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 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.