Download Sea jelly, cephea cephea, Shark Bay.

Sea jelly, cephea cephea, Shark Bay.
Marine biodiversity – hard and soft tropical corals.
30 Western Fisheries JANUARY 2008
WF Feature
Cnidarians:
Diverse and spectacular
All species in the Cnidaria phyla possess stinging mechanisms, though some
are to be feared much more than others. Thanks to Loisette Marsh of the
WA Museum for this authoritative article on cnidarians, complementing
our lift-out poster (centre spread). Photos: Clay Bryce.
C
nidarians include all simple animals
with a cup-shaped body, the wall
of which has two layers separated by a
thin (in hydroids) or thick (in sea jellies)
jelly layer called the ‘mesogloea’. All
body processes take place in a single
body cavity opening at the top that is
surrounded by tentacles. This opening
takes in food and expels wastes and
reproductive products.
Cnidarians have no brain and only a
simple network of nerves over the body;
however some box jellies have complex
eyes, with a lens and retina, and are able to
detect obstacles and avoid them.
The term ‘Cnidaria’ refers to the
cnidome or stinging mechanism these
marine animals all possess. The stinging
mechanism consists of a capsule
(nematocyst) containing a coiled thread
that explosively shoots out when the
trigger is touched and injects venom
or simply entangles its prey. Each
microscopic nematocyst lies within a
cell, but groups of them are often visible
as beads or rings on the tentacles; there
may be 1,500 nematocysts to the square
millimetre on a tentacle of the deadly box
jellyfish Chironex fleckeri.
Cnidarians have two distinct body forms:
polyps and medusa. One or other of these
body forms is dominant in each of the
four classes, Hydrozoa (hydroids and
bluebottles); Anthozoa (anemones, hard
corals, soft corals, gorgonians, black
corals and sea pens); Scyphozoa (saucer
jellies); and Cubozoa (box jellies).
Hydrozoans – the simplest cnidarians
Hydrozoans are the most basic cnidarians
and usually have both polyp and medusa
stages. Small feathery hydroids attach to
algae, seagrasses or hard surfaces. Some
have polyps enclosed in minute horny cups,
while others have unprotected polyps.
With a hand lens one can see a few
abnormal-looking polyps in which the cup
or ‘theca’ encloses an elongate structure
which develops tiny medusae. These float
off and mature in the plankton, developing
reproductive organs that release eggs
or sperm into the water column. The
resulting larvae settle to the bottom, where
each develops into a polyp that buds off
more polyps, growing into a plant-like
frond – thus completing an alternation of
a vegetative polyp stage with a sexually
reproductive medusa stage.
There are more complex hydrozoans such
as the pelagic by-the-wind sailor (Velella)
which has a triangular sail borne on an
oval chitinous disc, and Porpita which
has a chambered circular disc. Both have
feeding tentacles around the margin and
reproductive structures around a central
mouth under the float. Even more complex
are the Siphonophores, which include
the bluebottle.
Bluebottles consist of a colony of polyps
and medusae. One secretes the gas-filled
float above the water surface while others
form the long feeding tentacle, packed
with stinging cells and other shorter
tentacles. Just under the float are feeding
and reproductive polyps. In Western
Australia, bluebottles, Vellelas and
Porpitas are often washed ashore by
winter storms.
Some hydrozoans, the hydrocorals,
secrete a calcareous skeleton which is
delicately branched in the pink or white
Stylaster, sometimes found on Rottnest
reefs, or yellow-brown in the massive or
coarsely branched fire corals (Millepora).
Hydrocorals have minute pores for feeding
polyps surrounded by even smaller
pores for polyps specialised for stinging.
Medusae are directly budded off from tiny
cups on the surface. Millepora can give a
painful sting but is not dangerous.
Saucers and boxes
The Scyphozoa (saucer jellies) and
Cubozoa (box jellies) have the medusa
stage predominating and a very tiny,
inconspicuous polyp stage resulting from
sexual reproduction. In saucer jellies, the
polyp buds off saucer-like discs – each of
which becomes a medusa. With box jellies,
Western Fisheries JANUARY 2008 31
the polyp may bud a few other polyps but
each polyp gives rise to only one medusa.
Carybdea box jelly, Cockburn Sound.
Inset: Sting from Carybdea, Rottnest Island.
Two saucer jellies commonly seen in the
Swan River estuary are the white-coloured
Aurelia aurita, which has a fringe of short
tentacles around the margin; and the
large brown sea jelly with white spots,
Phyllorhiza punctata, which has much
divided mouth arms ending in filaments
but no marginal tentacles. The brown
colour of this saucer jelly is due to the
presence in the tissues of zooxanthellae
(microscopic single-celled algae), which
provide carbohydrates to the jelly and give
off oxygen by photosynthesis. The sting
from these jellies is negligible, but beware
of saucer jellies with long marginal
tentacles, as these can sting very painfully.
The deadliest of all
The Cubozoa contains the deadliest animal
of all, the large box jelly Chironex fleckeri,
the sting from which can kill in a few
minutes. C. fleckeri has very long, flat
tentacles and can have a bell as big as a
man’s head. A few stings in the Broome
area have been attributed to this species as
they resulted in permanent scarring from
the burn-like weals.
Several other species of cubozoans
with severe stings are found along the
north-west and Kimberley coasts. The
Stinging hydroid, Ashmore Reef, Kimberley WA.
32 Western Fisheries JANUARY 2008
smallest, but possibly most dangerous
of these is Malo maxima, a recently
described species. M. maxima produces
a severe suite of symptoms known as the
Irukandji syndrome, in which there is a
delayed onset of pain, then severe pain,
dangerously high blood pressure and
respiratory distress caused by fluid on the
lungs. The symptoms are life threatening
but can be readily treated in hospital.
