18 – Pelagic Cnidaria and Ctenophora
197
Nematocysts, the weapons of jellyfish, are basically
a capsule with a spring-loaded thread coiled up inside,
and an external hair-trigger at one end. The thread has
rows of spines near the base, and usually a relatively
smooth shaft along the rest of its length. When the hair
trigger is activated, the thread is discharged inside-out
(i.e. spines first), with tremendous force, about 40 000
times the force of gravity. Nematocysts can easily penetrate crab carapaces, fish scales, and human skin, so
delivering their subcutaneous dose of venom. Nematocysts discharge following a combination of mechanical
and chemical stimulation.
N COLLECTING AND PRESERVATION
In general, jellyfishes, especially in the GBR region, should
not be handled as their stings can be painful or even lifethreatening. If it is necessary to handle them, gloves
should be worn and safety precautions should be strictly
adhered to at all times. Jellyfish can be challenging to
photograph, with the diver having to manage lighting,
background, currents, and typically uncooperative subjects; however, truly beautiful images can be taken by a
skilled and patient photographer. It is important to ensure that the correct permits are in place before collecting
or handling any species in the GBR region. Specific collecting and preserving techniques for different groups
are available on the internet at www.medusozoa.com/
curating.html [Verified 13 February 2008].
Jellyfishes do not preserve well in most standard
preservatives, and must be preserved in dilute (2–4%)
formaldehyde to help retain their original form. Alcohol may be needed to preserve hard parts such statoliths in cubozoans.
N IDENTIFICATION
Some of the more conspicuous jellyfishes are easily
identifiable, even at a glance:
L The cubozoan box jellyfish, Chironex fleckeri
(Fig. 18.5D), is large, more or less transparent, and
has up to 15 fettuccini-like tentacles hanging off each
corner; a close relative, Chiropsella bronzie (Fig. 18.5C),
has spaghetti-like tentacles and rarely more than
seven or nine per corner.
L Irukandjis (Cubozoa, Fig. 18.5A, B) are also transparent, and may be as small as 1 cm or as large as 18 cm
in height, but always have box-shaped bodies with
only four tentacles, one extending from each corner.
L Scyphozoan blubber jellyfishes, Order Rhizostomeae,
Catostylus (Fig. 18.3A), Mastigias, Netrostoma
(Fig. 18.3D), and so on, are large and heavy, and often brightly coloured; they have eight frond-like
structures hanging down from the centre, and never
have tentacles hanging from the bell margin.
L Scyphozoan hair jellies, Cyanea spp. (Fig. 18.3C),
have a large flat bell, with a mop of hundreds of fine
tentacles clustered underneath.
L The scyphozoan mauve stinger, Pelagia (Fig. 18.3B),
has a small dome-shaped pink-purple body, with
eight fine marginal tentacles and four ribbon-like
pleated oral arms extending from the centre of the
underside of the body.
L Hydromedusae (Class Hydrozoa) have small transparent bodies, typically with numerous tentacles
around the margin; often they are not seen but
are felt as ‘sea lice’. Members of the Order
Leptomedusae (Fig. 18.1C) have flat bodies with
gonads on the radial canals, hollow marginal tentacles and external statocysts; Order Anthomedusae
(Fig. 18.1A) have tall bodies with gonads on the
stomach, and with hollow or solid marginal tentacles and no statocysts; Order Narcomedusae have
stiff tentacles emerging from the upper surface of
the bell, and with the gonads in pouches; Order
Trachymedusae have deep bodies with gonads on
the radial canals, and with solid marginal tentacles
and free statocysts; and Order Limnomedusae
(Fig. 18.1D) have flat bodies with gonads on the
radial canals or stomach, and have hollow tentacles
and enclosed statocysts.
L Blue bottles (Subclass Siphonophora), Physalia spp.
(Fig. 18.1B), have a floating bubble structure atop a
cluster of blue and clear tentacles and filaments,
with one or more bright blue main fishing tentacles
trailing out to a distance of about a metre.
