Firth of Clyde Forum Annual Conference 2007
The Colourful Lives of Britain’s Marine Animals, by Paul Naylor
Wonders close to home
British coastal waters contain the most wonderful array of marine animals, and this can surprise people who assume you have to visit exotic foreign locations to see such sights. Right here on our doorstep we have colourful fish, dazzling sea anemones and corals, and a whole host of other beautifully exotic creatures. There are charismatic animals too, and many of them don’t just look spectacular but also exhibit intriguingly bizarre lifestyles and behaviour. The octopus, for example, preys on crabs by enveloping them in its tentacles before injecting them with a fluid that not only paralyses the crabs, but turns their flesh into a soup that can be sucked out from the crabs’ body armour before this is discarded!
Just as you don’t need to go abroad to observe these wonders, neither do you have to don complex diving gear and venture far offshore. Many fascinating animals can be seen by snorkelling or even by exploring rock pools. The dramatic sight of predatory fish hunting smaller shoaling fish as they swirl to escape can be watched in waist-deep water. Nor are sights like these restricted to the expected locations such as south west England and western Scotland; they can be seen anywhere conditions allow clear water to occur (although a lot more patience and luck may be needed in some places than others).
The plankton feeders
The tremendous variety of our marine creatures can be appreciated by looking at food chains. Many of these start with plankton (that fabulously rich “soup” of minute plants and animals suspended in the seawater) and a wide range of animals feed by simply using filtration or an equivalent process to extract it. Barnacles use their grasping sieve-like feeding limbs, sponges and mussels use vast arrays of hair-like cilia to pump impressive volumes of water through their body cavities while other notable filter feeders, such as tube worms and brittle stars, employ their own brands of filtration equipment. Loch Creran is home to unique reefs composed of tube worms (of the species Serpula vermicularis). While the species itself is common elsewhere, nowhere in the world boasts the type of extensive reefs found at Creran. Brittle stars are found in huge aggregations around much of our coast, including many inlets, sea lochs and other areas in the west of Scotland. More mobile plankton feeders range from small sand eels to the enormous basking shark – at up to 12 metres long the second largest fish in the world and an increasingly common summer visitor to the Scottish west coast.
Grazers with “the knowledge”
Not all the animals near the base of the food chain are plankton feeders, there are grazers such as limpets and omnivorous scavengers like prawns. Limpets are a good example of how seemingly simple animals can have an interesting “story” to tell. Living in the intertidal zone, they graze when the tide covers them but have to clamp themselves to the rock to retain water when the tide leaves them high and dry. Having ground their shell and the rock to be a perfect fit in a particular place, they have to return to the same spot at every low tide. They do this by following a trail of mucus that they leave across the rock as they graze, and some sort of memory has also even been suggested.
Predators and prey
Of course filter feeders and grazers are not left in peace; they are hunted by predators. Barnacles may be broken open and chiselled off the rocks by crabs or be pierced by the “drillerkiller” dog-whelk.This important predator of rocky shores was found (in the 1980’s) to be extremely sensitive to pollution from TBT (Tributyl Tin) anti-fouling paint widely used on boats. Legislation restricting its use has allowed dog-whelk populations to recover. The feeding limbs of barnacles are bitten off by opportunistic young shannies, the small fish so common in rock pools. Older shannies use their strong teeth to scrape off and eat whole barnacles.
Mussels can be smashed open by large crabs but can actually defend themselves against dog-whelks on occasion. The thick shell of mussels takes the dog-whelk a long time to drill through and this can enable the mussel and its neighbours to attach so many of their anchoring threads onto the dog-whelk’s shell that it is turned over and unable to move or feed until it starves to death. Starfish are perhaps the most impressive predators of mussels. They hunch over the shell to grip it with as many of their tube feet as possible then manage to pull it open. Only the tiniest crack is needed between the mussel’s two shell halves for the starfish to insert its stomach (pushed out from its mouth) down into the hapless mollusc and start digesting it.
