THE BIOLOGY OF WOOD-BORING TEREDINID MOLLUSCS
447
caused by reduced inflow of fresh water or reduced rainfall causing an
increase in salinity (Baumhauer, 1860; Vrolik et al., 1860; Harting,
1862), high temperature and dry summers (Hoeven, 1861) or an unusual
increase in water temperature (Dons, 1940, 1949).
A. The inJluence of temperature on the activity and distribution of
shipworms
Temperature is a major influence on the activities and distribution
of shipworms and is a limiting factor in growth, reproduction and
distribution. There is a complex correlation between the biological
effects of temperature and salinity, the former can modify the effects
of the latter and change the salinity range of an organism (Kinne, 1963).
The temperature cycle is a mere reflection of the annual fluctuation in
mlar radiation and this factor affects animals in other ways as well,
such as influencing photosynthesis of the phytoplankton. Temperature
also influences precipitation, thereby affecting salinity. Species living
in different geographical locations are adapted to different ranges of
temperature and each species usually has an optimum temperature.
There are maximum and minimum temperatures as well. Lethal extremes vary greatly from species to species. Temperatures that are too
low for some species may be favourable or tolerable for others. The
minimum, optimum and maximum temperatures all vary with species
and even that of the same may change through periods of acclimatization. These enable different species to establish themselves in
every available life zone. Some species are characteristically restricted
to the cold waters of higher latitudes, others are typically inhabitants
of subtropical areas while a majority are denizens of the tropical regions
illustrating the general rule that tropical biotopes are typically rich in
species. Thus along the coasts of Norway 3 species of shipworms occur
(Nair, 1959), 5 species in the Mediterranean, 7 species from Hawaii and
Midway Islands, 16 species from the Philippine Islands (see Turner,
1966) and 22 species from the east and west coasts of India (Nair, 1968).
Limiting winter temperatures and minimal summer heat determine
boundaries of distribution towards higher latitudes. Limiting maximal
temperatures for survival occur in summer and minimal temperatures for
reproduction and growth in winter, these determining the boundaries
of distribution towards the equator. Uniformly high tropical temperatures hasten metabolic activities and accelerate growth rates,
leading to the attainment of sexual maturity at a surprisingly early
age. Some species breed almost continuously and several generations are produced, in rapid succession within a single year. The
speeding up of life histories favours the acceleration of the evolutionary
447
caused by reduced inflow of fresh water or reduced rainfall causing an
increase in salinity (Baumhauer, 1860; Vrolik et al., 1860; Harting,
1862), high temperature and dry summers (Hoeven, 1861) or an unusual
increase in water temperature (Dons, 1940, 1949).
A. The inJluence of temperature on the activity and distribution of
shipworms
Temperature is a major influence on the activities and distribution
of shipworms and is a limiting factor in growth, reproduction and
distribution. There is a complex correlation between the biological
effects of temperature and salinity, the former can modify the effects
of the latter and change the salinity range of an organism (Kinne, 1963).
The temperature cycle is a mere reflection of the annual fluctuation in
mlar radiation and this factor affects animals in other ways as well,
such as influencing photosynthesis of the phytoplankton. Temperature
also influences precipitation, thereby affecting salinity. Species living
in different geographical locations are adapted to different ranges of
temperature and each species usually has an optimum temperature.
There are maximum and minimum temperatures as well. Lethal extremes vary greatly from species to species. Temperatures that are too
low for some species may be favourable or tolerable for others. The
minimum, optimum and maximum temperatures all vary with species
and even that of the same may change through periods of acclimatization. These enable different species to establish themselves in
every available life zone. Some species are characteristically restricted
to the cold waters of higher latitudes, others are typically inhabitants
of subtropical areas while a majority are denizens of the tropical regions
illustrating the general rule that tropical biotopes are typically rich in
species. Thus along the coasts of Norway 3 species of shipworms occur
(Nair, 1959), 5 species in the Mediterranean, 7 species from Hawaii and
Midway Islands, 16 species from the Philippine Islands (see Turner,
1966) and 22 species from the east and west coasts of India (Nair, 1968).
Limiting winter temperatures and minimal summer heat determine
boundaries of distribution towards higher latitudes. Limiting maximal
temperatures for survival occur in summer and minimal temperatures for
reproduction and growth in winter, these determining the boundaries
of distribution towards the equator. Uniformly high tropical temperatures hasten metabolic activities and accelerate growth rates,
leading to the attainment of sexual maturity at a surprisingly early
age. Some species breed almost continuously and several generations are produced, in rapid succession within a single year. The
speeding up of life histories favours the acceleration of the evolutionary