OCEAN
249
'
These seasonal changes in the quantity ofplankton are
.
reected in the growth of all the larger animals in» the sea.
A very great number of the. small invertebrates living on the
sea
oor
might be termed annuals.
Spending their
early life in the drifting plankton communityin the spring,
they soon settle to the bottom and reach matUrity in the
course
of the summer.
The countless— young that never
survive to maturity have nourished other growing animals,
which in turn have fallen prey to the larger creatures. ‘
It is probably largely for this reason that we nd that most
of the sh in our northern waters make the greater part of
their growth during the summer.
Plankton is then present
.
in greatest quantity foipelagic sh. Such as the
herring and the mackerel which feed directly on it.
Then,
too, at its greatest is the toll of life among the young
molluscs and crustacea at the hands-of Such bottom—living
‘
sh as the plaice.
It is natural that during the period
of greatest feeding most growth takes place and this is
shown very distinctly on the scales of many sh.
A reliable
index to the age of these shes is a‘orded bythe number
of wide and narrow zones shown on their scales, the wide
:
zones
corresponding to the period of greatest growth
which generally takes place in the summer,
and the marrow
'
zones to the period of poor growth in the winter.
.
-
_
,_
Such then is the cycle of life in our northern seas.
But
remains to be mentioned how the animal and plant
life inthe tr0pical waters varies with the time of year.,
.
Here there
not
the marked temperature changes
that occur in temperate seas, ‘n‘either does the strength of
the sun’s light vary to such a marked extent. *At__ any
,
rate it is/ probable that at all periods ofthe year it is
,
Suciently strong to a110w active growth amongstthe drift—
_
ing plants; On account of this uniformity in temperature
and light in some regions Such marked changes in the
249
'
These seasonal changes in the quantity ofplankton are
.
reected in the growth of all the larger animals in» the sea.
A very great number of the. small invertebrates living on the
sea
oor
might be termed annuals.
Spending their
early life in the drifting plankton communityin the spring,
they soon settle to the bottom and reach matUrity in the
course
of the summer.
The countless— young that never
survive to maturity have nourished other growing animals,
which in turn have fallen prey to the larger creatures. ‘
It is probably largely for this reason that we nd that most
of the sh in our northern waters make the greater part of
their growth during the summer.
Plankton is then present
.
in greatest quantity foipelagic sh. Such as the
herring and the mackerel which feed directly on it.
Then,
too, at its greatest is the toll of life among the young
molluscs and crustacea at the hands-of Such bottom—living
‘
sh as the plaice.
It is natural that during the period
of greatest feeding most growth takes place and this is
shown very distinctly on the scales of many sh.
A reliable
index to the age of these shes is a‘orded bythe number
of wide and narrow zones shown on their scales, the wide
:
zones
corresponding to the period of greatest growth
which generally takes place in the summer,
and the marrow
'
zones to the period of poor growth in the winter.
.
-
_
,_
Such then is the cycle of life in our northern seas.
But
remains to be mentioned how the animal and plant
life inthe tr0pical waters varies with the time of year.,
.
Here there
not
the marked temperature changes
that occur in temperate seas, ‘n‘either does the strength of
the sun’s light vary to such a marked extent. *At__ any
,
rate it is/ probable that at all periods ofthe year it is
,
Suciently strong to a110w active growth amongstthe drift—
_
ing plants; On account of this uniformity in temperature
and light in some regions Such marked changes in the