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What do you think members of these orders have in common? Osmeriformes, Stomiiformes and Percopsiformes. It is also present in species belonging to the orders of Myctophiformes, Siluriformes and Aulopiformes. Some of these rather formal names might be unfamiliar. How about the Characiformes then, an order containing the glamorous and sharp-toothed Golden Dorado?
A bit obscure still? Belonging to the above group and sitting squarely on the hearth of the fly fisher’s home we have the Salmoniformes, the fly fishers’ original quarry if interpretation of Aelianus’ writing is to believed. They all have one and it might be said to be diagnostic for this group.
Enough of Linnaean word games. What all these orders have in common are family members who sport an adipose fin, that “mysterious little fatty fin” as it has recently been described. The fin is a rare attachment existing in only the above eight orders and absent in all other major and more recent groups. The Cypriniformes, the group containing carp and the multitude of other familiar species, do not have one. Neither do the Perciformes, the largest order of fish in the world and the largest family of vertebrates in the world. The fin’s presence among the groups mentioned above is also patchy. In the Siluriformes for example, an order containing in the region of 2,500 different kinds of catfish, some species display the fin and some do not. But in the Salmoniformes, the home of many a fly fisher’s hopes and aspirations, it is present in all three lineages, the whitefish (Coregoninae), grayling (Thymallinae) and of course salmon and trout (Salmoninae).
Some time ago and perhaps still among certain fly fishers, the presence of the adipose fin was an indicator of rightness, a fish that possessed an adipose was good and one that did not, in some Orwellian oversimplification, was bad. The carp, pike, and chub I used to catch with such glee were a proletarian group, a coarse, rough bunch of fish fit only for capture by young boys and those not enlightened to trout and salmon. Naturally such apartheid is wrong because grayling and whitefish, have an adipose too. Basing identification more broadly on the presence of the little knob would lead to all sorts of confusion if there were not more distinguishing signatures separating salmon and trout from the more mundane grayling and whitefish, remote members of the family or not.
It follows that, aside from a dubious identification tool, any mind curious about both form and function would ask a simple question—why is the adipose there at all? It is small, has no rays or muscular attachments and so cannot be wiggled for any purpose in the current. Yet the fin persists and so one asks, “Does it have a function? Does it perhaps aid balance, propel or maneuver a fish in some way? Is it a flow sensor, a widget of fitness, a lure for the opposite sex? Or is it really functionless; is it in fact a classic example of a vestigial organ in the same manner as the hind limbs of whales or the “feet” of some snakes?” A vestigial status is a commonplace assumption and as a consequence fisheries biologists have decided that snipping off this fin incurs no cost or long term harm to the amputee. Vestigiality is a complicated topic however and classifying any organ as vestigial has to be done with some caution, even for us.
In 1893 the anatomist Robert Wiedersheim suggested there were 86 vestigial organs in humans [1]. Then, without the scientific tools to allow the detailed deciphering of the role of these organs the list may have seemed accurate. Now many of the organs on his list have been re-evaluated.
The appendix, the classic human vestigial organ, is not immune from this revision. Recent work has suggested the appendage, a leftover from ancestors who ate more vegetable matter than we do now (or have done for some time) was originally part of the intestinal tract, possibly a caecum-like extension which provided a supply of cellulose digesting bacteria, plant wall material being hard to process otherwise. A study [2] examining the extent to which different animals did or didn’t have an appendix led the authors to suggest that far from being without a role the appendix may provide a useful service. Following a bad bout of the runs, when diarrhea caused by cholera or some other debilitating or deadly disease flushes out the bacteria flourishing in the intestinal tract, a remaining population residing in the safe haven of the appendix could recolonize the gut and get back to the good work of aiding digestion. Removing the organ from a healthy person living in a country with adequate medical care may make no discernable difference. By contrast, excising the appendage from a person living in dire poverty, existing on little and low quality food and without access to even vaguely reasonable medical care, the loss of an appendix might cause some difficulty. Might, the idea is not proven as there are no clear cases available to show a detrimental effect of appendix removal. Yet it demonstrates, as do many other examples, that organs once defined as vestigial, might not be quite so redundant.
Which leads us back to this mysterious fatty fin. If many vestigial organs once thought to have no function, thought to be degenerating examples of what once was useful, are now considered to be necessities, would this apply to the adipose too?
