|
There
are hundreds of worm species found on
Cornish beaches, almost all of which
all belong to the animal class Polychaetes.
A definite description of Polychaetes
is almost impossible due to the number
of diverse species. This successful
class has produced a huge range of species
enabling them to be found in every marine
habitat. Each species has evolved and
adapted to perfectly fit their specific
environmental niche.
There
are three main life strategies used
by polychaetes; sessile, burrowing,
or free moving. Sessile worms fix themselves
to hard surfaces and remain attached
for their lifespan such as in species
of Spirorbis. There are sand burrowing
worms like as the Lugworm Arenicola
marina. Free moving the most mobile
strategy that can include swimming such
as the ragworm. The life strategy that
a species exploits is reflected most
of the other
aspects of their life.
Polychaetes
have bodies with distinct segments.
The majority of the segments have a
pair of paddle like flaps parapodia
with tiny bristles chetae. The paropodia
are used for movement and in some cases
modified for respiration. The chetae
or bristles are where the polychaetes
get their common name, bristle worms.
Paropodia
in free moving worms tend to be large
as they are required for crawling on
the sea bed or even for swimming. Those
species that burrow and hence do not
require parapods for crawling or swimming
but instead burrowing, evolutionary
constraints has kept them small. In
many cases the parapods have been adapted
for movement in the burrow or to produce
a current into the burrow for respiration.
With
the diverse range of species the methods
of feeding will differ. Sessile species
are fixed and have to wait for food
to come to them. Then when it arrives
there is required a method of collecting
or catching it. A strategy used by the
Sand Mason Lanice conchilega are tentacles
which catch food particles which then
pass them to their mouth. Other sessile
species are filter feeders.
Burrowing
worms such as lugworms are the marine
equivalent of earthworms. They take
in sand, as it the sand passes through
the body any organic matter is digested
in the gut. Free moving a common strategy
with carnivores such as in the
ragworm Nereis, which need mobility
to search and catch their prey.
Reproductive
strategies vary depending on the species.
In most species the sexes are separate
however there are a few that are hermaphrodite.
The sexes can sometimes be easily told
apart as in the case of the Ragworm
Nereis diversicolor, where originally
both sexes are brown, but when they
reach breeding condition; the males
change to green while the females are
a much darker green.
Most
species have a planktonic stage. The
majority of polychaetes expel their
eggs and sperm into the currents where
fertilization takes place, the larva
joining the rest of the plankton. After
a period of determined by the species
the larva will return to the sea bed.
There are free moving species that improve
their reproductive chances by swarming
to breed such as Nereis virens.
In
Cornwall worms vary in size from a couple of centimetres to over
eighteen inches in the case of the king ragworms Nereis virens.
Worms
have many predators hence they remain hidden and are not often seen, more common being traces of their existence such as
their casts or egg cases.
|
