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The extraordinary life cycle of the lion’s mane jellyfish

Lion's mane jellyfish, Cyanea capillata, underwater clearly showing tentacles trailing in many directions. Colin Munro Photography

Jellyfish, or sea jellies as they are now often called (clearly they are not fish) are amongst the most ancient of multi-organ animals.  Fossils of jellyfish (or scyphozoans, to give them their scientific name) are found only rarely as they contain no hard structures within their bodies, which are 95% water.  However, under the right conditions fossils of soft bodied creatures will form; current fossil evidence suggests they first evolved at least 500 million years ago.

Lion's mane jellyfish, Cyanea capillata, underwater clearly showing tentacles trailing in many directions. Colin Munro Photography

The lion’s mane jellyfish, Cyanea capillata, is the largest known species. The bell of individuals in colder northern waters can reach two metres across.

The lion’s mane jellyfish (Cyanea capillata) common throughout the North Atlantic, epitomises this image of a large, slowly pulsing, gelatinous bell (or medusa) and long trailing tentacles that pack a powerful sting, but this is in fact only one stage of a complex life cycle.  Lion’s mane medusae begin to appear in April or May in the Northern Atlantic, but are quite tiny at that stage.  These jellies are voracious predators and grow rapidly through the summer.  By August the medusae are commonly one third to half a metre across, with trailing tentacles many metres long.  However there is considerable variability;  large specimens have been reported at over two metres across with tentacles up to 37 metres long, though these generally occur within the more northern parts  of their range.  As they grow large in late summer they will often drift, under the influence of wind and tides, in to sheltered bays where they may aggregate in large numbers. This is when sperm is release and egg fertilisation takes place.  In common with most scyphozoans (the taxonomic group to which jellyfish belong) the sexes are separate; lion’s mane jellies are either male or female.  Sperm is released from the mouth of male jellies and drifts in the current, some reaching female jellies, where the eggs are fertilised. Fertilised eggs are stored in the oral tentacles of the female, where thy develop in to tiny planulae larvae. Once fully developed the planulae larvae detach and, after drifting for a short time, settle on the seabed.  Here they metamorphose into a polyp, not dissimilar to tiny sea anemones or coral polyps (both of which are relatives of jellyfish).  These polyps then grow, taking on a layered appearance until they resemble a stack of wavy-edged pancakes.  Each one of these ‘pancake layers’ will then separate from the parent polyp, once again becoming free living and drifting with the currents.  The ‘pancakes’, more properly ephyra larvae, will grow throughout the summer into the giant lion’s mane jellies and the cycle is complete.  With a lifespan on only one year, during which they can grow to be as long (possibly even longer) than blue whale, lion’s mane jellies need to catch and consume considerable amount of prey.  Each trailing tentacle is packed full of vast numbers of stinging cells, known as nematocysts.  When touched these cells fire out a harpoon-like structure which pumps toxins in to the hapless victim (this is what causes the painful sting from jellyfish).  These toxins incapacitate the prey, which is then drawn up towards the mouth of the jellyfish.  A large lion’s mane may have over 1,000 tentacles trailing far behind them.  Many SCUBA divers in Scotland and Scandinavia have experienced the situation where, having completed their dive on a sunken wreck and returned to the buoy line they planned to ascent to the surface, only to look up and see numerous lion’s mane jellies strung out along the line.  As the current sweeps the jellies along so their tentacles catch on the buoy line, leaving the divers with the unpleasant prospect of ascending through thousands of jellyfish tentacles.

A diver warily watches a large lion's mane jellyfish (Cyanea capillata) drift past. Isle of Arran, West Scotland.

A diver warily watches a large lion’s mane jellyfish (Cyanea capillata) drift past. Isle of Arran, West Scotland.

Not every creature lives in fear of lion’s mane jellies however.  Leatherback turtles, the only species of marine turtle that can tolerate the cold waters these jellies inhabit, consume them with relish, apparently oblivious to the stinging tentacles.  Lion’s mane jellies can make up 80-100% of a leatherback’s diet.  When you consider that a full grown leatherback weighs up to 800kg and may consume up to its own weight in jellyfish daily (bear in mind jellyfish are 95% water) then that equates to pretty large numbers of jellyfish being eaten.

As summer wanes and autumn approaches the lion’s mane jellies begin to die.  This provides a feeding bonanza for many scavengers.  On the surface seabirds will peck away at the gelatinous bell, whilst those that sink are often torn to shreds by shore crabs (Carcinus meanus) and velvet swimming crabs (Necora puber).

