Scallops (Pecten maximus) are a national concern on the Isle of Man. We have some of the most protected scallop populations in the British Isles. Licensed boats can only fish during daylight hours in certain areas of the sea and not during the summer months when the scallops are breeding. The catch is landed into harbours around the island; creamy, pink shells in 25kg bags loaded onto pallets for the forklift truck to move them into wagons.
If you glance down into the harbour its usually possible to spot the white inside of a few discarded shells shining on the seabed below. These shells eventually wash across the bay and onto the beaches, but they don’t always arrive in the same colour as when they were discarded. Many of the shells are stained dark brown or black, colours we never see during dive surveys of scallops.
Shells are mostly made of calcium carbonate which is white in colour, mixed in with about 2% of protein. As molluscs develop they absorb minerals from their environment and secrete calcium carbonate from their mantle to create their shell. The protein makes the shell very strong, but lightweight and resistant to dissolving in water. Shells are self-repairing, and the mollusc can secrete more shell material as needed for repair or growth. Naturally occurring colour and patterns in shells is as a result of mineral ions incorporated into the shell structure. But that doesn’t explain the post-mortem colouration of the scallop shells.
Shallow burial of shells causes iron oxides to form in the tiny pits on the surface of the shell and causes brown staining. The black colour is usually due to microscopic crystals of iron sulphide. These crystals form in the absence of free oxygen which can occur if shells become buried deeper in mud or sand.
Although my local harbour is sheltered, it doesn’t provide the deep mud conditions required to blacken shells, but there is a much more common cause. Burial under just a few centimetres of seaweed rotting on the beach will provide suitable anoxic conditions for sulphide formation. Hence blackened shells on the beach is a relatively quick process occurring under mounds of kelp and wrack.
There are some mollusc species that live well buried into deeper sediment. The Ocean Quahog (Arctica islandica) is a subtidal species of clam that is renowned for it’s longevity. Some individuals have been recorded at over 500 years old. The shells of Quahogs have dark black colouration, but they have a long time to absorb the necessary pigment. Whilst the shell is buried in the sediment, a siphon to the water provides for food and oxygen to the creature below.
Naturally acquired pigment probably strengthens the shell. Colour patterns often align with spiral or axial sculpture. Instead of producing and transporting a thicker shell, it might be more energy efficient for molluscs to make pigments. Pigments impede propagation of a crack in the shell. The structural explanation also works for colour inside of shells. A good example is Mercenaria mercenaria (the quahog or cherrystone clam). The purple inside the shell, hidden when the animal is alive, lies along the edges of the shell, just where predator whelks are likely to attack. Strangely young Merceneria don’t make the purple pigment. Their shells are too thin to resist attack anyway, so they concentrate their efforts on growing a thicker shell and surviving to when their pigment strengthened shell is going to ensure a long life.
There are lots of other reasons for shells to have different colours. A favourite project for marine science students is to send them to look for colour variation in Flat Periwinkles (Littorina obtusata). In this case pigment is used for camouflage, allowing the winkles to hide amongst the bladder wrack on the shore. Pigments may serve as a warning to possible predators, or the pigmentation pattern may provide a template for future growth of the shell. But there doesn’t have to be a reason for pigmentation in all cases. Oxygenated mammalian blood is red, not for any evolutionary reason, but because that’s the chemistry of the situation.
Seals are collectively known as pinnipeds, which means from the Latin pinna (fin) and pes (foot). This classification includes the walruses, eared seals and true seals. The Isle of Man and the rest of the British Isles are home to resident populations of Grey seals (Halichoerus grypus) and Common seals (Phoca vitulina). Common seals (also known as Harbour Seals) are found in both the North Atlantic and North Pacific. About 35% of the European population of common seals lives in UK waters. By contrast Grey seals are only found in the North Atlantic, Baltic and Barents Sea. The entire world population of Grey seals is probably no more than 400,000 individuals, with about 40% of them living in UK waters. Although we tend to take seals for granted, we should perhaps appreciate how lucky we are to have them in the waters around us and see them so often.
There’s been a long understanding that the pinnipeds evolved from land based mammals. This concept in itself is a little strange, as the general gist of evolution is that our ancient ancestors left the watery environment for a life on land. But somewhere millions of years ago, some of the mammals returned to the sea to take advantage of the feeding opportunities that existed there. Whales and dolphins have definitely taken their return to the marine environment to the extreme and evolved to the point that they can no longer safely return to the land. When they do, the amazing guys from the British Divers Marine Life Rescue swoop in and work their hardest to throw them back into the briny again. In the pinnipeds we have a group of species who spend their time mostly in the marine environment, returning to land only when necessary. On land seals are ungainly, slow and clumsy, which made them an easy target for hunting. In the water, they are agile hunters, capable of diving to about 200 metres for up to 15 minutes.