Another Irukandji stinger, found off
80 Mile Beach, is Carukia shinju which
has severe but not life-threatening
symptoms similar to those of the
original Irukandji stinger Carukia barnesi,
found in north Queensland. There is still
much to learn about all the sea jellies off
north-western Australia and new species to
be described.
Swimmers may be familiar with the small
box jelly, Carybdea xaymacana, which is
about three centimetres high with tentacles
about 20 centimetres long. This species
is similar in appearance to Malo maxima,
but is common off sandy beaches at
Busselton and in the metropolitan area and
at Rottnest Island.
It has a mild to moderate sting, best treated
by dousing with vinegar to de-activate
the nematocysts, then by immersing the
stung area in water as hot as can be borne
(without scalding). If hot water is not
available apply ice packs. This treatment
applies to all box jelly stings.
Vibrant and varied
The Anthozoa are some of the most
conspicuous, colourful and familiar
cnidarians (such as sea anemones, hard
corals, soft corals and sea fans), but there
are many other groups of less wellknown anthozoans. Over 1,000 species
of anthozoans have been recorded from
Australia including over 200 species of
reef corals.
Anthozoans have their body cavity
divided by partitions or septa on which
the reproductive organs are borne. Eggs
and sperm are released through the mouth
either separately or in egg/sperm bundles
in some reef corals. There is no medusa
stage so the fertilised egg develops into
an oval swimming larva, which attaches
to something hard (even a floating thong)
and develops into a polyp – which in hard
corals secretes a calcareous cup around
itself, buds and branches to form a colony.
Anthozoans are divided into eight orders.
One of these is Actiniaria (anemones) with
about 200 species in southern Australia
and more in tropical waters. Most attach
to rock and are found from intertidal and
shallow water habitats to deep water. All
can move around on a muscular base.
Corallimorpharia (jewel anemones)
are more closely related to corals than
anemones. They characteristically have a
knob at the tip of their tentacles and are
found in southern Australia, west to King
George Sound.
Zoanthidea (zoanthids) have usually
finger-thick or smaller polyps joined at the
base or in some species completely fused
together. Most are tropical species but
some are found as far south as Margaret
River on limestone platforms.
Ceriantharia (tube anemones) are encased
in a tough fibrous tube, 20 centimetres or
more long, buried in muddy sand. They
have a double crown of long, tapering
tentacles that in some are mauve and white
and look like a field of flowers.
Physalia
physalis, or
Portugese
Man o’war
Another order, Antipatharia, contains the
black or thorny corals. These take the
form of whips, bushes or even trees. The
skeleton is made of a very hard, horny
material which, cut in sections and highly
polished, makes beautiful jewellery.
The Scleractinia or hard corals – which
include reef corals with zooxanthellae and
small cave corals without zooxanthellae
– have tentacles in multiples of six, while
the order Alcyonacea (soft corals and
gorgonians) have eight feathery tentacles
to each polyp as do the Pennatulacea or
sea pens.
Zooanthids on
yellow sponge,
Abrolhos Islands.
Western Fisheries JANUARY 2008 33
Coral Bleaching
Reef corals, some anemones and
reef top soft corals have their
growth enhanced by single-celled
dinoflagellates (zooxanthellae)
in their tissues, which carry out
photosynthesis within the coral tissues
and pass the carbon compounds to
the host coral. They also give a brown
or greenish colour to the corals and
soft corals, masking the natural colour,
which is visible when zooxanthellae are
absent. This coral bleaching occurs
when corals are stressed by heat,
cold or heavy rain on a reef at low
tide. Without zooxanthellae, the white
coral skeleton is visible through the
transparent soft tissue. Bleached soft
corals may be palest yellow. Corals
can recover if the stress only lasts a
few days; otherwise they die.
Reef corals are limited in depth, depending
on the clarity of water needed for light
penetration. At 30 metres the number of
species is much reduced, but a few
species can be found at 50 metres. Corals
that live in dim light in caves or in deep
water lack zooxanthellae. Many are
solitary or form small colonies, and some
are adapted for living in sand with spiny
props to support them.
Alcyonacea are also known as ‘octocorals’
because of their eight feathery tentacles.
Anemone cluster, Shark Bay
34 Western Fisheries JANUARY 2008
This order includes the soft corals, whose
soft body is supported by internal water
pressure and calcareous spicules, and
the gorgonians or sea fans, which have
the spicules fused into a calcareous
skeleton or in some a skeleton of flexible,
horny material.
Sea pen, Fremantle
While reef top soft corals are leathery and
of subdued colours, the soft corals and
sea fans of the deeper coral slopes come
in a dazzling array of colours and forms
and are host to a multitude of feather
stars, brittle stars, molluscs, fish and crabs,
many so well camouflaged that it takes a
keen eye to find them. Like sponges, some
soft corals have chemical defences against
predators and these are being investigated
for possible medicinal use.
The order Pennatulacea (sea pens) are
highly specialised octocorals that are
adapted to life in soft sediments. The
name sea pen refers to its resemblance
to a quill pen. They are composed of a
below ground muscular ‘peduncle’ that
anchors the colony and an above ground
part bearing polyps – with a calcareous
rod supporting the whole colony. Several
species are found in Cockburn Sound and
even at the mouth of Fremantle Harbour.
One sea pen with leaf-like branches is
sometimes washed up on local beaches
after winter storms and puzzles people
with its strange structure. Many sea pens
luminescence brightly when touched.
Overall the Cnidaria are one of the most
dominant, diverse and spectacular of the
marine phyla. g