Dying lion's mane jellyfish (Cyanea capillata) that has sunk to the seabed being eaten by a velvet swimming crab (Necora puber).

Dying lion’s mane jellyfish (Cyanea capillata) that has sunk to the seabed being eaten by a velvet swimming crab (Necora puber).

At the other end of the scale these deadly tentacles can provide refuge to some unlikely creatures.  Juvenile whiting (Gadus melangus) have long been known to swim underneath the bell of lion’s mane jellies, apparently unconcerned by the curtain of tentacles they weave between. In fact they have been observed to rush into the mane of tentacles when startled by predators.  A series of fascinating experiments by the Swedish zoologist Erik Dahl in the late 1950s showed that, compared to other fish species, juvenile whiting were able to adapt their movements such that even when surrounded by tentacles they rarely came in to contact with them.  Also, unlike other fish species, when they did brush against them it seemed to cause them little concern. Biopsies of the tissue of whiting where they had contacted tentacles showed that very few if any stinging nematocysts had fired into the fish’s body; this compared to hundreds per square millimetre for other fish species.  We still don’t understand the mechanism behind this protection. So does the lion’s mane get anything in return for the refuge afforded the young whiting?  Well another creature found on lion’s mane jellies is the tiny planktonic amphipod (a type of crustacean) Hyperia galba. Hyperia is, for the jellies, a rather irritating ectoparasite. It lives on the outside of the jellies’ bell, nibbling away at it.  Now whiting don’t appear to like the taste of lion’s mane jellies, instead they are rather partial to planktonic crustaceans; in particular (you’ve guessed this already) Hyperia galba.   It is these elegant little symbiotic collaborations that make nature so beautiful.

These, and many more of my images, can be found at colinmunroimages.com

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A race against the rising water

A black swan frantically tries to save her nest and eggs as the river water rises following torrential rains. Colin Munro Photography

A black swan tends her eggs in the nest she has constructed along the banks of the River Exe. Colin Munro Photography

A black swan tends her eggs in the nest she has constructed along the banks of the River Exe.


A black swan frantically tries to save her nest and eggs as the river water rises following torrential  rains. Colin Munro Photography

A black swan frantically tries to save her nest and eggs as the river water rises following torrential rains.

The human cost of the wettest summer in 100 years, lost income and damaged property, has been highin the Southwest.  The cost to wildlife has also been high.  The mute swans that gather on the River Exe in the centre of Exeter have failed to raise a single clutch this year.  At the end of September, an Austalian Black Swan (Cygnus atratus) attracted considerable interest as she tried to brood a small clutch of eggs along the riverbank.  The weather was warm, and drier than it had been for most of the summer.  perhaps she might be lucky.  However Sunday and Monday the 23rd and 24th were to test her to the limit.  Around 2am on the morning of the 23rd the rain becan to fall and the wind started to howl.  For more than 30 hours it rained, and as it rained the river rose.  By the early hours of the 24th she was already frantic, trying desperately to shore up her nest.  By 10am the nest was still there, but floating.  Although probably exhausted she moved incessantly, plucking reed blades of the bottom, trying vainly to build up her nest.  The eggs were still same, but became submerged when she sat on the nest.  She was engaged in a desperate race to raise the nest before the eggs lost too much heat. A black swan frantically tries to save her nest and eggs as the river water rises following torrential  rains. Colin Munro Photography

The black swan tries to move her eggs out of the water pooling in the centre of her nest as the river rises.

Although the rains had now stopped, at least temporarily, millions of gallons were still flowing down the river from high ground and so the river was continuing to rise. Passerby stopped to watch, and throw her bread, which is probably all she had time to eat since she had laid her eggs. No-one knew whether the eggs were fertile. She was the only black swan on the river all summer; tagging along at a safe distance with the larger mute swans that congregated along the quayside. But hybrids between mute and black swans were believed to have occurred in captivity. So it was just possible. And although the odds seemed against her, it was still possible her eggs main survive the flood.
This story will me expanded soon – and the whole story of the black swan and her nest will be told.
All images can be licenses from my Photoshelter website here Search black+swan

Flying crabs and flailing birdmen

Swimming crab, Liocarcinus depurator, swimming in mid-water. This crab is also known as the harbour crab, blue-legged swimming crab and sandy swimming crab. Colin Munro Photography.

All the images in this blog are available to license.  To view a gallery (license images or purchase prints of) these, and more of my North east Atlantic marine invertebrate images go here.  Alternatively you can search all my online stock images at my www.colinmunro.photoshelter.com  site through the search box (top right) here or on my main website here.  swimming crab images, Liocarcinus depurator images, necora puber images, stock images.

