​Comb jellies are a common sight around British waters, particularly the sea gooseberry (Pleurobrachia bachei). With a clear body and measuring only a couple of centimetres long, it’s easy to overlook these beautiful little animals apart from one thing, stunning rows of rainbow coloured hairs running along their body. With no skeleton and composed of 99% water, you’d be forgiven for thinking that these were very simple creatures, but actually they have some pretty neat stuff going on.

Firstly, let’s start with where the comb jellies fit in, they are not the same phylum as jellyfish. Jellyfish (Cnidaria) have complex lifecycles and stinging cells. Comb jellies (Ctenophora) have a simple life cycle and make glue to catch prey. The problem for scientists trying to study the evolution of the Ctenophores is that they don’t make very good fossils so the records are patchy at best.  Genetic studies indicate that Ctenophores are much older in origin than other animals, and predate the bilaterian animals (those with bilateral symmetry) by millions of years.

There are 2 major cell layers, the external epidermis and the internal gastrodermis. Between these layers is the mesoglea, a jelly like layer.  The Ctneophores have muscles running through them and the outer epidermis contains a basic nervous system known as the nerve net. Comb jellies are named for their unique feature, plates of giant fused cilia (small hair-like cells) which run in eight rows up and down their bodies.  These cilia are used to propel the comb jelly through the water.
Many comb jellies have a single pair of tentacles and often these tentacles are branches and give the illusion of many tentacles.  The branched tentacles are used to catch prey, but unlike the toxins from jellyfish, comb jellies use glue.  Special cells called colloblasts respond to touch by firing a spiral filament and releasing sticky glue. Once the prey is stuck the comb jelly reels in the tentacles and brings the food to its mouth. Most comb jellies are carnivorous and will eat anything. Until recently it was believed that comb jellies spat out the indigestible waste particles, but it now seems like they release them through pores in the rear end – which is probably part of the evolutionary puzzle in explaining when animals developed anuses.

What about the beautiful rainbow coloured pulses of light? Whilst some comb jellies can produce light by a special chemical reaction to give photoluminescence, our sea gooseberry can’t. The rainbow colouring is caused by refraction of light through the hair like cilia cells. Light travels at a constant speed in a vacuum, but when light encounters a material that is more dense, it slows and its pathway bends. The different wavelengths in visible light bend at slightly different angles.  This means that the cilia act like the glass prism you played with in physics classes as a schoolchild, splitting the light into separate wavelengths. The refractive index (how much the light will bend is almost the same for the comb jelly tissue as it is for the salty sea water. The hair like cilia amplify the refractive effect creating an iridescent pulse that is mesmerising to watch. The effect works across the visible and UV light wavelengths, meaning a fluorescent torch will also give a great image.
​
Lastly, there’s a few things you should know about comb jelly reproduction. Comb jellies are hermaphrodites (both sexes in one individual) and can spawn eggs and sperm freely into the sea, through their mouths. They do this on a continuous basis unless they are starved of food, when they will shrink down and stop breeding. But feed them up again and they will start spawning again. Rainbow coloured hair, sticky glue fishing nets and continual reproduction – what’s not to admire?

Over a year ago when we set up our marine tank, complete with filters and chiller, I envisaged that it would be mainly the rock pool animals that would survive. My inexperience at maintaining a marine tank would be compensated by the relatively hardy nature of rock pool creatures.  Any critter that has evolved to tolerate massive variations in temperature and salinity (rainwater run off dilutes the salt in rock pools, but evaporation makes them saltier) was probably going to survive my attempts to keep it alive.  As time has passed, we have become more adventurous about the creatures that we have attempted to keep, but the latest inhabitant of our tank is another step up…
We have a shark egg case. With a live lesser spotted catshark. And we can see it growing!

We see catsharks quite often on dives, in fact they’re probably the most common shark. Many non-divers are stunned when you tell them that there are 21 species of shark that are native to British waters, and probably another 19 or so migratory species too. The film Jaws was released in 1975 and over the past 40 years has had a massive negative impact on the general public’s perceptions about sharks.  Organisations like the Shark trust have been battling this perception ever since.  Even my own children make jokes about Jaws like sharks, when they have never seen the films and have had the privilege of diving and snorkelling with sharks. And now we have a live shark in the dive centre!
We have no idea if the shark in our tank is male or female. It’s currently less than 2cm long and inside it’s 7cm long protective egg case. At this stage there is more yolk than embryo shark. Shining a torch through the egg case allows us to see the embryo wriggling around inside the case. The hunt has been on for a gender-neutral name, and we’ve settled on Charlie the Catshark. We are much more used to seeing mermaids purses wash up on beaches than finding one alive and still growing, and managing to get it into an environment that we can observe its development. We don’t have any dog whelks in our tank.  They are voracious predators and you can bet that anything capable of drilling into a Periwinkle shell for a meal would have no trouble getting through the mermaids purse to eat Charlie.