Interest in the adipose among biologists has been around for some time. It has been noted that the fin is not fixed in its form—there is a temporal pattern of growth and enlargement associated with reproduction [3]. There is no equality of the sexes in this development, the adipose of males increases along with other secondary sexual characteristics (jaw length, the development of the kype, body depth and skin colour being a few) during sexual maturation. Females persist with their demure adipose and the contrast to the males’ represents a clear case of sexual dimorphism. But describing changes in form is a small part of the story. What function does this growth imply? Does it have some role in mate choice (in sexual selection the driving force of many weird and wonderful forms in nature of which the peacock’s tail is a classic example)? A number of studies suggest that it does, though naturally in biological systems the exact importance of its role is difficult to run down. Female salmon and brown trout show a preference for males with large adipose fins. They show a preference for other traits as well, body morphology and the general ability to establish and defend territories, but when all the individual attributes of competing males are separated into their component parts the size of the adipose is still a significant factor when females choose a male to fertilize their eggs. What is more, there appears to be some advantage to the offspring sired by these males as they are more active and probably better at defending territories [4]. This is all relatively straightforward if it wasn’t for those pesky males messing everything up. It is true that when females are left unmolested, left to view some males and choose which one to excavate a nest next to, the size of the adipose, among other measures, is a significant factor. However in the hurly-burly of the redds females seldom seem to get that chance. Males form dominance hierarchies and the largest and most aggressive male tends to dominate spawning events with females. That is not to say that the adipose fin doesn’t play a role, just that it is more difficult to decipher the contribution female choice actually has.
The complexity of the adipose’s role in mate choice and offspring fitness is confusing and ultimately unresolved yet its role in swimming has become clearer in recent years. A study [5] using juvenile steelhead trout measured tail beat frequency and amplitude at flow speeds between one and three body lengths a second. Having established what the steelhead did with their adipose fins intact the researchers then removed the fin and put the fish to the test again. In all but the smallest fish tail beat amplitude increased by about 10% on average, not a lot, but a significant change to the beat amplitude recorded prior to adipose removal. The fin then has a role to play in increasing the efficiency of sustained swimming. The authors also noted some small nerves running to the base of the fin, suggesting that its specific purpose may be to play a role as a flow sensor, perhaps signaling necessary adjustments to tail movement in turbulent flows. Whatever the exact function, fish with their adipose fin removed are likely to have higher energetic expenditure than those fish retaining the fin, a potentially costly outcome in a dynamic aquatic environment. A rather neat connection between the possible role of the adipose in mate choice described above and the clear advantage the adipose confers in sustained swimming arises from this finding. Male anadromous salmonids enter freshwater earlier than females, remain in natal streams for longer and are much more active during this period. A large adipose may be advantageous, an adaptation to these greater swimming demands, and if true, would provide a reason for the apparent preference of females for males with large adipose fins.
The adipose fin then would not seem to be a classic case of a vestigial organ. Though the importance of its role in sexual selection is unclear it certainly aids swimming efficiency. And that leads to yet another question. Why fin clip? Many fisheries routinely use the technique to mark stocked fish and these pen-reared fish are subsequently released (in vast numbers) to supplement dramatically dwindling populations of wild salmonids hit by the double effects of anthropomorphic exploitation at sea and river use changes in their natal streams. It can be argued that, from the evidence accruing, the removal of the adipose separates even further these hatchery fish from the wild originals they are meant to imitate, a process already well documented to have occurred in the rearing pens. There is no evidence that adipose clipping has an impact on returns, the great maw of the ocean can account for more than 90% of migrants after all. And fin clipping is a cheap, reproducible and robust way of identifying fish. Nevertheless, it is tempting to speculate that a hatchery smolt sent seawards would not encounter the costs of fin removal in the ocean but would on its return to the river. But, as the rivers of their return are characterized by dams and decreased flows, the very lack of turbulent water a fish with an adipose may have an advantage in, the costs may not be realized even here. We end up with a mutilated fish in a mutilated river. Some might say an apt description for the plight of anadromous salmonids in this modern world.
1. Wiedersheim, R. (1893) The Structure of Man: An Index to His Past History. Second Edition. Translated by H. and M. Bernard. London: Macmillan and Co. 1895.
2. Smith, H. F., Fisher, R. E., Everett, M. L., Thomas, A. D., Randal Bollinger, R. & Parker, W. (2009). Comparative anatomy and phylogenetic distribution of the mammalian cecal appendix. Journal of Evolutionary Biology. 22, 1984-1999.
3. Beacham, T. D. & Murray, C. D. (1983). Sexual dimorphism in the adipose fin of Pacific salmon (Oncorhynchus). Canadian Journal of Fisheries and Aquatic Sciences. 40, 2019-2024.
4. Peterssen, E. & Järvi, T. (2007). Characteristics of brown trout males influence growth and survival of their offspring. Journal of Fish Biology. 71, 493-509.
5. Reimchen, T. E. & Temple, N. F. (2004). Hydrodynamic and phylogenetic aspects of the adipose fin in fishes. Canadian Journal of Zoology. 82, 910-916.
Simon Blanford is a contributing editor who likes to think about how fish work but still manages to catch very few when he does go fishing.