Flying crabs
A crab is neither the most graceful nor aerodynamic creature in the sea.  Okay you probably knew that already.  At first glance it does not appear to be designed for flight, its squat, angular body, entirely encased in a thick, heavy shell.  A crab attempting to fly would seem as sensible as attempting to run a marathon wearing a suit of armour.  But then, as anyone who has watched the London marathon will know, people do attempt – and succeeded  – in running marathons in suits of armour.  So why shouldn’t crabs fly?

I have been a little loose with the term ‘fly’; okay, they fly underwater.  They are collectively known as swimming crabs.  This group includes such species as the blue crab (Callinnectes sapidus) which is found around the coasts of North and South America, the red-eyed and fearless velvet swimming crab (Necora puber) that is common on shallow rocky reefs around the coast of UK and much of Europe, and the blue-legged swimming crab (Liocarcinus depurator) also common in shallow waters around UK and Europe but preferring sandier areas.

A velvet swimming crab, Necora puber (previously known as Liocarcinus puber) adopts a defensive posture as it moves across a maerl gravel seabed. Colin Munro Photography.

the distinctive wild red eyes and aggressive posture of a velver swimming crab (Necora puber). Note the broad swimmerets. Image No. MBI001256

A common feature of all these crabs is the adaptation of the fifth walking leg for propulsion through the water.  Crab legs are mostly fairly spindly affairs, ending in points on which they tippy-toe across the sea bed.  The final articulated segment of swimming crab legs is flattened and splayed into a paddle shape.  Additionally they are edged with long thick hairs, effectively widening the paddle blade.  These swimming legs are known as pleopods (from the Greek plein, to sail or to swim, and pods – legs) or swimmerets.

Swimming crab, Liocarcinus depurator, swimming in mid-water.  This crab is also known as the harbour crab, blue-legged swimming crab and sandy swimming crab. Colin Munro Photography.

Swimming crab, Liocarcinus depurator, swimming in mid-water. Image No. MBI001254.

I started this by stating that crabs were not really designed to fly (or swim for that matter).  This is true.  Swimming marine creatures conjures up images of graceful fluid movements.  That is not swimming crabs.  Generally swimming crabs swim when disturbed, as a means of escaping real or perceived danger.  They launch themselves off the seabed, flailing wildly as if convulsing through being wired up to high voltage electricity.  For seconds, or at most a few minutes, the crab will move erratically through the water.  As it tires its legs will slow; the crab will drift back down to the seabed, to scuttle away hopefully haven shaken off its pursuer.

Swimming crab, Liocarcinus depurator, swimming in mid-water.  This crab is also known as the harbour crab, blue-legged swimming crab and sandy swimming crab. Colin Munro Photography.

Close up of a swimming crab, Liocarcinus depurator, swimming in mid-water. Image No. MBI001254CP2

Watching a crab frantically waving its legs in an attempt to defy gravity, as it descends inexorably back to solid seabed I am always struck by the uncanny resemblance to another quirky British tradition (apart from wearing ridiculous costumes for marathons) that of the birdman competition.  This is an annual event held in several seaside towns, most notably Bognor Regis, where the great British eccentric emerges to don one-piece stripey swimsuits, circa 1900, batman masks, vinyl capes and wings that appear to be constructed from broom handles and ostrich feathers.  Suitably attired they launch themselves off the end of the town jetty.  Arms flaying wildly in an attempt to defy gravity (now you see where I’m coming from) they perform a graceless parabola and they too, descend inexorably to the sea below.

The first birdman competition (according to Wikipedia) occurred in Selsey, West Sussex, in 1971.  How long have crabs been launching themselves off the seabed, attempting to escape the limitations imposed by a million years of evolution, no-one knows.  Perhaps they have been sitting, half-buried, on the seabed, watching and pondering on ill-designed creatures in strange garb plunge in to the sea as they attempt to escape their own limitations.  Perhaps flying crabs are a recent phenomenon, the dreamers inspired by eccentrics in sleepy seaside towns.

Sex and death in the seagrass

A common or European cuttlefish, Sepia officinalis, trapped in monofilament bottom set nets, Brixham Harbour. Each year nets are set around seagrass beds (Zostera marina) where cuttlefish come in to breed. Colin Munro Photography
A common or European cuttlefish, Sepia officinalis, trapped in monofilament bottom set nets, Brixham Harbour.  Each year nets are set around seagrass beds (Zostera marina) where cuttlefish come in to breed. Colin Munro Photography

A common or European cuttlefish, Sepia officinalis, trapped in monofilament bottom set nets, Brixham Harbour. Each year nets are set around seagrass beds (Zostera marina) where cuttlefish come in to breed. Image No. MBI000305.

