MICHELLEHAYWOOD
Barbed riposte
Where skin is exposed to water , the UK diver is at risk of jellyfish stings . Michelle Haywood investigates the biological forces that power this predatory weapon
Every diver in British waters will encounter lion ’ s mane jellyfish ( Cyanea capillata ) at some point in their dives . lion ’ s mane are the largest jellyfish known and can have tentacles extending more than 30 metres from the bell , which itself can be up to a metre in size . Tentacles are easily broken off , especially when they snag on shot lines , so it ’ s not unusual to find the tentacles and not see the jellyfish itself .
Jellyfish tentacles are covered with stinging cells called nematocysts . Nematocysts are little capsules on the tentacle surface . They have elastic walls and a small operculum that acts as a lid . Inside the capsule is a protein tube with a stylet with spiked barbs like a fish-hook on the end . This protein tube contains the venom and is attached to a coil of threads . Before the nematocyst is triggered , these threads hold the barbed head in place .
Nematocysts can be triggered by touch . The operculum pops open and the dart is shot out , unravelling the coiled threads . As the whole system is stored with considerable pressure , the barbs are capable of penetrating even crustacean cuticles . Studying nematocysts firing with high-speed cameras ( over a million frames per second !) shows that discharge of nematocysts can fully deploy in as little at 700 nanoseconds or 0.0007 of a second . Although the mass of the nematocyst is very small ( around 1 nanogramme or 0.000001 of a gramme ), the pressure at the site of impact is more than 7 Gigapascals , which equates to the same impact as some bullets . The energy for the firing comes from the collagen and proteins in the capsule wall , which act as a molecular spring .
The barbed head sticks into the prey like a microscopic needle and the venom is injected into the prey ( or the unsuspecting diver ). Jellyfish obviously aren ’ t intending to sting divers , most of whom are far too big to be digested anyway . The venom is intended to paralyse and kill their small prey ; the tentacles will then move the prey to the mouth and the nematocysts will reset . Unsuspecting divers brushing against the nematocysts will trigger them to fire , and exposed human skin is easily penetrated .
The venom needs to be fast acting ; venom needs to stop the prey from escaping . The exact composition of jellyfish venom varies for each species . It is a cocktail of haemolytic ( attack blood cells ), neurotoxic ( attacks nerve cells ) and cytotoxic ( cell killing ) proteins and non-protein toxins . That being said , some jellyfish venom has been investigated for anti-cancer and anti-inflammatory activity . Other components have shown promise as pain killers . However , elucidating which protein out of the cocktail has a therapeutic function is difficult . There are hundreds of possibilities to test . Jellyfish aren ’ t the only toxic organism from which we look for new medicines ; similar studies have been carried out with snake venom .
For most divers , the sting from a jellyfish is minor . There is a small amount of localised pain , maybe some itching and skin irritation and the possibility of blisters or wheals . However , for a small minority there may be progression to nausea , vomiting and breathing difficulties . The original identification of anaphylaxis came from the study that looked at dogs being stung by the Portuguese Man-O ’ -War ( Physalia physalis ). The scientists discovered that a second injection of the same venom didn ’ t provoke protection ( phylaxis ) but actually was more toxic than the first exposure , so
������������������
����������
������ ������
��������
����������
������ ������
�����������
�������
�������
they christened the effect anaphylaxis ( the opposite of phylaxis ). They won the Nobel Prize in Medicine .
Once the venom has been injected into the poor diver , the diver ’ s immune system swings into action . The immune system recognises the complex mixture of venom components and acts to compartmentalise and destroy the foreign material . There are so many different components in venom that this reaction involves pretty much every immune cell type and a number of chemical messenger molecules . This explains the immediate itching and skin irritation , as white blood cells race to the scene of the venom injection .
The redness occurs as the blood vessels become leaky and allow fluid into the affected tissue . Toxins in the venom activate pain receptors in the skin . Some of the barbs may be left in the skin and that chitinous material creates a sustained reaction .
In some people , all these reactions can become widespread or systemic , leading to more serious effects . As with all immune reactions , subsequent exposures may provoke a faster and bigger response that occurs away from the original skin lesions . This suggests that in some people the immune response to the toxin works slightly differently . But until and unless it happens to you , you ’ ll never know ! �
��������
25