeding period in heated parts of
Southampton Water (Pannell et al., 1962), whilst in the same area a
predominantly low latitude estuarine copepod Acartia tonsa Dana has
increased in abundance (Conover, 1957 ; Raymont, 1964) and a related
warm water species Acartia grani G. 0. Sars has appeared for the first
time (Raymont, 1964). In addition, Ansell (1963b), Ansell and Loosemore (1963) and Ansell et al. (1964a) have shown that Venus mercenaria
(L.) in heated localities in Britain had advanced spawning times,
protracted growing seasons and showed significantly greater total
annual increments in shell length and total weight than specimens found
in unheated areas. More prolonged breeding periods have also been
reported for a number of species, including fouling and boring forms,
in heated docks at Swansea, S. Wales. These include two species of the
bryozoan Bugula (Ryland, 1960), Balanus amphitrite, Corophium
acherusicum A. Costa, Limnoria tripunctata Menzies, Botryllus schlosseri
(Pallas), Diplosoma listerianum (Milne Edwards), Ascidiella aspersa
(0. F. Muller) and Ciona intestinalis (L.) (Naylor, 1965). The patterns
of growth and reproduction of Ciona (Figs. 6 and 7) and Ascidiella
were of particular interest, for normally in Britain these two species
often occur together and reproduce as annuals. At Swansea, however,
they each produced more than one generation each year and in the
warmest parts of the dock competition between the two seemed to be
such that Ascidiella was most abundant in winter and spring whilst
Ciona was more abundant in summer and autumn (Naylor, 1965). The
maximum size reached by Ascidiella was also much smaller than would
be expected in Britain (see p. 70).
The extension of breeding periods and alternation of abundance of
species in this way not only leads to changes in the relative abundance
of different species but also demonstrates that the problem of fouling
on dock structures and ships could be considerably worsened by heating.
For instance, barnacles and mussels appear to have increased in density
in Southampton Water as a result of only slight heating (Pannell et al.,
1962). In Copenhagen Harbour, too, it has been shown that the growth
rate of Mytilus is linearly related to the age of the animals expressed in
E. NAYLOR
temperate marine waters are better documented. As in freshwater localities (van W e t , 1957; Coutant, 1962; Mann, 1965) there is evidence that
some organisms show an increase in numbers and their seasonal cycles of
abundance are shifted or extended. In Britain, Crisp and Molesworth
(1951), for instance, recorded breeding and settlement of Balanus
amphitrite from March to September in artificially heated areas, and the
absence of breeding for the same species within the normal range of sea
temperatures (see also Fig. 5 ) . Another barnacle, Elminius modestus,
also showed a more extended breeding period in heated parts of
Southampton Water (Pannell et al., 1962), whilst in the same area a
predominantly low latitude estuarine copepod Acartia tonsa Dana has
increased in abundance (Conover, 1957 ; Raymont, 1964) and a related
warm water species Acartia grani G. 0. Sars has appeared for the first
time (Raymont, 1964). In addition, Ansell (1963b), Ansell and Loosemore (1963) and Ansell et al. (1964a) have shown that Venus mercenaria
(L.) in heated localities in Britain had advanced spawning times,
protracted growing seasons and showed significantly greater total
annual increments in shell length and total weight than specimens found
in unheated areas. More prolonged breeding periods have also been
reported for a number of species, including fouling and boring forms,
in heated docks at Swansea, S. Wales. These include two species of the
bryozoan Bugula (Ryland, 1960), Balanus amphitrite, Corophium
acherusicum A. Costa, Limnoria tripunctata Menzies, Botryllus schlosseri
(Pallas), Diplosoma listerianum (Milne Edwards), Ascidiella aspersa
(0. F. Muller) and Ciona intestinalis (L.) (Naylor, 1965). The patterns
of growth and reproduction of Ciona (Figs. 6 and 7) and Ascidiella
were of particular interest, for normally in Britain these two species
often occur together and reproduce as annuals. At Swansea, however,
they each produced more than one generation each year and in the
warmest parts of the dock competition between the two seemed to be
such that Ascidiella was most abundant in winter and spring whilst
Ciona was more abundant in summer and autumn (Naylor, 1965). The
maximum size reached by Ascidiella was also much smaller than would
be expected in Britain (see p. 70).
The extension of breeding periods and alternation of abundance of
species in this way not only leads to changes in the relative abundance
of different species but also demonstrates that the problem of fouling
on dock structures and ships could be considerably worsened by heating.
For instance, barnacles and mussels appear to have increased in density
in Southampton Water as a result of only slight heating (Pannell et al.,
1962). In Copenhagen Harbour, too, it has been shown that the growth
rate of Mytilus is linearly related to the age of the animals expressed in