L Ctenophores all have eight bands of cilia along
the body. Pleurobrachia-type comb jellies (Order
197
Nematocysts, the weapons of jellyfish, are basically
a capsule with a spring-loaded thread coiled up inside,
and an external hair-trigger at one end. The thread has
rows of spines near the base, and usually a relatively
smooth shaft along the rest of its length. When the hair
trigger is activated, the thread is discharged inside-out
(i.e. spines first), with tremendous force, about 40 000
times the force of gravity. Nematocysts can easily penetrate crab carapaces, fish scales, and human skin, so
delivering their subcutaneous dose of venom. Nematocysts discharge following a combination of mechanical
and chemical stimulation.
N COLLECTING AND PRESERVATION
In general, jellyfishes, especially in the GBR region, should
not be handled as their stings can be painful or even lifethreatening. If it is necessary to handle them, gloves
should be worn and safety precautions should be strictly
adhered to at all times. Jellyfish can be challenging to
photograph, with the diver having to manage lighting,
background, currents, and typically uncooperative subjects; however, truly beautiful images can be taken by a
skilled and patient photographer. It is important to ensure that the correct permits are in place before collecting
or handling any species in the GBR region. Specific collecting and preserving techniques for different groups
are available on the internet at www.medusozoa.com/
curating.html [Verified 13 February 2008].
Jellyfishes do not preserve well in most standard
preservatives, and must be preserved in dilute (2–4%)
formaldehyde to help retain their original form. Alcohol may be needed to preserve hard parts such statoliths in cubozoans.
N IDENTIFICATION
Some of the more conspicuous jellyfishes are easily
identifiable, even at a glance:
L The cubozoan box jellyfish, Chironex fleckeri
(Fig. 18.5D), is large, more or less transparent, and
has up to 15 fettuccini-like tentacles hanging off each
corner; a close relative, Chiropsella bronzie (Fig. 18.5C),
has spaghetti-like tentacles and rarely more than
seven or nine per corner.
L Irukandjis (Cubozoa, Fig. 18.5A, B) are also transparent, and may be as small as 1 cm or as large as 18 cm
in height, but always have box-shaped bodies with
only four tentacles, one extending from each corner.
L Scyphozoan blubber jellyfishes, Order Rhizostomeae,
Catostylus (Fig. 18.3A), Mastigias, Netrostoma
(Fig. 18.3D), and so on, are large and heavy, and often brightly coloured; they have eight frond-like
structures hanging down from the centre, and never
have tentacles hanging from the bell margin.
L Scyphozoan hair jellies, Cyanea spp. (Fig. 18.3C),
have a large flat bell, with a mop of hundreds of fine
tentacles clustered underneath.
L The scyphozoan mauve stinger, Pelagia (Fig. 18.3B),
has a small dome-shaped pink-purple body, with
eight fine marginal tentacles and four ribbon-like
pleated oral arms extending from the centre of the
underside of the body.
L Hydromedusae (Class Hydrozoa) have small transparent bodies, typically with numerous tentacles
around the margin; often they are not seen but
are felt as ‘sea lice’. Members of the Order
Leptomedusae (Fig. 18.1C) have flat bodies with
gonads on the radial canals, hollow marginal tentacles and external statocysts; Order Anthomedusae
(Fig. 18.1A) have tall bodies with gonads on the
stomach, and with hollow or solid marginal tentacles and no statocysts; Order Narcomedusae have
stiff tentacles emerging from the upper surface of
the bell, and with the gonads in pouches; Order
Trachymedusae have deep bodies with gonads on
the radial canals, and with solid marginal tentacles
and free statocysts; and Order Limnomedusae
(Fig. 18.1D) have flat bodies with gonads on the
radial canals or stomach, and have hollow tentacles
and enclosed statocysts.
L Blue bottles (Subclass Siphonophora), Physalia spp.
(Fig. 18.1B), have a floating bubble structure atop a
cluster of blue and clear tentacles and filaments,
with one or more bright blue main fishing tentacles
trailing out to a distance of about a metre.
L Ctenophores all have eight bands of cilia along
the body. Pleurobrachia-type comb jellies (Order