Sea hares (Aplysia punctata) are a common find on dives around the south, west and northern British Isle. They are usually around 7cm long but can grow up to 20cm in length. The colour of Sea hares varies from olive, brown, red and purplish black depending on the algal diet. At the head end, two slender rhinophores stand up like the ears of a hare, hence their common name.
At first sight, Sea hares would appear to be an easy target as a meal, there’s no shell, no spines like an urchin, no claws and they move at a fairly slow pace. Sea hares have a mucous coating containing acid and other nasty compounds which might deter some predators, but their party piece is to release a cloud of sticky, purple ink when attacked by hungry predators.
The cloud of purple ink is in fact a mixture of two secretions. On the back of the Sea hare the central structure is called the mantle with an opening called the foramen. Just under the surface there’s the last remnants of what was probably a shell in the Sea hare’s evolutionary history, a protein disc which acts as an internal shell. On the roof of the mantle is the Purple Gland above the gills. The Purple Gland is responsible for storing and secreting the ink. The building blocks for noxious chemicals are obtained from their algal food, particularly from red algae, metabolised and stored here. This strategy is quite common in marine gastropods and a number of these substances are being actively tested for pain killing, antibacterial, antiviral and anticancer activity. It’s powerful stuff.
Beneath the gill on the floor of the mantle cavity is the Opaline Gland. This gland secretes a white liquid that becomes viscous upon contact with water. If you’ve ever seen the videos of Hagfish (Myxini sp.) producing copious volumes of slime as a way of evading predators, then you will already have seen opaline in action.
The ink and the opaline are secreted into the cavity in the mantle where they mix and are expelled towards the predator. The ink has an intriguing role in that it has been shown to be a phagomimetic decoy (phago = eat, mimetic = to mimic). Some species of lobster will drop the sea hare and try to manipulate and eat the ink cloud, thinking that it is food. Whilst testing this idea scientists found another ink effect. The lobsters tried to rub the ink off their antennules. The opaline in the ink blocks the receptors and response to food odours, thereby preventing the predator from recognising that Sea hares are food. It’s the lobster equivalent of a stuffy nose. Whilst the lobster is busy removing the sticky ink, the Sea hare can make its escape. So, in this aspect the ink is rather more than just a cloud to hide the escape of the Sea hare, it is an active cloud which has the effect of blinding the predator.
It can take quite a bit of stimulation to persuade a Sea hare to produce ink. The threshold depends on factors such as the environment (living in a turbulent environment makes inking less likely), how full the gland is and what the stimulus is (inking occurs more rapidly near anemone tentacles that it does in response to an electric shock). If a Sea hare has full ink glands then it will release nearly half of its ink at the first stimulation. Each subsequent stimulation will release 30-50% of the contents. It will take at least 2 days to replenish the gland.
Whilst aquarium owners like the grazing tendencies of Sea hares, they panic about the effect that any inking has on the fish and other residents inside their tank. The jury is out on how much effect the Sea hare ink can have. Sea anemones retract their tentacles, but the evidence for the toxic effect of the ink inside an aquarium is limited, perhaps because the Sea hares aren’t feeding on red algae that are the source of the most potent toxins.
It’s a popular misconception that lobsters are red in colour. That’s because most people have never seen a live lobster, and what gets served up in their lobster thermidor is distinctly red in colour. It’s one of my pet hates when media shows lobsters as red, right up their with the reporter talking about the oxygen tanks worn by the scuba divers. Grrr! As divers we are well positioned to know that lobsters are a dark shade of blue-green, but it’s not just the shell that’s blue, so is the lobster’s blood.
Technically the lobster doesn’t have blood in the way that we would understand it. They have haemolymph, a fluid equivalent to blood that circulates inside arthrpods. Haemolymph is mostly watery with some salts and nutrients dissolved in. There are some cells known as haemocytes, but in arthropods these play a role in the immune system of the organism. There aren’t any red blood cells containing haemoglobin to carry oxygen like we have. Instead invertebrates have some special proteins in their haemolymph called haemocyanins that transport oxygen for them.
Haemocyanins are metalloproteins that have two copper atoms which can reversibly bind to a single oxygen molecule. Oxygenation causes a colour change from the colourless deoxygenated form to the blue oxygenated form. Haemocyanins are only found in molluscs and arthropods but aren’t limited to marine animals and can be found in tarantulas, scorpions and centipedes too. Haemocyanins turn out to be rather more efficient than haemoglobin at binding to oxygen in cold environments and when the oxygen pressure is low. Sounds ideal for the average marine crustacean.
Lobster haemolymph has been shown to have antiviral properties, but only in the uncooked state. In research it was shown to be effective against the viruses that cause shingles and warts. In fact, there is an American based company that has developed a blood-based cream for treating cold sores and skin lesions, although it’s yet to get approval from the regulatory authorities.
