Hangers-on, lumps & body-snatchers
IT WAS A PHOTO OPPORTUNITY among a trip of photo opportunities. One of those pictures real photographers enter into competitions and do well with, and certainly exciting for a happy-snapper like me.
On the dark sand at Lembeh in Indonesia, a large, almost luminously brick-red Synapta sea cucumber sat foraging. These alien-looking cucumbers are pretty special in their own right. Their bright bodies have a slightly transparent, bubble-wrap look, with a glitterlike, red-orange glow of speckles on the skin.
The cucumber’s green, feather-shaped tentacles brush the sand, curling towards the mouth and filtering out food from the sediment. On this cucumber sat a lovely bright red emperor shrimp, with orange and vivid purple accents. A favourite of photographers, this group of shrimps is often found with cucumbers, urchins and anemones, foraging for food scraps and keeping their hosts clean.
This particular shrimp, however, offered something even more interesting than simply an easy, photogenic subject. It had a tell-tale lump along the side of its carapace, over the top of its gill chamber, stretching out the thin armoured exoskeleton.
The lump was an isopod, one of a group of crustacean parasites known for latching onto hosts and drinking their blood, or taking chunks of their flesh.
AMONG THE MOST SUCCESSFUL GROUPS of animals on earth, there are around 10,000 species of isopods. Unusually for crustaceans, they even have a rather successful terrestrial group – the familiar woodlice.
One key to isopods’ success is parasitism; another is brooding. Unlike most crustaceans, which lay thousands of eggs, isopods and a handful of other crustacean groups have a brood pouch, hatching fewer eggs inside and protecting the young as they develop through the first few vulnerable moults.
This hapless shrimp not only had a parasite latched onto the rich blood supply to its gills, but it was providing sustenance to a brood that would be released when the shrimp moulted.
Hiding inside the protection of a shrimp’s shell makes sense for blood-feeders. Like ticks, their bodies can swell with a good feed, so normal exoskeleton armour would be too restrictive.
The most successful of these tick-like isopod parasites are known as epicarideans. Female epicarideans do not grow an exoskeleton shell or even a full complement of limbs, and in some species are simply balloon-like bags, barely recognisable as crustaceans.
The more “normal” males are tiny, and often live as parasites enfolded by and sustained by the females.
More conspicuously seen by divers are the more conventional, shelled isopods that attach to the gills and heads of reef fish.
Years ago, I photographed a butterflyfish on a night dive in the Turks & Caicos Islands. I was amazed by how dopey and easy to approach it was.
Many reef fish settle down closer in to the reef to rest at night, but even compared to the other butterflyfish this individual was lethargic. Presumably the parasite was really taking its toll.
If you look closely at the picture, you can see the dwarf male isopods on the snout of the butterflyfish.
IWAN, LEMBEH DIVE GUIDE SUPREME, was extremely happy to find me a rare Rhinopias scorpionfish. I felt a bit bad for not being more excited.
For most photographers Rhinopias is the holy grail of photo subjects, but I’m not much of a photographer, so for me it was just another scorpionfish, moderately interesting insofar as being just a little different from the dozens of others we’d seen.
It became more exciting when I found an isopod latched to his “fin-pit”.
Many isopods swim out to attach at night, when the fish are hiding closer to the reef. Many of them drop off before daybreak and hide in the reef, but the line between temporary and permanent parasite, even the line between parasite and predator, can become blurred.
Some isopods are more voracious, and sprint out to latch onto prey, taking chunks out of them until they succumb.
In some cases these are piranha-like swarms with hooked legs. In other cases large, heavy-jawed isopods individually take on small fish prey.
Most reef-divers are familiar with one particular group of parasitic isopods, although we may not know it.
Gnathid isopod larvae make an extremely successful living attached to the gills of reef fish. When you watch cleanerfish at a reef cleaning station, most of what they pluck from the gills of the grouper and snapper are isopods.
Another approachably dopey fish proved to be another isopod victim.
As I reported in divEr in late 2010, I found a clownfish in Lembeh with a tongue parasite. This type of parasite was previously reported only in snappers, where, although it ate and replaced the tongue of its host, it didn’t kill the fish.
However, unlike the large and robust snappers, it appeared that the normally full-of-beans clownfish was weak and lethargic. Sure enough, when I returned a couple of days later the parasite had left the clownfish, which lay weakly under the anemone, dying.
The guides confirmed that they had seen these parasites on other clownfish, and a friend returning recently from Lembeh had taken photos of another unusually dopey clownfish with a tongue parasite.
LOOKING INTO THE EYE of a whale takes your breath away, makes your heart race and lives with you.
For me, the first time was in the Turks & Caicos Islands. Those huge, flat brown eyes mesmerise, and it’s not until you swim around the whales that the spell is broken, and you notice that they are swarming with parasites.
