Oarfish vs ocean sunfish: a contrast in tails, part 1

The oarfish (Lampridiformes: Regalecidae: Regalecus glesne, https://www.inaturalist.org/taxa/196745-Regalecus-glesne and https://www.earthtouchnews.com/wtf/wtf/in-photos-fishermen-release-juvenile-oarfish-back-into-the-ocean-off-baja-california/ and https://www.dreamstime.com/called-king-herrings-source-sea-serpent-sightings-cutout-white-ground-giant-oarfish-regalecus-glesne-isolated-background-image157964154 and https://www.britannica.com/animal/oarfish) and the ocean sunfish (Tetraodontiformes: Molidae: Mola mola, https://www.inaturalist.org/taxa/49601-Mola-mola and https://phys.org/news/2019-03-boulder-sized-sunfish-ashore-australia.html and http://www.oceanlight.com/spotlight.php?img=03325 and https://www.angelfire.com/mo2/animals1/tetra/oceansunfish.html)
are unusual fishes of the deep sea.

Both species

• are widespread in the tropical to temperate oceans, although rarely seen,
• have small mouths,
• eat jellyfish, suspended crustaceans, and other relatively small organisms,
• lack a swim bladder,
• are scaleless but have armoured skin,
• have jelly-like flesh, and
• are among the least brainy of fishes relative to body mass.

Despite the semblance of the ocean sunfish to a disembodied head, its brain appears to be no bigger than that of the oarfish.

However, the two species make for a potentially revealing comparison between the longest tail and the shortest tail among fishes.

The oarfish has a tail that can be longer than a difficult-to-believe 10 metres (https://en.wiktionary.org/wiki/oarfish#/media/File:Giant_Oarfish.jpg and https://www.deccanherald.com/content/376357/oarfish-catch-revives-memories-sea.html). By contrast, the ocean sunfish, despite weighing more than a female hippopotamus, lacks a conventional tail (https://www.inaturalist.org/observations/57882247 and https://www.inaturalist.org/observations/22018690 and https://www.inaturalist.org/observations/1602138 and https://www.inaturalist.org/observations/414931) and even has a much-reduced spinal column within the body.

So here we have similar lifestyles despite the greatest conceivable difference in the development of the axial skeleton of vertebrates. Contemplating this structural divergence but functional convergence, my eyes have been opened to the true nature of tails in even the most normal of fishes.

The oarfish and ocean sunfish are extreme in opposite ways, as follows:

• The oarfish is the longest of all the tens of thousands of species of ‘bony fishes’ (Osteichthyes), mature specimens reaching approximately 15 m. By contrast, the ocean sunfish is exceptionally short for its mass and – despite being the heaviest of all ‘bony fishes’ at up to 2.3 tonnes – has a cartilaginous instead of bony skeleton.
• Both species have small pectoral fins and lack a conventional caudal (tail) fin. However, the pelvic fins are streamer-like in the oarfish and absent in the ocean sunfish, with neither appearing to be functional for locomotion.

Turning now to their modes of reproduction: both species reproduce by laying many small eggs in the open sea. In the case of the ocean sunfish the number laid – 300 million or more at a time – exceeds that of any other vertebrate, as does the difference in size between hatchling and adult. In both cases, the larvae look quite different from the adults with body proportions not drastically different from those of other fishes.

However, as their bodies grow towards maturity, the directions in which this growth occurs are so different that it is difficult to reconcile these as functionally similar fishes in the same oceans.

Human appreciation of where the tail of fishes begins and ends tends to be poor, even in ‘normal’ fishes such as goldfish, trout, or perch. This is possibly because, as a tailless species, our minds are ill-primed.

In general, bony fishes differ from most other vertebrates in the sheer size of the tail relative to the rest of the body, and in the elaboration of the tail into not one but several separate fins and fin lobes. Apart from the fins, the bones and muscles of fish tails also comprise a major, horizontal axis and a minor axis at right angles to it. Any part of the fish posterior to its vent, in theory, should be ‘caudal’, i.e. ‘of the tail’.

The tail in its entirety thus includes

• the left and right surfaces of the caudal peduncle,
• each lobe of the conventional caudal fin (which can be surprisingly complex in some fishes), and
• all other fins located posterior to the vent.

In some fishes this means a whole series of ‘dorsal’ fins separate from the true dorsal fin of the body of the fish, and up to several anal fins.

Because textbooks seldom present this material clearly, it is easy to 'underlook' the fact that the muscle we eat in the form of fish fillets is, more often than not, that of the tail, not the body.

The anal and ‘posterior dorsal’ fins form a vertical axis within the tail skeleton. These fins are poorly named, because both are in fact part of the tail and should properly be called caudal fins of sorts. In the case of cartilaginous fishes (Chondrichthyes, https://en.wikipedia.org/wiki/Chondrichthyes), the anal fin is a particular misnomer because the vent in these fishes is in fact quite separate from the anal fin, being located between the pelvic fins.

This is highlighted by the fact that in many bony fishes the tail, including all its various fins, has a total surface area exceeding that of the head and body (including dorsal, pectoral, and pelvic fins).

Of the major lineages of vertebrates, only the dinosaurs rivalled the bony fishes in the sheer proportion of the body contributed by the tail. However, few dinosaurs can rival fishes in the elaboration of the tail into eight or more facets.

A first step in appreciating the true ‘tailiness’ of fishes is to find the posterior orifice, where the tail logically begins. This vent, the orifice of the cloaca, tends to be located centrally on the ventral surface of commercial fishes rather than on their hindquarters.

Viewed against the template of normal fishes, the tails of both the oarfish and the ocean sunfish seem to have fallen off the edge of the drawing board.

For example, neither species has retained the conventional caudal fin that is taken for granted in most fishes. In the oarfish, the fleshy part of its tail has been lengthened so much that its conventional caudal fin has been attenuated to oblivion. In the ocean sunfish, the conventional caudal fin has also been lost. However, in sealing off the truncation, this species has produced a false, non-propulsive tail (called a clavus) out of the adjacent parts of its remaining, unconventional, tail fins.

If there could be said to be a ‘hypertail’ among fishes, the oarfish has it. The tapering tail of a 15 m specimen would be at least 11 m, making it the longest tail of any living animal - probably including the blue whale (Balaenoptera musculus, https://en.wikipedia.org/wiki/Blue_whale) which has a tail up to approximately 11 m long.

The next longest fish tail, that of the mature whale shark (Rhincodon typus, https://www.inaturalist.org/taxa/52188-Rhincodon-typus), is less than 7.5 m long despite comprising more fins than that of the oarfish and belonging to an incomparably more massive fish. This supremacy of the oarfish in tail length must bring some sort of advantage, but it is unclear what exactly this might be.

Based on our currently poor knowledge, this seems an excessive focus on one anatomical feature at the expense of more essential organs. For example, in the oarfish, a long pocket of the gut extends several m past the vent into the tail, in contrast to normal fishes, which fit all of the internal organs into the main body cavity in front of the tail.

to be continued in https://www.inaturalist.org/journal/milewski/66987-oarfish-vs-ocean-sunfish-part-2#...

Posted on June 8, 2022 07:44 PM by milewski