Whilst we watch Charlie grow we will see the yolk sac diminish in size. It’s easy to understand that the yolk must contain ‘nutrients’ to allow this growth; amino acids to make proteins for muscle and cartilage, and fatty acids which are vital to make cell membranes and to metabolise for energy. Shark egg yolk can contain over 50% fat which is roughly similar to chicken egg yolk. Most of the fatty acids are unsaturated fats which is important to keep them mobile in the low temperature of the sea. There are no carbohydrates. It’s hard to get carbs into an efficient storage molecule in such a limited space. There are other substances in the yolk vital to Charlie’s development especially hormones which will drive growth and sexual characteristic development.

We are going to have a long time to wait. It will take 9-10 months before Charlie chews his/her way out of the egg case. At that point, Charlie will be around 15cm long and will be heading back to the sea before he/she has a chance to eat everything else in the tank.  My money is on the shrimp being the first casualties. But really a marine tank in a dive centre is no place for a young shark to grow up. For now, Charlie is safe inside the egg sac and the other inhabitants in our tank are safe from Charlie.
​
We have a rare chance to capture the attention of local children. We often have young visitors into the dive centre to hold a hermit crab, small urchin or starfish, but now we can show them a shark and talk about conservation. We are hoping to catch them before the cultural references to sharks start to become ingrained. The latest viral hit song ‘Baby Shark’ will help too. A combination of an ear-wormy irritating song that kids adore plus the chance to watch Charlie grow. This is a hearts and mind battle we have a good chance of winning. Do do do do do (a quick online search required if you haven’t seen/heard it yet – and my apologies!).

In May 2017, a female Orca (known as Lulu) was found washed up dead on the shoreline on the Isle of Tiree in Scotland. Lulu was one of the last remaining members of Britain’s only resident Orca population, which now only comprises 8 individuals, 4 males and 4 females. This west coast community had been monitored by researchers for over 25 years.

Orcas are very social animals with the Orca mother at the centre of the pod. Her children, including adult sons, stay together throughout their lives. Female Orcas start reproducing around 13 years of age and during her lifetime a female will have 4 to 6 calves and stop reproducing around 40 years of age. Pregnancy lasts about 17 months and is followed by a period of mothering in which other members of the pod will assist with babysitting duties.

Different organisms have evolved to have different reproductive strategies. At one end of the spectrum there are organisms which produce a huge number of offspring, in some cases the parent then dies or never reproduces again. This Big Bang reproduction is a good strategy if the organism has a high probability of dying early. If the chances of surviving are low, then it makes sense to invest your energy into a Big Bang reproduction when conditions are right and thus ensure your genes carry on into the next generation. Many fish and some insects employ this strategy including salmon.

At the other end of the spectrum are the organisms that have a single or only a few offspring but at multiple times in their lives. Humans along with other large mammals use this strategy, known as iteparous reproduction. These organisms have a low probability of dying early, so it is expected that the offspring will have a good chance of survival to maturity. Any species that takes years to reach breeding maturity and has a limited number of offspring is vulnerable to any factor that impacts on breeding success.

In between these two extremes are organisms like coral, who breed en masse at limited times, generating lots of offspring but only when the moon is in the right phase.  These organisms have a constant risk of dying throughout their lives and so a constant reproduction strategy is a good idea, balanced against the resources needed for reproduction to happen.

Researchers are certain that there have been no new calves born into Lulu’s pod in the last 25 years. The entire pod is ageing and it is likely that during our lifetimes there will be no resident Orcas in British waters. A post mortem on Lulu has provided the clues that man-made polychlorinated biphenyls (PCBs) are likely to be the culprit. There has been concern over the persistence and biological effects of PCBs for many years. Scientists have established that physiological effects can be measured with around 30 milligrammes of PCBs per kg of body tissue. Lulu’s level was 975mg per kg. Furthermore there was not evidence that Lulu had ever been reproductively active or ever had a calf.

PCBs are a group of chemicals that were originally made from 1929 until they were banned in 1979, by which time it is estimated around 700,000 tonnes had been produced. They are chemically stable, non flammable, have high melting points and are poor electrical conductors. For many years PCBs were used as insulators in transformers and capacitors and cable insulation, plasticizers in paints and plastic, and pigments and dyes. Does anyone remember the carbonless copy paper? For the younger readers, you’ll need to look up carbon copy (and then you can understand why we cc people on a email) and understand what a revolution it was when carbonless copy paper arrived.