The clues to the pinnipeds evolutionary past are clear in a number of ways. Their forelimb has five webbed fingers, with claws that are used for grooming and fighting. This five fingered (pentadactyl) limb structure is a common evolutionary feature, linking many vertebrates including reptiles, birds, mammals and amphibians. Just let that sink in for a moment. You can see the same bone structure in pretty much every group of animals with bones. The humerus at the top, an elbow where the radius and ulna join, a wrist connecting to fingers. It’s there in the bats wing (with elongated fine boned fingers and skin stretched over them), it’s there in frogs (although the ulna and radius have partly fused), and cats and dogs and tigers and crocodiles and in us..
In seals the flipper bones that would be the equivalent of your arm are shortened, so that what appears to be their armpit is in fact their elbow (front flipper) or ankle (hind flipper). Their metatarsals (fingers) are elongated compared to ours and the skin in between gives them something akin to swimming gloves. Close interaction with a seal will reveal that they can still bend their webbed fingers to grip and hold onto objects or, if you are lucky enough, onto you as you are diving. Their flippers are well adapted to propel them through the water. When swimming quickly, the hind flippers are used in a side to side motion, and the front flippers are held against the body. If you have watched seals turn under water, you’ll know that they stick out a front flipper to perform sudden changes of direction. Cruising speed for seals is about 2 to 3 knots, but when hunting seals can move at an astounding 20 knots (that’s probably faster than most club ribs!).
Seals are part of the Caniformia (dog like) sub order of the Carnivora group of Mammals. In fact, most divers that have had encounters with seals will tend to describe them as being like big puppies. Despite this, there have been many studies suggesting that seals are in fact more closely related to bears than they are to dogs. Perhaps the fact that we are more likely to have encountered and interacted with dogs rather than bears gives rise to our misconception? Remember that Grey seals are the largest living carnivore in Britain, can grow up to 2.3 metres and weigh over 300kg and treat these amazing creatures with the respect they deserve. When you get to shake hands with a seal next time, count his fingers and say hello to a very distant cousin.
Sellafield is located across the Irish Sea on the Cumbrian coast and is approximately 32 miles from the Isle of Man, on a clear day you can just about see it. The main activities at the plant include reprocessing of spent fuel from nuclear power reactors and storage of nuclear waste. There are no longer any nuclear power plants in operation at the Sellafield site. It was built in the late 1940s to manufacture plutonium for atomic bombs and Sellafield is one of the most radioactive places on earth. In its prime the plant was releasing eight million litres of contaminated waste into the sea every day. In 1957 the plant became the site of the worst nuclear accident in Great Britain's history, The Windscale Fire. This was a blaze that raged for three days, releasing radioactive gases into the air. The discharge of low level liquid wastes from the Sellafield site in the north west of England is the most significant source of artificial radioactivity in the Irish marine environment.
Now the site is mainly used for nuclear fuel reprocessing, and this and other activities gives rise to the discharge of low level radioactive materials in the form of liquids and gases into the environment. These discharges are regulated by the UK authorities and limits for releases are set by the Environment Agency of England and Wales (EA). Liquid radioactive waste is discharged from the plant into the Irish Sea via a pipeline, about 3 km from land. Gases are released from the plant via a number of chimneys (referred to as ‘stacks’). Discharges into the Irish Sea peaked in the mid-1970s and have dropped significantly in recent years. This is as a result of improved waste treatment facilities at Sellafield, which convert much of this radioactive waste into a solid for long-term storage.
As a result of the discharges from Sellafield, low levels of artificial radioactivity can be detected in sediments, seawater, seaweeds, fish and shellfish taken from the Irish Sea. A wide range of marine samples are collected and analysed on a regular basis by both the EA and the Manx Government. This monitoring can show where the radioactive particles become concentrated. As expected many particles end up in sea bed sediment, so there are sometimes slight increases when the winter storms have been especially ferocious and stirred up the seabed. Generally, levels are falling from their peak in 1998.
There are several radioactive isotopes that are monitored, Technetium-99, Caesium-137 and 134 and Cobalt-60. Of these, Tc-99 is regularly tested for by catching lobsters. Tc-99 concentrations in our local lobsters have declined from a peak of around 400Bqkg-1 in February 1998 to average 10 Bqkg-1 during 2015. These Tc-99 concentrations are lower than the levels found in lobsters caught off the Cumbrian coast. The EC recommended maximum permitted level for Tc-99 in seafood which is 1250 Bqkg-1, so these lobsters are safe to eat and regularly eating seafish will only make a minor contribution to your overall radiation exposure.
