Ode to a Jellyfish [49]

By Richard Birdsey

After an number of questions on the chat-line about dangerous jellyfish and to build on Trevor Gardner’s article in the last edition of the magazine, I thought Club members might like to know a bit more about these rather interesting animals. Most of us have had a memorable encounter with a jellyfish at some stage of our lives. My two are being hit full in the face by one hurled by my big sister when I was five, the other being stung on the hand by a box jelly whilst handling a trawler net and crying from the shock of it almost as much as I did when I was five.

Jellyfish are an extremely important part of marine food webs and are both predators and prey to a huge range of organisms. They are a surprisingly adaptable and widely distributed species found in all the oceans of the world as well as estuarine and freshwater environments. Most of our encounters with them are in shallow marine waters though they are also found many thousands of meters down at the bottom of the deepest oceans. Many species inhabit both fresh water and marine environments at different stages of their life cycle; like salmon they breed and grow in fresh water and live as adults in the sea.

Family Ties

Jellyfish are members of the phylum Cnidaria (pronounced ‘ni-dair-re-ah’) coming from the Greek word ‘cnidos’ meaning ‘stinging nettle’. Relatives of the jellyfish are many and varied and include sea anemones, sea whips, corals, sea fans, sea squirts and other forms. Cnidarians can have one of two adult body forms: polyps (e.g. corals) have a hard or semi-hard body and are attached to surfaces, or soft-bodied medusae which are free-swimming as adults but might be attached to surfaces in the juvenile stages of their life cycle.

Two sub-groups within the Cnidaria contain most of the jellyfish species. Scyphozoans are ‘true’ jellyfish and by far the most diverse and common group. They come in all colours, shapes and sizes from a few millimeters through to giants two metres across with tentacles 40 metres long. The other group are the Cubozoa, which, as the name implies, includes the much feared Australian Box jelly, Chironex fleckeri. This latter group is differs from Scyphozoans in that they are square shaped, have four discrete bunches of tentacles on each corner of their body (or ‘bell’) and they have excellent eyes! Yep, their eyes contain lenses, corneas, and retinas, very similar to yours. More on this later.

Anatomy 101: Where Jelly Ain’t Jelly

While jellyfish come in a huge range of forms, their body construction is reasonably similar. The main body of a jelly is called the ‘bell’ and consists of three layers. The outer layer (epidermis) covers the external body surface and an inner layer called the gastrodermis lines the mouth/gut cavity. A layer of thick elastic jelly-like substance called mesoglea (‘middle jelly’) is sandwiched between the other and inner layers. This inner goo has fibers running through it which is acts as a ‘chassis’ to give the animal shape and attach muscles to — particularly as jellyfish are 95% water!

A cavity inside the bell (the coelenteron) fulfils a number of purposes. It is a gullet, stomach and intestine connected to the outside world with a single opening which is both mouth and anus. A number of oral arms located near the mouth transport food captured by the tentacles. Other bits include muscles, a simple neural net (but no brain), chemosensory pits (sort of a nose) to detect prey and gonads.

Slimy Sex

Well sort of. The bluey-purple rings you see in the top of common jellyfish are gonads. And yes, there are boy and girl jellyfish. The typical jellyfish life cycle alternates two different body forms. The adult is the common free-swimming medusa, while the larvae take on a polyp form and are attached to the bottom. During spawning sperm are pushed into the male’s coelenteron and on into the outside water where they swim into the females ‘mouth’ and fertilise the eggs (a potential source of very bad oral sex analogies, but we won’t go there). At this stage the fertilised egg becomes a planktonic larvae called a ‘planula’. Some species keep planula in their coelenteron or in small brood pouches on their tentacles or arms before releasing them. Once released the planula swim down to the bottom or some other convenient surface and form polyps. These polyps develop into juvenile jellyfish and after a few weeks bud off the polyp and grow into an adult medusa. Jellyfish live for about three to six months, though some studies suggest they can live longer.

Slow But Sure

While adult jellyfish are largely at the mercy of currents in the ocean, they do have a surprising amount of ability to direct where they are carried. Simple muscles in the bell enable the jelly to contract the bottom skirt of the bell pushing water out through the narrowed opening. While appears to be a simple and relatively crude form of propulsion it is surprisingly effective.

When I worked on a Chironex project in North Queensland one of the more interesting tasks was to capture live specimens. Box jellyfish could sense the net coming (thanks to those excellent eyes) and could really take off. No kidding!

While their ability to move horizontally is limited to prevailing wind and currents, it is their vertical movement through the water column that really determines their movements. By moving in and out of current layers in the water column they can effectively maintain their position or move on or off shore. Some species (such as Box jellies and sea wasps) will follow the tide in over shallow water hunting prey. Other will migrate to deeper water during the day and come up to feed at night — probably to avoid shallow-water predators.

Anatomy of a typical adult medusae

Huntin’ & Fishin’

While it is hard to think of mere jellyfish as carnivores and predators, that’s exactly what they are. They capture and kill zooplankton and other planktonic animals. Larger or more active predatory species such as the Cubozoa can kill small crustaceans and fish. I remember seeing a small fish put in a tank with a Box jelly we’d captured and within 5 seconds — wham — dead.

Jellies are beautifully adapted to prey on other animals. As previously mentioned they have ‘noses’ and ‘eyes’ to detect prey. And what’s really cool is that not only can they ‘look’ outwards scanning for prey or predators, being semi-transparent jellies can look inwards at what is going on inside their body (I’m glad I can’t do that). However, the best-known and most feared adaptation is the tentacles, or more accurately the thousands of stinging cells they contain. It is important to point out that these weapons are used for attack and defense. When humans are stung it is a case of mistaken identity or defense — though that is small consolation when you are dancing around screaming and swearing!

The stinging cells are called nematocysts and consists of a capsule containing a hollow barbed ‘spring’, a venom sac and a trigger mechanism at the mouth of the capsule. Thousands of nematocysts are concentrated in the epidermis of the tentacles and oral arms. The nematocyst fires when the trigger hairs brush against something that appears edible (like a kayaker for instance). The trigger hairs are chemo-sensitive and can discriminate between non-food objects (e.g. wood or material) and something edible. Once triggered, pressure in the nematocyst releases the coiled barb into the target and toxins flow down the hollow tube. The toxins are not poisonous but paralyse the victim. The cumulative effect of thousands of nematocysts simultaneously discharging can be imagined. The oral tentacles then pass the paralysed prey to the mouth and into the coelenteron for digestion.

I’m not going to go into the details of managing stings (see Trevor’s article in the last issue for a comprehensive treatment of this issue) but I will mention a couple of things:

The use of vinegar to confuse the chemo-sensitive trigger hairs and stop the discharge of the nematocysts is still being debated. Some research suggests that vinegar stops the nematocysts discharging in some species, in others if causes them to discharge.

The penetrating power of the nematocyst also varies from species to species but is often sufficient to go through clothes commonly used by paddlers such as thermals, thin lycra and cotton. Wear a proper ‘stinger suit’ or thicker clothes if to protect yourself, especially around the upper body.

Some people are naturally more sensitive to venom than others and are at greater risk. Smaller people such as women and children may also be at greater risk as their smaller body mass concentrates the venom.

Some researchers suggest that a lack of body hair increases the risk factor as hair fends off some of the tentacles, reducing the number of nematocysts discharging into your skin. This my be a myth but is a good excuse not to shave your legs or take performance-enhancing testosterone — you’ll certainly paddle faster!