So, we’ve established that copper coloured proteins give lobsters their blue blood, but that’s not the explanation for the shell changing colour when the lobster is cooked. For this we have to look at another protein called crustacyanin, and its best buddy astaxanthin. Astaxanthin is a carotenoid pigment so it absorbs blue light and gives off red/orange colour. Astaxanthins are mostly synthesised by microalgae and enters the marine food chain. In shellfish the astaxanthin becomes concentrated in the shell. Its likely that astaxanthins have a string antioxidant and anti-inflammatory effect and may protect against age-related degeneration. Astaxanthins are also found in some sponges, starfish and in species of octopush and cuttlefish. They give the muscle tissue of the salmon its characteristic colour.
Crustacyanin is a very interesting chromoprotein, that changes colour depending on whether it is in water or dehydrated. When it’s bound to the lipid like astaxanthin it has a distinctly blue colour. While a lobster is alive, crustacyanin stays bound tightly to astaxanthin, so tightly that the astanxanthin’s light-absorption properties are quashed and the complex appears to be blue-green in colour. That all stops when the lobster hits the boiling water.
Crustacyanin is not heat stable, so the boiling temperatures cause it to unravel and lose its grip on astaxanthin. The true colours of the astaxanthin then shine through in the red lobster shell. In fact, they were there all the time, you just couldn’t see them. It’s the same mechanisms for cooking shrimp too. Flamingos rather cleverly digest the crustacyanin protein to release the astaxanthins that colour their feathers soft pink.
In 1973, four hostages were seized in a bank robbery in Stockholm, Sweden. A convict on parole attempted to rob the bank but as the siege situation developed he negotiated the release of his friend from prison to help him. The hostages were held for 6 days in the vault. When the siege was finally over none of them would testify against their captors and they even started raising money for the defence’s legal team. Baffled by the responses of the hostages, further assessment was sought.
A Swedish psychiatrist examined the hostages and described ‘Stockholm Syndrome’. In cases where Stockholm syndrome is present, victims start out as powerless, but go on to develop positive feelings towards their captors. Sympathy for the cause and goals an often follow hostages back into their real life. This can cause cognitive and social problems, and a feeling of dependence on the captor.
Stockholm syndrome probably arises as a coping mechanism. The victim wants to survive, and that is a stronger instinct than hating the person who has created the hostage situation. A positive emotional bond will help survival, but there’s a danger of being spotted as a fraud. So the victim ends up believing that they really do like their captor.
You’re by now, probably wondering why I’m writing about this topic…and whether you’ve accidentally picked up a copy of Psychiatrist Monthly. But actually, Stockholm syndrome is an extreme example of how an imbalance of power in any relationship can have a massive influence on how the parties behave. For this reason, educational establishments (from schools to universities) have very strict guidelines about appropriate relationships between teaching staff and students. Teaching staff (or instructors) occupy a position of power over the student. Instructors grade work, give personal feedback, ensure standards for the course are met. And students will hold the instructors in high esteem because of their position and experience. Most educational establishments would require a member of teaching staff to remove themselves from teaching any student who they were having a relationship with. In this case the student may be so overwhelmed by the instructor’s attention that they may feel unable to say no, concerned about the impact on their progress. And the student will normalise this behaviour in their future life.
When would-be instructors first attend a BSAC Instructor Foundation Course, we run a session on what the ideal instructor would look like. Usual (and valid) responses include knowledgeable, patient, approachable, organised and skilful. Rarely does anyone mention ethical. In PADI’s instructor manuals, there’s a small section on ethics, although it seems to deal more with the ethics of business than the relationship between an instructor and a student. And yet, instructors are in a position of extreme power. On smaller courses, especially technical ones, there may only be one instructor for 2 students. That instructor will play a variety of roles during the course, mentoring and assessing the student.
Diving at all levels in built on trust. Instructors that build a positive relationship with a student will achieve more as they work to develop the student’s skills. But whilst this relationship can be hugely beneficial, they are in a situation where a massive imbalance of power in possible. The risks for student infatuation with the instructor are real. Ask any experienced instructor and they will be able to tell you of the students who came a little too close. In the stress of the course with the worry of whether you’ll pass, those evolutionary survival mechanisms kick in. The need to survive outweighs the desire to fight back against the demands of the course (and the instructor delivering it). Scuba instructors get idolised, and that’s not always a healthy situation. Stockholm syndrome may be at the extreme end of the scale, but the potential imbalance of power exists. Treat carefully my fellow instructors.
Michelle has been scuba diving for nearly 30 years. Drawing on her science background she tackles some bits of marine science. and sometimes has a sideways glance at the people and events that she encounters in the diving world.