Sponges do their best to make themselves unpalatable, as they have no other option for protection, but particular hunters such as the sea lemon (a sea slug) are happy to feed on them. Many of the colourful sea slugs specialise in preying on animals with defences that deter generalist predators. Some sea slugs even transfer the stinging cells of hydroids, without triggering them, into their bodies for use in defence against their own enemies.
Brittle stars are hunted by large starfish. Swift crawlers over the seabed, brittle stars often seem to be able to get out of the way of slow-moving sunstars. The rapid seven-armed starfish, however, is a more fearsome proposition. Prawns, along with small fish, can be captured by sea anemones that belie their status as effective predatory animals with an almost flower-like appearance. The tentacles of sea anemones carry thousands of stinging cells that, when triggered, shoot out sticky or poisonous threads that ensnare and paralyse their victims which are then pulled into the anemone’s central mouth. The blue “beads” around the rim of the small red anemone so common on the shore give it its “beadlet” name, and are used in aggressive action against other beadlet anemones. There is competition for the best spots on a rock and two rivals will blast each other with the concentrations of stinging cells loaded in their “beads” until the weaker one retreats.
Sand eels are food for a huge range of predatory fish and other animals such as the multi-skilled cuttlefish. They are also the staple diet of numerous sea bird species, which is why changes to their distribution are having such a serious and widespread effect on sea bird population.
Hunting the hunters
Of course the animals that eat filter feeders and grazers have their own predators to fear too. Crabs, for instance, are preyed upon by fish such as sea scorpions and by cuttlefish and octopus. They are also often eaten by crabs of other species or their own – so cannibalism is a regular occurrence. All crustaceans have to moult their armour suits to grow and, during the period between shedding their old armour and fully hardening their new one, they are particularly vulnerable. This explains why crabs can be seen being eaten by others that one might think are too small to subdue them.
Marine food chains are spectacularly long, much longer than those generally found on land. An example could run as follows: plankton, barnacle, dog-whelk, shore crab, velvet swimming crab, cuttlefish, conger eel. With all the different potential links (a conger may “cut out the middle men” and eat a shore crab for instance) the term “food web” is actually much better.
Family life
Marine animals don’t simply have to find food and avoid becoming someone else’s food of course, they also need to reproduce and there is a huge range of strategies here too. For example, while numerous fish mate by a female and male shedding eggs and sperm into the water near each other, there are intriguing variations. The males of many shallow water fish species take on nest building and guarding duties alone, adopting striking breeding colours while they do so, with the females simply making a brief appearance to lay eggs. Male pipefish and sea horses take paternal devotion even further, with the female laying her eggs into their belly pouches where they are nurtured until being “born” as miniature fish. Some fish, including the spectacularly coloured cuckoo wrasse, are all born as females with a few developing into males that are dominant in a breeding group. When a male dies, a senior female in his group may actually change sex and takes over his role.
Sea slugs are all hermaphrodites; each individual acting as both male and female to its partner so both will lay an egg string after mating. One type of sea slug, the sea hare, takes this a step further and forms mating chains with each individual acting as a male to the one below it and a female to the one above.
In partnership
In this fascinating environment, even partnerships between species are common. Smaller fish will clean parasites from larger fish and sea anemones gain scraps of food from the crabs they live on while providing extra protection in return. Some relationships seem quite mysterious with no certainty as to what, if anything, one party gains.
What we can do to help
I hope this brief summary has shown what a wonderful and complex world exists around our shores, and how important it is that we take better care of it. Only a horrifyingly tiny fraction of the UK’s sea area (less than 0.001%) is fully protected from all damaging activity. This approach would never be tolerated on land but, in the sea, “out of sight” is far too often “out of mind”. We can all play a part by reducing water usage and waste production, by using our power as consumers to encourage more sustainable fishing and by supporting organisations that are campaigning for vital marine protected areas. Last, but certainly not least, we can share our enthusiasm and love for the sea.
For more information on Paul’s work, including his new book the 2nd edition of Great British Marine Animals, visit his website at www.marinephoto.org.uk or email him on paul@marinephoto.org.uk
