Divernet
All life - well, a whole bunch of colourful British marine life - can be found on one shore dive in Cornwall. John Liddiard turns his attention to a very popular crevice

PORTHKERRIS REEF, ALSO KNOWN AS DRAWNA ROCKS, is a shore dive I know very well. Every spring finds me there, helping my local dive club take beginners on their first few dives.
I often return later in the year, usually for boat diving, but a quick spin round the reef can still be a pleasant bonus at the end of the day.
There is plenty to see on the reef, and naturally it varies with the seasons. But no matter what there is to see overall, the highlight of the dive is often just one crack in the rocks, about 3m long and between 2cm and 5cm high.
I used to refer to it as the crack in the back of the reef where the shrimps and blennies live. Then, one year, some bright spark referred to it as the Crack of Life, a description so obviously appropriate that it stuck.
Now, whenever I ask a beginner what they want to do on the dive, which exercises they need to complete and what they want to see, the Crack of Life is a frequent request...


hspace=5 FINDING THE SITE
The Crack of Life runs almost horizontally along the rock, at the back of the big rock at the south end of the reef.
This rock is separated from the rest of the reef by a canyon just wide enough to swim through, a nice bit of scenery in itself. An easy plan is to take a compass bearing on the canyon and head out from shore under water.
Go through the canyon, rising slightly to clear the rock across the outside end. Still facing out to sea, descend a few metres following an obvious square cut that opens out. Where this meets the seabed and the rocks, keep the reef on your right and you are already at the Crack of Life.


hspace=5 PRAWN / COMMON SHRIMP
Also known as the common prawn, these small crustaceans can sometimes be difficult to spot, because they are almost transparent except for faint brown stripes on their bodies and paired brown and yellow or orange bands round their arms and legs.
For most of the day they stand at the edge of the crack, ready to dart back by flipping their tails beneath them if threatened. At night, they stroll out to roam the rocks, foraging for food. They can also swim, using their legs. Like many crustaceans, they will eat almost anything they can chop up small enough with their pincers.
In the summer after mating, the female will have a cluster of a couple of thousand eggs guarded on the underside of her tail.
These hatch to swimming larvae that form small clouds for a couple of weeks before settling down to walk the reef.
Like crabs and lobsters, these prawns grow by shedding their shells, to a full size of about 5cm. They can live three to four years.


hspace=5 EDIBLE CRAB
While the face of a tompot blenny is always happy and smiling, the face of an edible crab always looks grumpy. It wears a permanent frown.
Edible crabs are not at all fussy about where they live, and can be found just about anywhere we care to dive in UK waters, at any depth and on any type of seabed. They can live for up to 20 years and grow up to 30cm across the carapace.
When threatened, they have various defence strategies, from burying themselves in the sand to hunching their shoulders to wedge themselves into a gap in the rocks, with their big claws in front of their faces.
A full-grown edible crab would never fit into the Crack of Life, so the inhabitant I found was just a baby a few centimetres across. If it grows much more, it will either have to move out to larger accommodation or be stuck there.
Like the velvet swimming crab, the female uses her tail tucked beneath her body to hold and protect eggs.


hspace=5 CREVICE SEA CUCUMBER
Crevice sea cucumbers wedge their bodies into cracks in the reef and wave their tentacles outside to catch particles in the water. If you watch closely, they will curl one tentacle at a time into their mouths, and suck on whatever they have captured.
Its a bit like licking your fingers after getting them all gooey with the barbecue sauce on a plate of ribs.
In the Crack of Life, the crevice sea cucumbers live right at the end, where the crack is very narrow and their bodies are protected.




hspace=5 GOLDSINNY WRASSE
The goldsinny is one of the smaller species of wrasse we commonly encounter as divers, growing up to 15cm long. Its body is a brown-gold colour, fading to white on the underside, with a single black spot on each side towards the top of the body and just before the tail.
Like all wrasse, one male will control a territory with a harem of females. Should the male die or there be too many females, one of the females changes sex to become the next male.




hspace=5 TOMPOT BLENNY
An inquisitive nature and a cheeky, endearing smile make tompot blennies a favourite with many divers. Growing up to 25cm long, they like to live between stones and in cracks and crevices in reefs or wrecks at pretty much any depth within the usual air diving range.
The cracks are merely a home, and tompot blennies venture out in search of food and often just to be inquisitive when a diver settles nearby and does not threaten them.
They have small rasp-like teeth and, when beckoned and teased with the bare end of a divers finger, make a tickling exploratory nibble.
In addition to the smile, other characteristic features are the branching antler-like antennae above each eye. The shape of these antennae is one means of distinguishing them from the similar, but less common, Yarrells blenny, where the antennae are more like puffs of sponge on sticks.
The tompots colour is a mottled orange and brown with broad vertical stripes to blend in well with rocks covered in a turf of hydroids and bryozoans.




hspace=5 SQUAT LOBSTER
Squat lobsters hide in crevices or burrows during the day, never venturing further than would allow them to dart back to safety if threatened.
At night, they may range further to forage for food, nibbling away at almost anything they can cut or pull apart with their pincers.
Five species are commonly found in UK seas, some easier to distinguish than others. The residents I found in the Crack of Life were tiny, only 1cm or so long in the body, and smaller than the prawns. Coloured bright red with some blue spots, I think they are Galathea intermedia.
The spiny squat lobster Galathea strigosa is also red with blue highlights and lives in rocky habitats, but grows up to 12cm long, so the ones in the Crack could also have been babies of this species.
While squat lobsters look like tiny lobsters, marine biologists reckon that they are more closely related to hermit crabs, substituting a rocky crevice for a borrowed shell.