Whale lice are not actually lice, and arguably barely parasites. They look a little like isopods, flattened and with hook-like limbs for hanging onto their hosts, but they are mainly amphipods, another group of brooding crustaceans but normally more flea-like in shape.
Whale lice are passed on from the parents as the whales are born. Whales produce huge amounts of skin, which they slough off far faster than land mammals do, to keep themselves streamlined. Most of the food for the whale lice is surplus to the whales’ needs.
The other rather startling parasites I saw on the Turks & Caicos humpbacks were ping-pong-sized barnacles, their shells in a shocking bright yellow.
The adults don’t look it, but barnacles are also crustaceans. Like barnacles everywhere, their surprisingly athletic, shrimp-like larvae zip through the water until they find a suitable surface such as turtle or crab shell onto which to cement themselves, or whale skin into which to burrow and anchor.
They attach via their shoulders, then for their adult lives they grasp into the plankton with their legs from the safety of their hosts and their shells, straining out food.
Keen photographer Alan Larsen recently sent me a photo that started out as a nice macro opportunity, and asked me to identify what was happening.
A gorgeous goby was sitting photogenically on a sea-pen, but it looked as if it had been tagged by a supermarket pricing gun. The paired “sausages” hanging off the sides of the tiny fish were actually members of another, even more successful group of crustaceans – the copepods.
Most copepods are a couple of millimetres long, and form the majority of the zooplankton.
This is the most successful group of animals on Earth, forming immense swarms in every ocean. These mighty atoms zip through the water, and provide almost all of the food for most ocean carnivores, including whales, basking sharks and manta rays.
However, a few species of copepods have turned the tides on some of their would-be predators, latching onto their skin and burrowing in.
Like many isopods, these copepods cease to resemble their relatives once they have become adults.
They lose their segmentation and their heads, buried inside the hosts, and become anchor-like, their paired ovaries hanging outside the skin of their hosts.
They become egg factories, converting the blood of the host into eggs, which are gradually dropped off the conveyor-belt production line.
If you look closely, you’ll see that many, if not quite most, fish have these copepod parasites, often around their mouths or fins. They are usually tiny in relation to their host, and probably cause little significant harm.
Greenland sharks even have a copepod parasite on their eyeball! Oddly, almost all of these giant sharks have one – but on one eye only. The parasite scratches the eyeball, making the shark effectively blind on that side, but it is slightly luminescent, so it may offset this apparent handicap and its parasitism by attracting prey to the host.
MANY DIVERS AROUND THE UK have seen crabs with what look like sponges growing beneath them.
I noticed with distaste pink and red slug-like, almost leech-like attachments under the abdomens of Antarctic crabs when I first went down there. They were obviously parasites, but dissections and literature searches revealed them to be far more interesting – and far more horrific – than I had guessed.
This most insidious, most unpleasant, and perhaps most impressive of all the crustacean parasites is a group of barnacles. Most barnacles whizz along as larvae, then burrow into the skin or shell of their hosts simply to gain a vantage point from which to gather plankton, causing little harm to the host.
A quarter of all barnacles, however, have evolved this burrowing to a truly nasty level of parasitism. These barnacles are called rhizocephalans, and they may be the ultimate parasites.
The females of the parasite larvae are somewhat syringe-like. They dart a joint in the leg of a host crab, shrimp or lobster, then inject their cells inside the host exoskeleton.
From this point onwards, they cease to resemble any kind of crustacean.
They grow wormlike through to the abdominal gonads, eating host tissue and replacing it with their bodies. When they get to the oil-rich gonads, it’s feast-time. They grow explosively on the rich food source, replacing the gonads with a truly disgusting mass of tubes.
The crab will never reproduce, but it is not killed by its host.
A REMARKABLE THING HAPPENS HERE. Whether male or female, the crab’s abdomen broadens in a very feminine way, and the crab seeks out higher ground to broadcast spawn.
It seems that the dastardly barnacle actually produces synthetic crab hormones, and the crab – whatever its gender – behaves like a berried crab with a clutch of eggs.
In place of crab eggs, however, the barnacle has punched a tube out through the abdomen and extruded its own huge egg mass, which the crab wafts as if it were its own.
Tiny male parasites are attracted to pheromones released by the slug-like mass, and they fertilise the barnacle eggs as they are released into the plankton.
Eventually the parasite eggmass drops off, and the crab is left weakened, neutered and scarred, but alive.
In a dose of ecological justice, the rhizocephalan egg-sacs are themselves parasitised. Liriopsis is another sac-like isopod, rather like the one I had seen inside the emperor shrimp’s carapace.
I feel more sympathetic towards this tick-like hyper-parasite, however, as it drains the rhizocephalan egg-sac in a vampire-like way.
It doesn’t help the crab much, but it somehow makes me feel better.