Electric eels can produce 500-volt shocks to kill their prey; Great White sharks can slice through the hindquarters of a seal in one bite; and the blue-ringed octopus has poison enough to kill 26 humans in a matter of minutes. Yet despite these large and menacing creatures, the most venomous marine animal known on the planet remains the box jellyfish.
If you are stung, your chance of survival or even getting yourself to the shore is virtually zero, according to the Australian Institute of Marine Science.
Sometimes the victim somehow manages to get ashore only to die within a few minutes as friends look helplessly on. Fortunately for Costa Rican swimmers, that particular menace remains confined to waters off the Australian coast. Locally, the majority of jellyfish stings produce nothing more than mild pain and irritation, which experts say is best treated with baking soda and seawater.
(Contrary to popular belief, most agree urine is not a proper remedy.) The greater threat, however, may be to other fish and the industry that relies on their well-being. Jellyfish (which along with invertebrate creatures like coral and sea anemones are known as cnidarians) feed on larvae and fish eggs.
According to jellyfish expert Dr Karina Rodr�guez, this can pose harm to fish species when applied in large numbers.Perhaps even more concerning, researchers have found in recent years increasing numbers of jellyfish �blooms� � large swarms of hundreds of the gelatinous creatures that can take over vast stretches of sea.
The blooms occur when a precise combination of temperature, shifts in ocean currents, and changes in marine nutrients come together. Many have attributed the growing phenomenon to human-induced causes including over-fishing, climate change, and marine contamination. In water with depleted oxygen levels, for example, jellyfish can tolerate the compromised conditions better than larger fish.
As fish numbers decrease, the jellyfish proliferate to fill in the ecological gap.
(Jellyfish) are considered plague species due to their capacity to survive on what they have,� says Dr Rodr�guez. �Faced with issues like contamination - organic human waste from black water, for example, they begin a trophic (feeding) dynamic.
The biologist says �explosive reproduction� occurs when they begin consuming surrounding nutrients rapidly. Yet when observing the sea dweller�s locomotive patterns, one is pressed to see how the seemingly inert creature can create such a stir.
Rather than swimming, jellyfish move by opening and closing their bodies in rhythmic motions that create an advantageous ocean current which propels their drift through the water. For this reason, says Dr Rodr�guez, they can be indicators of ocean currents.
They are highly dependant on the marine currents because sometimes their swimming capacity is not enough and they use the water currents to float and move to other place, says Dr �lvaro Morales Ram�rez, of UCR's Centro de Investigaci�n del Mar y Limnolog�a (CIMAR).
It is the same principle of the vacuum plunger used to unplug toilets, where the absence of air produces the strength to unplug drains�the jellyfish lower the bell and contract their muscles and shift very actively. They usually rest by floating, and then move again.
The ocean creatures have no respiratory organs and breathe by an exchange of gases, expelling carbon dioxide naturally through the skin and absorbing oxygen from the surrounding environment. Instead of brains they have what is known as a subdermic network, or a �nerve net.� Sensorial organs perceive changes in the environment.
Yet according to Dr Morales, a primitive eye was discovered recently in a certain species. Dr Rodr�guez says the discovery, published in Nature Science magazine, pointed to a capacity to see and even detect color.
In Costa Rica, an investigation Dr Rodr�guez conducted between 1999 and 2001 in the Pacific revealed 83 species, 21 of which were new registers. About 57 per cent of those were gathered in the Gulf of Papagayo.
There is a great richness, she says. We gathered more in one year than other countries gathered in ten years.
A type of Cubozoa jellyfish (the generally more venomous class which box jellies are part of) was found inhabiting certain mangroves in the Gulf of Nicoya, although with venom less potent than its Australian counterpart.
In Bah�a Culebra, the biologist says, many small jellyfish live adhered to the bottom of rocks and when the tide recedes they become detached. Dr. Rodr�guez says there has been little research in the Atlantic, but notes in 2004, ten people were hospitalized during an upsurge in the Lim�n province.
Jellyfish are made up of a bell or umbrella-shaped sacks that store cellular content and a gelatinous layer called the mesoglea. Tentacles dangle from the bell (Polar regions report some with tentacles up to 90 feet, or 30 meters) covered with thousands of microscopic features called nematocysts. When contact is made, these features uncoil to inject toxins into its prey.
In humans, Dr Rodr�guez says, reactions will vary depending on personal sensitivity and allergic tendencies, as well as the species and age of each jellyfish. Younger ones tend to be more venomous. Worldwide there are over 3000 species of cnidarians and more than 350 genera of jellyfish specifically. (In Chinese cuisine, tentacles are removed and the dehydrated creatures are put into soup.)
They fall generally into two categories: those with a visible cellular umbrella (a common Costa Rican version is the balas de ca��n, or cannonballs) and those in a microscopic form that have merely a thin layer without cellular content. There is also a very small umbrella-less type, according to Dr Rodr�guez, called hilos de oro, or golden threads, that travel in colonies.
They are jellyfish but morphologically very distinct from the others, says Dr Rodr�guez.
One or both of the latter may be attributed to a phenomenon Costa Rican swimmers often experience when suddenly surrounded by prickling sensations. It is one many describe as an unmistakable occurrence:
You can�t see them but you can feel them.
Ecotourism and Nature 