hspace=5 VELVET SWIMMING CRAB
The most striking features of velvet swimming crabs are their vivid red eyes, ball-shaped and on stalks for 360 vision. When photographing them, make sure that you have red-eye reduction turned off, or your picture could be disappointing.
The rest of the body is a velvety textured green-brown, with black lines running up the legs. The swimming part of the name comes from the back legs, which flatten out into paddles to help these crabs move faster through the water, though they dont really swim - its more an extended leap.
When approached they are particularly aggressive, rising up, spreading their claws wide and slashing at anything within range, no matter how big. Larger velvet swimming crabs, growing up to 8cm across the carapace, will even sometimes come out of hiding to attack. Having said that, the small ones in the Crack of Life prefer the security of the rock.
Underneath, they have a vestigial tail. Females use this to hold eggs during the breeding season.


hspace=5 CONNEMARA CLINGFISH
As the name suggests, clingfish can cling to the rocks using a sucker that is evolved from their pelvic fins, usually clinging upside-down beneath a rock. They are found down to 30m in the south-west of the UK and Ireland, growing to 8cm long, and come in various colour schemes, including the orange-brown of those in the Crack of Life.
A very similar species, the shore clingfish, also known as the Cornish sucker, is found in rock pools and shallow water.
The key difference is that the shore clingfish has a pair of blue spots on the back of its head, and a pair of small tentacles just forward and between its eyes.


COMMON STARFISH
hspace=5 The common starfish is one of many types of starfish that can be found just about anywhere in UK waters, so it is hardly surprising that a few small ones have found their way into the Crack of Life.
Common starfish have five arms and grow up to 50cm across, but are often considerably smaller. They move about by walking on the hundreds of tiny tube-like feet that project from the undersides of their arms. Their eyes are clusters of light-sensitive cells on the ends of these arms. When you see a starfish with the tips of its arms raised up, it could be lifting its eyes for a better view.
Starfish eat by climbing over their meal and extruding their stomach to envelop it. They can use this technique to feed on just about anything that will stand still long enough for them to digest, from a dead whale to shellfish like mussels, though they use their arms and feet to pull the shell open first. Shellfish are a firm favourite, and a nearby starfish is one of the things that is sure to set a scallop flying.
When attacked by a predator, starfish can shed an arm to get away. The lost arm then regenerates, so occasionally you will see starfish with mis-matched arms.
Contrary to popular myth, you cant grow a new starfish from an arm - the central disc is needed for that.


THE ONES THAT GOT AWAY
There were none this time round in the Crack Of Life, but other fish I have seen living there in the past include topknots, which are flatfish; butterfish, which are long and thin like small eels; and leopard gobies, which are grey with spots.


PHOTOGRAPHING THE CRACK
I used a housed SLR camera with macro lens and dual strobes, but many compact cameras with a good macro capability and a single external strobe or video light would be equally capable of taking pictures in the Crack of Life. The critters stay still, so shutter-lag on compacts are not an issue.
The first problem is focusing on something deep in the crack. Autofocus needs good light with which to work, and is easily confused by the rocks above and below. If your strobe does not have a built-in aiming light, a separate dive light will be needed to spotlight the subject while focusing.
When autofocus struggles, rather than going to manual focus I use autofocus to home in on any convenient target at approximately the right distance, then lock the focus and point my camera at the real target, moving it in or out until the focus is correct.
The next problem is getting the flash light onto the subject. Any built-in camera flash would most likely leave the subject in shadow, so an external flash is needed. It needs to be held pointing into the crack such that the subject is not in shadow. I used flexible flashgun arms to get the positioning right. Another strategy would be to hand-hold a flash, or even to get your buddy to hold it for you.
The final problem is also to do with lighting. With the walls of the crack in the foreground, so closer to the flash, any automatic light metering is easily confused.
I tried each subject first using automatic metering with positive exposure compensation, then again with completely manual control. Both worked, but the subject that came out best with each method varied.
Before the instant feedback and huge capacity of digital cameras, photographing the Crack of Life would have been an enormous task spanning many dives. With digital, the whole lot took 45 minutes, and the massive number of megapixels gave our Art Editor Tom plenty of scope to crop in tight and enlarge the interesting parts of each picture.

FACTFILE
DIVING & AIR: Porthkerris Divers, 01326 280620, www.porthkerris.co.uk
ACCOMMODATION: Camping at Porthkerris or B&B or static caravans nearby on the Lizard. Falmouth Tourist Information Centre, 01326 312300, www.go-cornwall.com, www.carrick.gov.uk
FURTHER INFORMATION:Admiralty Chart 154, Approaches to Falmouth. Ordnance Survey Map 204, Truro, Falmouth and Surrounding Area. Seasearch Observers Guide to Marine Life of Britain and Ireland by Chris Wood. Hamlyn Guide To Seashores and Shallow Seas of Britain and Europe by Andrew Campbell. Marine Wildlife of Atlantic Europe by Amanda Young. British Sea Fishes by Frances Dipper.