Now it’s not true to say that lobsters are immortal, but once they reach adulthood they don’t have many predators except humans. Good lobster fishery management sets minimum landing sizes for lobsters, ensuring that they are at least able to breed once before being caught. Small lobsters can get out of pots through the escape hatch or they are returned to the sea anyway.
Just as lobster pots discriminate against small lobsters, they also prevent very large lobsters from getting in. Consequently, larger lobsters do tend to live a very long time. The lifespan of European lobsters has been estimated at between 30 and 50 years. Large lobsters have lived through the peak discharges from Sellafield, unlike their smaller 3-4 year old counterparts who got caught in lobster pots and tested. Lobsters have a fairly high affinity for Tc99 and they accumulate the radioactive particles in their bodies. But the only real predator for the large lobsters is, you’ve guessed it, divers.
Something to think about the next time you wrestle a monster lobbie from under a rock
Some years ago I craved having a tropical fish tank. I’d had coldwater fish starting with the short-lived goldfish I won at the carnival hoopla stand, but tropical fish seemed like they were more interesting. The big problem is that a fish tank is a bugger of a thing to move and at that time in my life it became a chore and a burden. I relocated 6 times with the fish in bags inside a coolbox, hence it was with some relief that when the last fish died I emptied out the last of the water and put the tank away, promising that when life was a little less hectic I’d get it back out and set it up again. About 6 months later disaster struck when I cracked the glass at one end, but I didn’t get rid of the tank, just planned on repairing it. One of those tasks on my endless to-do list.
And then my goals changed. Stuff the guppies and their frilly tails, why not set up a coldwater marine tank? After all I spend a large amount of my life underwater, why not bring some of the great critters back? Several times a year I visit schools on the Isle of Man and bring a variety of sea creatures in to meet the children and explaining something about their lifecycles. I’ve developed a habit of going and collecting little stuff anyway.
A chance conversation with one of our club members who wanted to rehome one of his tanks, ended up in him loaning me a pump and a chiller unit as well as a fish tank without a crack in the end. At 10am we were having a brew in the dive centre and by 2pm I was stood ankle deep on the slipway filling a cleaned out sofnolime container with seawater. Our marine tank was installed and populated within 24 hours. And if I thought the tropical freshwater tank was hard work, I had another shock coming. Weekly 50 litre seawater changes are just hard work.
I now spend my time thinking about the ecological balance of the tank much more than I ever bothered with guppies. When you keep tropical fish there is loads of info about how many fish per litre of water etc, for British marine life tanks there isn’t the same guidance. A small edible crab was a disaster and massacred poor Kevin the Masked Crab within 24 hours. Kevin had a dodgy past himself, and was often seen amputating limbs from small brittle stars, so he was called Kevin the Killer Crab, but we had grown fond of him and it was sad to see parts of his exoskeleton scattered around the tank.
Our current population includes about 10 hermit crabs, who mainly seem to fight over shells and ignore the rest of the inhabitants. We have two small shore crabs, although one of them is getting a little larger and consequently even hungrier. I’ve a feeling he’ll be heading back to the shore next weekend. We’ve ended up with about 30 North Atlantic Prawns who pounce any food in the tank, and will come to your hand if you put it in. Small Purple Henry starfish, a juvenile scallop, a small common sea urchin, some limpets, Top Shells and Periwinkles complete the scene. We’ve had small fish (they get eaten).
The current star of the show is our Leach’s Spider Crab (Inachis phalangium). Leachy has a small triangular carapace which will reach a maximum of 3cm. I picked him as he ran across a sandy patch between rocks. I’ve seen small spider crabs before, but never really bothered too much about them. Leachy’s small size made him a target for the tank. After a short trip in an old ice-cream container, he was released in the tank. On the same day, another diver brought in 3 small Snakelocks Anemones.
It turns out that Leach’s Crabs have a commensal relationship with Snakelocks Anemones, the crab benefits but not to the detriment of the anemone. Females stay with their anemone and males will rove around looking for a mate and then return home. They are beautifully camouflaged, with legs covered in sponges and algae. This isn’t by chance, Leach’s Spider Crab actively collects sponges and algae and attaches them to specially shaped spines on their legs and carapace. The sponges are unpalatable and stop predators from attacking the spider crab. The algae form a part of the diet, which also includes food debris from the anemone and the mucus from the tentacles. Our intrepid little Leachy has beautifully evolved to fit into his ecological niche. Admittedly, that’s not meant to be in a dive centre tank, but on the plus side, none of his natural predators are there. We’ve so far avoided large fish or octopus. Periwinkles occasionally find their way out past the pipe work so we’d have no chance of keeping a cephalopod and Leachy is safe for now and I’ve learned a lot more about him.
Michelle has been scuba diving for over 20 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.