The Great Barrier Reef
254
size and type of sediment, presence of suitable reefal
substrate for the borers and nestlers, hard substrates
for the encrusting species to settle on, and suitable algal
substrates. Factors such as exposure and water currents
are important for filter-feeding organisms. Species living in sediments need to have stable sediments so high
energy beaches are typically low in the number of species and individuals. As all polychaetes are soft-bodied
they need protection from predatory organisms, either
by secreting a tube into which they can rapidly retract
or habitats in which they can burrow and avoid predation. A few species have developed anti-predator strategies, for example, the numerous spiny chaetae of
amphinomids (Fig. 22.2B–D) may make them unpalatable to fish and other predators. However, examination
of fish and bird gut contents reveals that polychaetes
are an important prey item for many species and obviously even when buried in sediments they can be
preyed upon. Lacking an external skeleton or shell they
provide an easy to digest source of food.
Polychaetes exhibit an amazing diversity of reproductive strategies, including both sexual and asexual
reproduction. While most polychaetes are dioecious,
that is, with separate males and females, some species
are hermaphrodites, and others may be males at particular times of their life and then become females
subsequently. Species may live for a few weeks or
months to many years, some breed continuously over
several months, whereas others are restricted to
spawning on a single day. Some species actively mate,
with the male fertilising the eggs as they are laid. In
some the fertilised eggs are then placed under the
scales. The eggs of some other species are brooded
within their tubes, or placed in the chamber below the
operculum, others lay their fertilised eggs in brood
capsules that are then attached to the substrate where
development occurs. In some cases, the developing
embryos are cannibals, eating some of the other
embryos, but in other cases the large yolky eggs have
sufficient nutrients for them all to hatch as miniature
adults. Other species are broadcast spawners with
gametes being released into the water column where
fertilisation occurs. In these cases, free swimming larvae, or trochophores, are produced that spend anything from a few hours to many weeks in the plankton
before settling and metamorphosing into adult worms.
Some larvae feed while in the plankton whereas other
species, which produce large yolky eggs, do not. Most
of the terebellids, for example, produce large yolky
eggs and the pelagic larval stage is almost non-existent. Crawling benthonic larvae are produced that settle near where they hatch. In all cases it is important
that males and females are ripe at the same time, and
that spawning is synchronised— polychaetes have
well developed endocrine systems to co-ordinate this.
(Box 22.1 and Box 22.2.)
In summary, while not too many polychaetes will be
observed when diving on the reef, on closer inspection
tentacles can be seen spread over the substrate, as well
as the expanded branchial crowns of sabellids and serpulids. At night more tentacles can be seen spreading
out over the substrate or being held up fishing in the
water column, but all can rapidly retract back into their
burrows. Only when sediment samples are collected
and sorted under the microscope, or if pieces of dead
coral substrate are split open to reveal borers and
nestlers, will the diversity of polychaetes be revealed.
However, you must have a permit to collect such
samples (see Chapter 12). Even though most of the
diversity of this group is not readily visible, its members play an important part in the functioning of coral
reef ecosystems, with roles including bioturbating sediments, breaking down organic matter, and settling and
boring into coral substrate (see Chapter 8). They are also
important prey items for a wide variety of organisms.
N CLITELLATES
The clitellates consist of both the oligochaetes and the
hirudineans. While oligochaetes are often thought of
as being terrestrial, many species occur in both freshwater and marine sediments and while few studies
have been carried out they are known to be common
in marine sediments on the GBR. Species tend to be
small, <1 cm in length, and thread-like (Fig. 22.3K),
and often bright red in colour due to the blood pigment haemoglobin. They play an important role in
meiofaunal communities in the breakdown of particulate matter and they themselves are predated upon by
many animals.
254
size and type of sediment, presence of suitable reefal
substrate for the borers and nestlers, hard substrates
for the encrusting species to settle on, and suitable algal
substrates. Factors such as exposure and water currents
are important for filter-feeding organisms. Species living in sediments need to have stable sediments so high
energy beaches are typically low in the number of species and individuals. As all polychaetes are soft-bodied
they need protection from predatory organisms, either
by secreting a tube into which they can rapidly retract
or habitats in which they can burrow and avoid predation. A few species have developed anti-predator strategies, for example, the numerous spiny chaetae of
amphinomids (Fig. 22.2B–D) may make them unpalatable to fish and other predators. However, examination
of fish and bird gut contents reveals that polychaetes
are an important prey item for many species and obviously even when buried in sediments they can be
preyed upon. Lacking an external skeleton or shell they
provide an easy to digest source of food.
Polychaetes exhibit an amazing diversity of reproductive strategies, including both sexual and asexual
reproduction. While most polychaetes are dioecious,
that is, with separate males and females, some species
are hermaphrodites, and others may be males at particular times of their life and then become females
subsequently. Species may live for a few weeks or
months to many years, some breed continuously over
several months, whereas others are restricted to
spawning on a single day. Some species actively mate,
with the male fertilising the eggs as they are laid. In
some the fertilised eggs are then placed under the
scales. The eggs of some other species are brooded
within their tubes, or placed in the chamber below the
operculum, others lay their fertilised eggs in brood
capsules that are then attached to the substrate where
development occurs. In some cases, the developing
embryos are cannibals, eating some of the other
embryos, but in other cases the large yolky eggs have
sufficient nutrients for them all to hatch as miniature
adults. Other species are broadcast spawners with
gametes being released into the water column where
fertilisation occurs. In these cases, free swimming larvae, or trochophores, are produced that spend anything from a few hours to many weeks in the plankton
before settling and metamorphosing into adult worms.
Some larvae feed while in the plankton whereas other
species, which produce large yolky eggs, do not. Most
of the terebellids, for example, produce large yolky
eggs and the pelagic larval stage is almost non-existent. Crawling benthonic larvae are produced that settle near where they hatch. In all cases it is important
that males and females are ripe at the same time, and
that spawning is synchronised— polychaetes have
well developed endocrine systems to co-ordinate this.
(Box 22.1 and Box 22.2.)
In summary, while not too many polychaetes will be
observed when diving on the reef, on closer inspection
tentacles can be seen spread over the substrate, as well
as the expanded branchial crowns of sabellids and serpulids. At night more tentacles can be seen spreading
out over the substrate or being held up fishing in the
water column, but all can rapidly retract back into their
burrows. Only when sediment samples are collected
and sorted under the microscope, or if pieces of dead
coral substrate are split open to reveal borers and
nestlers, will the diversity of polychaetes be revealed.
However, you must have a permit to collect such
samples (see Chapter 12). Even though most of the
diversity of this group is not readily visible, its members play an important part in the functioning of coral
reef ecosystems, with roles including bioturbating sediments, breaking down organic matter, and settling and
boring into coral substrate (see Chapter 8). They are also
important prey items for a wide variety of organisms.
N CLITELLATES
The clitellates consist of both the oligochaetes and the
hirudineans. While oligochaetes are often thought of
as being terrestrial, many species occur in both freshwater and marine sediments and while few studies
have been carried out they are known to be common
in marine sediments on the GBR. Species tend to be
small, <1 cm in length, and thread-like (Fig. 22.3K),
and often bright red in colour due to the blood pigment haemoglobin. They play an important role in
meiofaunal communities in the breakdown of particulate matter and they themselves are predated upon by
many animals.