Sometimes the staggering number of coincidences required for an animal's life cycle to succeed defy belief.
Take, for example, the pelagic nudibranch Fiona pinnata. For starters, it lives in the open ocean, usually far from land, and it can't swim or float in water by itself. Pretty obviously the first order of business is to find a floating object on which to live. These improbable animals start out their lives as (capable of floating) larvae and may be able to delay changing to their (non-swimming non-floating) adult form for a while until they find a suitable vehicle. Still, it's a matter of a few more than five days or it's game over for the larvae. And the open ocean isn't over-supplied with floating objects, specially from the point of view of a minute floating larva.
Having found the floating object, Fiona must then find food. Since it can't leave the floating object for obvious reasons, the food needs to bump into the floating object or else be present on it in the first place. Fiona specialises in feeding on gooseneck barnacles, although it will eat other floating animals such as stinging-celled by-the-wind-sailors and blue buttons. Since these aren't that abundant in the open ocean, the floating object really needs to have resident barnacles. As if that wasn't enough, Fiona then needs another Fiona to mate with so that new little Fionas can continue to ply their absurd lifestyle on the high seas.
Fantastically improbable as this seems, it was just such a combination of animals that we came across when we found a floating bottle in Ponta do Ouro bay. The bottle had clearly been at sea a while and was covered with gooseneck barnacles and what looked like eggs. Its other residents only became apparent after much closer inspection. First off, a minute octopus. Possibly this was the creator of the eggs, though it might also have been a juvenile of one of the many species of poorly known octopuses. Closer inspection of the eggs revealed the big eyes and rounded bodies of embryonic octopuses. Then there were larval crab forms, taking a break from the dangers of floating free in the plankton. And goosneck barnacles, continually sieving the water for food with their modified legs.
Gooseneck or goose barnacles have an interesting cultural history. In mediaeval times, it was thought that barnacle geese developed from these 'eggs'; hatching from what was thought to be a 'goose tree'. The confusion came about because barnacle geese (as they are still known) don't nest in northern Europe, but were only seen to appear at the beginning of the European winter. Various authorities claimed to have seen the geese emerging from the barnacles, and the tree confusion came in because goose barnacles are frequently found attached to floating wood. So, what could have been more obvious? The goose tree produced barnacle goose eggs which then hatched into barnacle geese. The bonus was that since they were considered 'neither flesh nor born of flesh' these geese could be eaten on religious fasting days. Cynics suspect this might have been the force driving the story all along.
Be that as it may, like Fiona, gooseneck barnacles must first find a floating object for attachment before they can start their adult lives. Feeding isn't as problematic, since they are filter feeders, but reproduction again requires another gooseneck barnacle arriving on their floating object.
Our bottle had gooseneck barnacles in profusion, and once we'd got over cooing about the octopus, and the cute baby crabs, we saw that not only had the gooseneck barnacles been joined by a scale worm, but that not one, but two Fionas were enjoying their floating feast. They'd made good use of their time too and had laid several chubby egg ribbons. One hopes the hatching larvae would have had the good sense to stay on the bottle.
Floating objects like our bottle are often the substrate of thriving little ecosystems. Not only that, but fish will tend to congregate under them, in search of a little shelter. Which brings us to another unlikely life cycle choice. The Sargassum frogfish. Since frogfishes are not particularly streamlined, they would not be anyone's first choice of a fish with a pelagic lifestyle. But the Sargassum frogfish has evolved to live in the floating mats of Sargassum seaweed which populate the Sargasso Sea and other places. There it lives in the tangled mats as an ambush predator and, presumably because life can be tough in the seaweed jungle, eats other fishes and occasionally its own species to survive. When mating time comes around, the female lays a raft of eggs stuck together by mucus. The raft sticks to the seaweed and is fertilised by the male. The hatchlings, when they emerge, of course, have to beware of any hungry adults in their vicinity.
Animals which create their own floats are also among the improbable life cycle select. For starters there's the Portuguese man o'war or blue bottle. This animal (or colony of highly specialised individuals depending on how you look at it) creates a gas-filled float all the better to sail the open ocean. Below the float trail deadly blue tentacles equipped with ferocious stinging cells to trap and kill whatever floats or swims past.
The stings of the man o'war are not a problem to another pelagic nudibranch though. The sea swallow, Glaucus atlanticus, a rather more sea worthy beast than Fiona, gulps air into its stomach to enable it to float stomach up, on the surface of the ocean. Once floating and using its long and beautiful cerata as float aids, it needs to bump into blue bottles for its food and also for its defences. Because when feeding, sea swallows select the most virulent of the man o'war's stinging cells, pass them unharmed through their digestive system, and store them in specialised sacs, ready for deployment against anything which might seek to harass the sea swallow. An improbable life cycle indeed.
text: Georgina Jones
photo: Jenny Strömvoll
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