Far out to sea there float patches of ‘seaweeds’ called sargassum – forms of macroalgae that possess floating bladders. Although sargassum is particularly associated with the Sargasso Sea, east of the Caribbean, it is widespread across the oceans in lesser concentrations. And in association with these current-riding macroalgae are two epipelagic fishes that form a stranger-than-fiction triad with the voyaging photosynthetic raft that they seem to accompany. One fish is perhaps the most mobile of all finned creatures; the other has virtually no mobility other than the drifting of the ‘seaweeds’ to which it clings; and the third member – the sargassum itself – is simultaneously one of the most mobile of plant-like organisms and among the few examples of seafaring vegetation.
Our two fishes are the biplane flying fish and the sargassumfish. These contrasting identities coexist at the sea surface. Both breed by laying eggs among the clumps of floating ‘seaweeds’. Both bind their eggs to these macroalgae by means of sticky ribbons or blobs, to prevent the eggs drifting off. And in both species the newly-hatched fishes look different from their parents, and spend their early life as plankton which may drift some way from the ‘seaweeds’. But, as they grow, the two fishes diverge in body shape – particularly in the form of their paired fins. We refer here to the pectoral and pelvic fins that represent the counterparts for forelimbs and hindlimbs in fishes.
Regardless of the wing-like form of certain fins, no species of fish is capable of flying in the sense of beating its fins propulsively in the air. Nor does it seem that any species of fish has devoted its fins to aerial locomotion without retaining their partial use in water. But the biplane flying fish sails as close to true flight as any fish does. Indeed its gliding is perhaps the most accomplished form of flight to be found among the dozens of species of fishes, in various families, that have specialised to some extent for taking a short leave from the water to escape predators. Indeed, this fish has not one but two pairs of wing-like fins. Its extremely large pectoral fins and moderately large pelvic fins are all held out horizontally during gliding. And because the pelvic fins are located abdominally (in contrast to many lineages of fishes in which they originate just below the pectoral fins), they complement the pectoral fins to produce a four-winged gliding form. These four foils do not, however, all snap into their gliding position at once. When the biplane flying fish first emerges into the air, with its tail still beating just below the surface, the pectoral fins are immediately spread to give lift. And then once the tail leaves the water, the pelvic fins also spread out and the fish remains airborne for what seems like a breathlessly long time.
How much active control is involved in this gliding flight on an aerofoil consisting of the outstretched pectoral and pelvic fins? To generate sufficient power to leave the water in their extended leap, flying fishes beat their tails up to 70 times per second. Once airborne, the biplane flying fish can continue for a remarkable total of up to 400 metres with its gills in the air. During this flight, they can move faster than 70 kilometres per hour, and rise up to six metres (and perhaps even 10 metres judging from some landings on boat decks) above the sea surface. Although the fins do not beat in the way bat wings do, the biplane flying fish uses its muscles to move them just enough for changing its direction, braking or swerving to some degree in avoidance of an emerging obstacle, and accelerating repeatedly or at least prolonging the flight by catching successive gusts of surface wind. This latter tactic can enable the fish to stay airborne for up to 45 seconds.
We move now to the second of our unlikely triad of aquatic locomotors, the sargassumfish. This fish has used similar beginnings to arrive at a different way of life. It returns from its planktonic juvenile stage to the floating ‘seaweeds’. Here it develops into a slow-moving form that swims so little that it appears to abandon the principle of streamlining. Its pectoral and pelvic fins become as radically different from typical fish fins as in the case of flying fishes, but for the purpose of grasping the algal substrate as opposed to flying above it (see Figure 1). The fin membranes are reduced and the fin base is articulated into a ‘limb joint’ so that the fins can be used for grasping the floating vegetation in a flexible way. Combined with exceptional camouflage in both colouration and skin textures, this means that the sargassumfish looks like part of its algal surrounds and awaits its prey in this disguise.
Figure 1. The sargassumfish, showing articulated pectoral and pelvic fins which can support the body and grasp objects in ways convergent with the limbs of vertebrates other than fishes [image © Robin and the Honey Badger].
Not only are the modes of locomotion of the biplane flying fish and the sargassumfish radically different, so too are their foraging methods. The biplane flying fish eats plankton by means of a small mouth, covering large distances by swimming actively for most of its life, and taking to the air when chased by predatory fishes such as the mahimahi. By contrast, the sargassumfish is reluctant to move except for one action that would be spectacular if our eyes could actually register it: when it suddenly swallows prey that has strayed within reach. Indeed, the sargassumfish, like other frogfishes, has an enormous gape and can, by suction, swallow prey that is in extreme cases larger than itself. It does this by a combination of suction and jet-propulsion, opening its mouth at the same time as forcefully expelling water backwards from the gill openings – the whole movement being too rapid for the human eye to follow. Notably, some of its relatives have been timed to take as little as 10 milliseconds for the entire sequence from lunging forward to swallowing a fish completely, which means that the prey fish simply disappears in far less than the blink of an eye. It consequently seems safe to say that the whole act of the sargassumfish engulfing its prey is about as rapid as a single tail beat of the biplane flying fish just before takeoff: in the order of 1/50th to 1/100th of a second.
