Oarfish vs ocean sunfish: a contrast in tails, part 2

...continued from https://www.inaturalist.org/journal/milewski/66986-oarfish-vs-ocean-sunfish-part-1#

Apart from squeezing organs into unusual places, the oarfish has also had to develop apparently novel modes of locomotion, presumably owing to its unwieldy tail.

Unlike normal fishes, swimming by the oarfish is not achieved by a sideways beating of the tail, but rather by a subtle rippling movement of the long ribbon-like ‘dorsal’ fin that runs from the head down to the tip of the tail. (This ‘dorsal’ fin consists of approximately 400 rays and is actually a seamless composite of a true dorsal fin and an unconventional caudal fin, see video clip of this rippling http://www.youtube.com/watch?v=lvRqqwBoyx8&feature=related.)

Because nervous control of the ‘posterior dorsal’ fin is presumably conveyed down the length of the tail by the spinal column, it seems likely that the spinal column runs virtually to the tip of the tail. This anatomical detail is relevant: despite the apparent vulnerability of this long spinal column, many captured individuals of the oarfish bear scars on, or have lost portions of, the tail.

It seems therefore that an oarfish can survive the loss of considerable portions of its posterior half, including some of the spinal column, to predators (the International Wildlife Encyclopedia volume 12, page 1599).

The only other fishes that show a similar adaptation are certain eel-like forms of the deep sea. Notacanthiformes, particularly Notacanthidae (https://en.wikipedia.org/wiki/Notacanthidae), are bizarre in having a leptocephalus larva (https://en.wikipedia.org/wiki/Leptocephalus) up to 1.8 m long, which actually shrinks when metamorphosing into the adult, and in being able to regenerate much of the shed tail in a way resembling certain legless lizards.

Moving from the sublime to the ridiculous (or possibly the other way around):
If there could be said to be an ‘infratail’ among fishes, the ocean sunfish has it. In contrast to the long trailing ribbon shape of the oarfish, the body of the adult ocean sunfish has a disconcerting cut-off appearance. Its style of locomotion is as odd as that of the oarfish, being similar to a turtle or penguin swimming on its side. Both fins first flex to the left, then flex to the right, as the ocean sunfish swims normally.

See the video clip at http://www.youtube.com/watch?v=g7ZQwlwbrQo.) but beating the symmetrically projecting, pointed sections of the dorsal and anal fins in place of flippers.

These flipper-like fins are actually only parts of the original fins, the other sections having been distorted in the opposite direction to meet each other in the form of a fusion called the clavus. Thus the fringe on the tail-end of the ocean sunfish is even less like the conventional caudal fin than it may appear. This is one of the few fishes lacking any caudal vertebrae (except during the larval stage, at up to 2.5 cm long).

Yet another oddity of the ocean sunfish is that, within the vertebral column of its trunk, the central nervous system itself projects less than 3 cm behind the brain. Please remember that the entire fish is up to 3.3 m long when mature. Although the larva starts with a vertebral column of unremarkable proportions, most of the bones atrophy during their growth. A further oddity is that the kidneys lie not in the centre of the body but right behind the brain.

The result of this loss is that the spinal column of the whole tail and much of the body vanishes, while sections of the two remaining fins of the tail are fused to form a weak substitute.

The ocean sunfish not only has an extremely short tail, but it is also one of the few fishes in which the vertical axis of the tail (from the tip of the anal fin to the tip of the ‘posterior dorsal’ fin) is the major axis of the tail.

Thus, if the tails of normal fishes represent a balance of dimensions on a crossing system of axes, the oarfish has favoured its horizontal axis well beyond most eels.

Meanwhile, the ocean sunfish has virtually abandoned its horizontal axis and has reinvented the normal orientation of the fish body by adopting the vertical as its main axis.

Both extremes appear absurd. This is because, based on our current understanding, it seems that the oarfish has metres of redundant spinal column and associated flesh, while the ocean sunfish is in debt of the same parts of its anatomy.

As if the perversity of all of this is not enough, the few observations of normal posture of the oarfish show it upright during its normal behaviour (http://www.blueworldtv.com/behind-the-scenes/sea-stories/encounter-with-an-oarfish and https://www.marinebio.org/species/oarfishes/regalecus-glesne/).

This means that what is usually illustrated as a horizontal tail is held vertically as the fish hovers or swims slowly by means of a counter-propagating wave on its ‘dorsal’ fin. Not only does the oarfish appear not to use its tail in the usual side-to-side movement of other fishes, but even its relationship to horizontal versus vertical is confused – raising the possibility that it is actually the oarfish that has the ‘infratail’.

It is hard to imagine a more graphic example of fishes with like function but unlike form. In a sense, the oarfish’s tail, the longest of all living tails, is functionally redundant. This is because the oarfish has metre after metre of apparently dispensable tail flesh, which appears – although this needs confirmation – to be incapable of beating like a normal fish tail but capable of healing after amputation in appeasement of a predator.

By contrast, the ocean sunfish virtually amounts to a giant, sutured self-truncation, developing from larva to adult in such a way as to resorb its tail except for two fins that most of us would not recognise as belonging to the tail in the first place. It is as if Nature has made a game of taking lateral thinking in opposite directions, to two points just beyond the plausible.

And so both the hypertail and the infratail may find themselves tailless in the same ocean: the ocean sunfish having resorbed its tail within a skin so tough that even a harpoon would not penetrate it, and the oarfish appending a tail so superfluous that – if hypothetically harpooned – most of it might be forfeited without the death of the fish.

And we are left gazing at the wedge of ordinary fish on our dinner plate with a new appreciation of its normality.

Posted on June 8, 2022 08:18 PM by milewski