PCBs aren’t very soluble in water, but they are easily soluble in organic solvents and in body fat. They accumulate through food chains and long lived species will end up with high levels. PCBs are still being released into the environment today from poorly maintained waste sites, leaks from electrical transformers and incinerators using a temperature too low. By the 1960s PCBs could be found at trace levels in people and animals around the world, not just in heavily populated areas, but in the Arctic too. The health effects are widespread; PCBs cause cancer, reduce the function of the thyroid and the immune system and cause neurological deficits. Importantly for survival of Orca, PCBs reduce conception rates in females and sperm count in males and reduce the birth weight of calves.
​
Lulu’s death was caused by becoming entangled in fishing ropes not by PCBs. However, you don’t often hear about Orcas becoming entangled. They are very intelligent, nimble and aware creatures. It’s highly likely that the PCBs in Lulu’s body impaired her ability to navigate a common hazard, but to be brutally honest the PCBs had killed her chances of reproducing years ago. Her body may have washed up in 2017 but her pod have been dying for years. Humans changed the chances of survival and sadly there is no hope of saving them now. 

Ever since the footage from Blue Planet II hit our screens, there has been a growing awareness of the devastating impact of plastic on the marine environment. Anyone who has taken part in any of the Great British Beach Cleans organised by Marine Conservation Society volunteers will have already know much of the debris collected every year is plastic, from fish crates to drinks bottles, rope to the cotton bud sticks with cigarette butts thrown in for good measure. Whilst the MCS encourage volunteers to collect every item in a 100 metre stretch for their survey work, its certain that there are some plastic particles that are just too small to be picked up. These are the microplastics.

Microplastics are particles less than 5mm in size, ranging down to 5μm. That’s the Greek letter mu which represents micrometers, one million times smaller than a metre, a thousand times smaller than a millimetre. This size range is why these particles are called microplastics. Microplastics are not as easy to count. A collaboration between the University of Portsmouth and a charity called Just One Ocean is running a scheme to encourage people to survey their beaches for microplastics. There’s a carefully designed citizen science method to allow data from around the UK shores to be submitted and collated. We don’t fully understand the scale of the problem yet, but scientists from a number of disciplines are expressing concerns that it’s much larger than anyone suspected.

Microplastics have many sources. Large plastic pieces tumbling around in the marine environment will gradually break down into smaller and smaller pieces. Washing synthetic clothes in a washing machine releases microfibres into the waste water. If you run a tumble dryer you will know that you have to clear out the lint from it. That is just the larger fibres that get trapped. Smaller particles will enter the air blown out from the dryer. Microplastics have been found in the air, soil and water across the world. Since the 1960s, plastic particles were deliberately put into toothpastes and body care products as microbeads. They helped scrub the plaque off your teeth or acted as an abrasive layer to remove dead skin in facial scrubs. Those plastic beads were useful for about 2 minutes (because we all brush for the recommended time don’t we?) and when you rinsed and spat, those plastic beads were set free on the world. And the chances are they will exist long after you have gone.

We also find microplastics in food and bottled drinks. Filter feeding marine life picks up the particles from the environment. Mussels tested from 8 locations round the UK and samples bought from 8 UK supermarkets were all found to contain 70 microplastic particles per 100g of meat. Beer, honey and sea salt have also been shown to be good sources. Some studies have shown that over 80% of tap water samples contained microplastic and bottled water was even worse. One sample of bottled water had over 10,000 plastic particles in a litre. Science has a funny way of examining such issues, they’ve even looked at human faeces to gauge the levels that we are consuming. They found 20 microplastic particles for every 10g of stool sample.

So, what’s the big issue? Indestructible little bits of plastic that are so small you can’t see them with the naked eye? And they obviously pass through your intestine because they are too big to be absorbed. Why should anyone care? And to a certain extent we don’t fully know yet – and that may be the scariest aspect of this story.

When plastics are made, they aren’t a pure substance. Thousands of different chemical additives can be thrown into the recipe to manipulate the properties of the plastic. As large plastics breakdown, these additives are released. The other concern is that microplastics are very good at adsorbing (not absorbing) certain chemicals onto their surface. You might have heard of a few of these; PCBs, PAHs and DDT are on the detected list. We’re reasonably certain that probably 90% or more of the plastic microparticles that you’ve ingested have passed through your intestinal tract. But what scientists cannot quantify yet is whether they transferred damaging levels of toxins to you on the way. The data so far suggests possible disruption of your gut microbiome and inflammation. The consequences of long term repeated exposure to the toxic chemicals carried by the microplastics is still being assessed. It doesn’t bode well. We know from other species how devastating persistent organic pollutants can be.
​
There are estimated to have been 8.3 billion metric tonnes of plastic made since the 1960s, of which over 4.9 billion metric tonnes has been discarded into the environment. This is a problem that will not be easily solved and we are all at risk.

​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.