Although we have chosen to focus on the biplane flying fish because it is a four-fin, not two-fin, flying fish, and the sargassumfish because it is one of the few frogfishes that lives epipelagically, these two identities represent whole groups of fishes that manage, to varying degrees and extents, to ‘fly’ in rapid motion or to ‘run’ in slow motion. For example, the closest relatives of the sargassumfish have paired fins that are so limb-like that they can move short distances along the sea floor in a way similar to tetrapods. These frogfishes have two gaits in such situations. They can use just the two pectoral fins to creep ‘bipedally’, albeit with the ‘forelimbs’ rather than the ‘hindlimbs’. Alternatively, they can occasionally use all four ‘limbs’ in the slow-motion equivalent of a canter, in which the body is supported by the pelvic fins while the two pectoral fins are slowly swung forward in unison. Frogfishes thus ‘run’ in the same sense that flying fishes ‘fly’ – albeit at very different speeds. And the sargassumfish, although retaining the characteristic reluctance of all frogfishes to swim, has modified its pectoral fins in keeping with its floating substrate of ‘seaweeds’ – by using finger-like fin rays not merely to stand but manually to clutch the constantly moving strands (see Figure 2).
Having examined the sargassumfish, we now turn our gaze to sargassum itself. One of the odd facts about the sea, usually taken quite for granted, is its general lack of visible vegetation at the surface. It is true that the long, dense kelp found anchored to the rocky bottoms along wintery coasts reaches the surface in the form of writhing fronds. And, of course, the mangroves of tropical muddy shores are true trees growing in the sea, which form a green canopy just above water level at high tide. However, the simplest option is the exception rather than the rule for large photosynthetic organisms. This option is to buoy the photosynthetic surfaces with air-filled bladders that grow naturally as part of the foliage, to abandon dependence on a holdfast to the substrate, to allow the vegetation to float unattached at the sea surface, and to drift passively with the currents.
Figure 2. The sargassumfish not only clutches ‘seaweeds’ with its armlike pectoral fins, but uses the fin rays as webbed fingers – despite the fact that these bear no homology with the true digits of amphibians [image by www.daveharasti.com]
Most species of sargassum start life, like kelp, held fast to the sea floor or rocks of the tidal zone. Their detachment by storms later leads to a life of passive propulsion that appears almost accidental, perhaps some marine equivalent of a zombie existence. However, two species of sargassum are holopelagic, reproducing vegetatively and never attaching to the sea floor during their life cycle. For vegetation to float is not in itself as remarkable as it may at first seem, for even grasses and sedges, with their airy stems, can form extensive floating meadows on warm, nutrient-rich waters of e.g. the Amazon and the Nile. In these rivers, they are loosely attached by a buoyant mat of fibrous roots to the herbage rooted in the riverbank, and float free, raft-like, for some distance during seasonal floods. However, where sargassum is different is that it surrenders completely to the four winds, moving wherever the ocean currents take it – thereby becoming the only obvious example of ‘boating’ as opposed to merely ‘floating’ vegetation. Although sargassum is scattered across the waters, it is only likely to be noticed where it has pooled at the becalmed centres of large-scale gyres of the ocean currents.
It is not understood why flying fishes depend on a floating substrate for their breeding. Perhaps part of the reason is that flying fishes are fast-growing and short-lived (as is their predator the mahimahi) and thus depend on the relatively rich supply of plankton that congregates to forage on the detritus of the ‘seaweeds’. Another clue to solving this puzzle is that the eels of European and North American rivers travel immense distances to breed in the Sargasso Sea. Although this has not been explained, it may have to do with the rich fallout of organic matter from the accumulated rafts of sargassum. Larval eels, unrecognisable compared to the adults, are leaf-like ‘leptocephali’ which absorb food directly through their transparent skins and would collect the rain of detritus from sargassum simply by hovering in the water column below the ‘seaweeds’.
Although the biplane flying fish and the sargassumfish differ radically in their approaches to locomotion and their strategies to evade predators in the same habitat, it is probable that they occasionally wind up eating each other. This is because flying fishes eat planktonic animals of which the larval stage of the sargassumfish is potentially a part, and the biplane flying fish could conceivably loiter close enough to an adult sargassumfish to be snapped up by it. Such meetings might even amount to a kind of bizarre temporary entanglement of the most wing-like and the most leg-like appendages known among fishes – with the swim bladders of the sargassum itself completing the image of a triad in action. We challenge Nature photographers to capture such a moment in the wild .
The sargassumfish, as a member of a bottom-living lineage of fishes, would seem to have achieved the improbable by living close to the surface of the sea. However, like flying fishes but in its own way, it actually transcends this sea-air boundary. As if it has observed the vanishing-act of the flying fishes and fathomed the underlying principle, the sargassumfish surprises us one more time. When threatened from below, it is known to flip out of the water on to any sufficiently solid raft of sargassum to hide in the aerial dimension. Here, although stationary, it seems to imitate the flying fishes by ‘holding its breath’ until danger has passed and it flops back into the water. Which leaves us in some doubt as to which fish actually spends more time out of water: the biplane flying fish with its flagrant defiance of surface tension, or the sargassumfish, which is as hard to spot when perched on the sargassum as it is when entangled within.… See the hidden half of this Biological Exposition by subscribing here