Intelligent life. Few divers who have ever encountered an octopus or cuttlefish underwater cannot have been struck by a sense of an alien intelligence staring hard at them, assessing whether you are friend or foe and what you are likely to do next. That cephalopods (octopi, cuttlefish, squid and their kin) are bright has now become common knowledge; yet this is still something that sits uneasily with their molluscan nature. Great apes, whales and dolphins are easier to accept; they are mammals and so not that distantly related to us. But cephalopods? They are not even vertebrates; first cousins to slugs, snails and slipper limpets. Their blood is greeny-blue not red as the oxygen carrying molecule is copper-based rather than the iron-based haemoglobin in all mammals. They really are shape-shifting aliens from inner space. And yet, when a cuttlefish rises out of the sand or seagrass to hover in front of you, you get a real sense of cogs turning and a logical decision-making process taking place. That may, of course simply be down their binocular vision; we tend to almost intuitively associate this with intelligence (it is designed, after all, to assess distance, form mental 3D images of the World and judge when and how to strike at prey). That there is real intelligence behind those eyes is most elegantly demonstrated by the octopus, which has joined the elite group of animals that have demonstrated the use of tools to manipulate their environment in the wild. The veined octopus has recently been filmed collecting and stacking discarded coconut shells halves to use as a shelter. So far it is the only invertebrate to do so.

Cuttlefish breeding. The seagrass beds of South Devon have long been a favourite dive habitat of mine. They provide gentle, sunlit dives where one can float along in the hope of encountering a pipefish, mating sea hares and, at the right time of year, cuttlefish arriving to lay eggs. Watching a female cuttlefish lay eggs is a fascinating experience. Each pointed black egg is attached to a tuft of seagrass or weed one at a time. The female will hover before it, then after a few minutes contemplation will move forward to firmly grasp the stem with her tentacles and pull it towards her. After a minute she draws back again, to reveal a new shiny, pointed black eggs bound to the stem by a band extending out of the egg case. This process with continue for over an hour, until the stem is wrapped in what resembles a bunch of black, pointed grapes.

A common or European cuttlefish, Sepia officinalis, trapped in monofilament bottom set nets, Brixham Harbour.  Each year nets are set around seagrass beds (Zostera marina) where cuttlefish come in to breed. Colin Munro Photography.

A female common or European cuttlefish, Sepia officinalis, trapped in monofilament bottom set nets, Brixham Harbour. A male hovers nearby and will eventually also be entangled. Other males hover in the background. Image No. MBI000270.

While the eggs are laid a gang of males hover in attendance. When cuttlefish mate the male transfers the spermatophores (sperm packages) to within the females buccal cavity, using his modified fourth arm (the hectcotylus). The hectocotylus is then used to break open the spermatophores, releasing the sperm which is then temporarily stored within the female’s buccal cavity. If she subsequently mates with another male it will direct jets of water into her buccal cavity to attempt to flush out earlier sperm deposits. Consequently the most recent mate will hover above her, warding off other males that will hang around, to ensure that it is his genetic material that is passed on rather than that of his upstart rivals.

Cuttlefish in nets. Cuttlefish are, of course, a valuable catch for fishermen. They are caught in trawls out at sea and also in set nets deployed around their inshore breeding grounds. Cuttlefish grow quickly and most die shortly after breeding, so those caught after they have laid their eggs will not have any real effect on future populations. Unfortunately however, nets are often laid around the edges of breeding grounds such as seagrass nets, thus cuttlefish are caught not only when leaving the breeding ground but also when arriving, before they breed. The nets do not kill them, but they are often quite badly damaged as they twist and the fine line cuts into their flesh. Indeed I often wonder how they can be sold after their flesh is so ripped up. If a female is caught then inevitably a number of males arriving will hang around her until they too are caught. A trapped female may remain their for up to twelve hours before the nets are hauled, so the potential for catching large numbers of breeding cuttlefish is quite high. So far, this practice does not seem to have noticeably affected local cuttlefish populations. It is nonetheless rather disconcerting to watch such lovely animals twisting and turning for hours on end.

Update. In the past couple of years I, and other local divers, have seen notably fewer cuttefish hanging around in the shallow bays of South Devon during the breeding season.  This may simply be due to factors like the lousy weather we’ve had during the past couple of summers; it may also be due to the almost impenetrable ring of nets set on the edge of these bays.

