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A rare pea-sized jellyfish and the surprisingly deadly six-spot burnet moth are out and about at this time of year and under the spotlight in this week’s blog.

Exploring Shetland’s sea caves and rocky reefs

With the help of Hughy the seal, Lisa Kamphausen and the Heriot-Watt Scientific dive team set out to the tiny island of Mousa to survey the condition of one of Scotland’s SACs (Special Areas of Conservation).

A tiny seaslug (nudibranch) on red seaweed – life underwater is as much as rich as above the waves, but much harder to observe and monitor.

“Hughy! Come here boy! Hughy!” A second later a huge seal jumps out of the water and grabs a mackerel out of Alan’s hand. Hughy has been on standby for our return to port every day this week, and never fails to get a fish or two out of Alan the skipper.

Fancy a snack? Hughey the seal eying up a fish.

As Alan ties up the boat, we unload a big pile of empty dive cylinders, bags of samples collected during the day, tape measures, compasses, quadrats, cameras, a few soggy leftover sandwiches, and a team of scientists from Heriot-Watt University, SNH, and National Museums Scotland onto the pier. Smiles all round after another successful day’s surveying, and a star performance by Hughy.

Intertidal survey team in action. To study the rocky reefs the team recorded all plants and animals along a line running from the top of the intertidal to 100m beyond the waterline. Divers took over from where welly boots flooded.

Alan is taking us out on the boat for a survey of sea caves and rocky reefs around Mousa’s shores. Having lived and fished here all his life he knows the place better than anyone. We are out here for just three weeks and hope to catch enough calm weather to be able to make detailed observations of at least five sea caves and five rocky reefs, before finishing the survey off with a week of remote video work. The survey is part of SNH’s Site Condition Monitoring programme, to assess the condition of Scotland’s SACs, and compare it to how things were in the past.  Alan has taken out the team who conducted the previous survey as well, eight years ago. Our initial impression is that things have not changed much, and the caves and reefs remain in good shape. More smiles.

To assess the status of the caves and rocky reefs, divers from the scientific dive team make records of the physical environment, animals and plants, the biotopes and associated communities, and take video footage and photographs.  Today the divers made an interesting find in a shallow bay just by the Mousa ferry pier: they came across a stalked jellyfish which they think is of a species only recorded in the UK once before, in Caithness. To confirm whether the pea-sized carnivorous animal is indeed Haliclystus sapinx they need to examine the specimen they collected more closely in the laboratory.

At the end of the day two of us did a quick dive to check up on a maerl bed site which we may study in detail if we unexpectedly finish the other sites ahead of schedule – or more likely, on a day when swell and surging waves make working in the caves and reefs impossible. We found the maerl bed covered in translucent seasquirts, and happened across an octopus who looked a little surprised.

We now have two weeks left to spot the pod of killer whales which is circling Shetland at the moment. And we hope that Hughy will tell his fellow seals, some of whom like to hang out in the caves we come to survey, that we are friendly and don’t need to be chased out of the caves. So far the trick seems to work. We better make sure he gets his mackerels tomorrow as well.

Obligatory team photo in front of the Broch of Mousa.

You can listen to an interview with two of the team members on BBC Radio Shetland here.

Species of the month – the six-spot burnet moth

Athayde Tonhasca explains the deadly beauty of burnet moths.

Six-spot burnet moths feeding on a flowering thistle head. ©Lorne Gill/SNH

For over 430 million years plants and animals – particularly insects – have been tangled in an evolutionary tug of war. Through the inexorable process of natural selection, a plant produces chemicals which are repellent or harmful to insects. These insects then develop the ability to overcome the plant’s defences, which in turn puts pressure on the plant to come up with ever more powerful chemicals, and so this arms race goes on.

As a consequence of these adaptations and counter-adaptations, plants have accumulated an arsenal of more than 300,000 products, known as secondary metabolites, which repel or poison potential enemies. Among these chemicals, cyanogenic glucosides – CNglcs for short – are particularly efficient. When an insect rips through plant tissue, CNglcs immediately react with other chemicals to produce hydrogen cyanide, a substance highly poisonous to most animals, including humans.

Self heal and a six-spot burnet moth on the Vatersay machair. ©Lorne Gill/SNH

However, in an ingenious twist of plant-herbivore coevolution, CNglcs have not only become harmless to a few millipedes, centipedes and insects, but have even turned into phagostimulants (substances that induce eating).

Why would this happen?

Through several behavioural and physiological adaptations, these herbivores have developed the ability to sequester CNglcs from their food plants, that is, ingest and store them in their bodies, without triggering the chemical reactions that create poisonous hydrogen cyanide. By accumulating CNglcs, these species then put their toxic chemicals to their own use as defences against predators.

No group illustrates this process of chemical manipulation better than the moth family Zygaenida (forester and burnet moths), of which the six-spot burnet, Zygaena filipendulae, is the best known representative in Scotland.

A six-spot burnet moth caterpillar and bird’s foot trefoil. ©Lorne Gill/SNH

Six-spot burnet larvae feed only on bird’s-foot trefoil (Lotus corniculatus), from which they sequester CNglcs. But that’s not all; the larvae can also produce their own CNglcs from amino acids in their host plant. As a result the caterpillars, and later the adults, are extremely unpalatable, which could partially explain why burnet moths can afford to fly about during the day: few predators would dare to take them. And they will have been well warned by the moths’ black-red contrasting colouration, which is a common pattern among poisonous animals.

A six-spot burnet moth caterpillar at Forvie NNR. ©Lorne Gill/SNH

But the six-spot burnet moth’s association with poison is even more remarkable. Researchers have found that females release gaseous plumes of hydrogen cyanide to attract males, and refuse to mate with those with a low content of CNglcs. If a male is perceived to be suitably toxic and accepted, it then transfers some of its own chemicals to the female during mating. It is believed that this ‘nuptial gift’ is then relocated to her eggs to help protect them from predators.

When life gives you lemons, make lemonade; but when life gives burnet moths cyanide, they make chemical weapons.