Images. All the images in this blog are available to license.  You can search all my online stock images at my www.colinmunroimages.com  Cuttlefish images, Sepia officinalis images, fishing images, stock images.

Greeting for the Festive Season – Snow comes to Exeter

Greeting for the Festive Season – Snow comes to Exeter

Hungry geese paddle across the snow-covered footpath on Exeter's Quayside

Hungry geese paddle across the snow-covered footpath on Exeter's Quayside


This image is for sale at Alamy, click here and search for geese, snow, Exeter.

Snow has finally arrived in Exeter, just in time for Christmas! Despite sub-zero conditions that have lasted for weeks we have escaped the heavy snowfalls that have paralysed much of the UK. This morning I woke to a couple of inches in light, fluffy snow covering my boat, and pretty much everything else. Just enough to look pretty without causing too much disruption.

Close up of an inquisitive goose in snow, Exeter Quayside

Close up of an inquisitive goose in snow, Exeter Quayside


This image is for sale at Alamy, click here and search for geese, snow, Exeter.

Close-up of inquisitive goose standing in snow, Exeter Quayside

Close-up of inquisitive goose standing in snow, Exeter Quayside


This image is for sale at Alamy, click here and search for geese, snow, Exeter.

www.colinmunrophotography.com

The swans of Exeter

The swans of Exeter
Mute swans (Cygnus olor) congregate under Cricklepit Bridge

Mute swans (Cygnus olor) congregate under Cricklepit Bridge

One of the advantages of living on a boat is that you get to see a lot of aquatic life go past. Exeter is famous for its mute swans (Cygnus olor) with congregate in large numbers on both the River Exe and the Exeter Ship Canal. The swans have become very used to the tourists and locals strolling along the river and canal side in the centre of town, so much so they even nest next to the footpath in the heart of town. At this time of year pairs of swans can be seen cruising around guarding clutches of fluffy grey signets. The adults will shepherd the signets along, occasionally pulling bits of weed off the botton for the youngstesr to feed on, or paddling furiously with their webbed feet to stir up weed in the shallows for them.

Mute swan cygnet (Cygnus olor) only a few days or weeks old.

Mute swan cygnet (Cygnus olor) only a few days or weeks old.

Mute swan cygnets (Cygnus olor) feeding in shallows.

Mute swan cygnets (Cygnus olor) feeding in shallows.

Mute swan cygnets (Cygnus olor) swimming.

Mute swan cygnets (Cygnus olor) swimming.

A pair of mute swan (Cygnus olor) cygnets swimming, Exeter Ship Canal.

A pair of mute swan (Cygnus olor) cygnets swimming, Exeter Ship Canal.

As always my images are available to license and as fine art prints. If you’d like to use one of my images for publication please contact me. If you’d like a print of one of the images drop me an email stating image number and print size (costs for prints can be found on my fine art prints pages, e.g. Fine Art prints of Devon. Email me.
To my main website www.colinmunrophotography.com

Spring is in the air. Seaslugs mating: Polycera faeroensis.

Spring is in the air. Seaslugs mating: Polycera faeroensis.
The seaslug, or nudibranch,  Polycera faeroensis mating.  Like all nudibranchs, Polycera faeroensis is a simultaneous hermaphrodite.

The seaslug, or nudibranch, Polycera faeroensis mating. Like all nudibranchs, Polycera faeroensis is a simultaneous hermaphrodite. Image MBI000678

Spring is in the air, the sea is getting warmer – slowly – and the birds and bees, and most things beneath the waves too. Polycera faeroensis is a very common seasulg in British waters, and although colourful is often overlooked due to its small size, large individuals are no more than 4.5 centimetrs long. Like all nudibranchs they are simultaneous hermaphrodites – possessing both male and female sex organs at the same time (sequential hermaphrodites have either only male or only female sex organs at any given time). Copulation works both ways (reciprocal copulation, as it is termed). As the sex organs always appear to be on the right side or their bodies, Polycera faeroensis nudibranchs copulate head to tail. The missionary position has not caught on in the nudibranch world. One might think this was already exciting enough for any mollusc, but some nudibranchs, (such as the related Palio dubia found around the northern Uk shores) add a touch of S & M to their sex lives. Unlike many nudibranchs, Palio dubia does not have a complete vaginal opening. Thus copulation occurs by hypodermic injection; the barbed penis (or penile cirrus as it is properly termed) simply punctures the body wall into its mating partner. Ouch!
As always my pics are available to license – if you’d like to use one just email me