„A small fleshy fin lacking rays or spines but reinforced by actinotrichs posterior to the soft dorsal fins (rarely a hard ray or a few soft rays may be developed in the adipose fin of certain catfishes)” (Coad & McAllister 2005).

That is how Brian Coad and Don McAllister define the adipose fin in their ‘Dictionary of Ichthyology’. - Alright. But what is it about this mostly nondescript ‘additional fin’, where does is come from, and above all: what is its purpose? Situated between the dorsal and the caudal fin of diverse fish species, origin and function of this fin remain a mystery of fish anatomy up to this date. 

Occurence

An adipose fin can be found in the following eight orders of teleosts:

Ontogenetically, the adipose fin is - like the other median fins of a fish - most likely derived from the larval finfold. Phylogenetically, it is considered a plesiomorphic character rather than having developed independently for several times. (Fiedler 1991). Britz (2004) refers to the adipose fin as a possible autapomorphy of euteleosts, however, its homology between the particular taxa being uncertain. The discovery of fossile gymnotids (Lecointre & Nelson 1976), salmonids (Jordan 1905) and stomiatiods in deposits of the cretaceous, each possessing adipose fins, dates its origin back to the mesozoic (Garstang 1931). Although no obvious function could be assigned to the adipose fin until today, it has been retained ever since. Possible reasons are still discussed controversially, as are the considerations regarding its use for the particular fish taxa. Among others, pleiotropic gene effects have been cited as a potential explanation for the pertinacious maintenance of this seemingly functionless organ (Kosswig 1965).

As already mentioned at the beginning, the adipose fin is situated dorsally between the dorsal fin and the caudal fin. Opinions differ as to whether its contemporary position is that of an original first or second dorsal fin (Sandon 1956; Kosswig 1968). Only some fishes, like in the genus Chauliodus (Viperfish), even possess an additional adipose fin, which will then be located directly before the anal fin. Members of the Malapteruridae (electric catfishes) are special in possessing an adipose fin as the only dorsal fin at all, while lacking the regular rayed dorsal fin. 

There are still huge difficulties in attempting to interpret the benefit and function of the adipose fin. Speculations and hypotheses, however, are numerous. Among these, the support of flow detection (Reimchen & Temple 2004) as well as contributions to hormone regulations or additional stabilization and thrust in larval swimming (Gosline 1971) have been discussed. But above all deliberations, at least one assumption appears to be plausible, as was aptly delineated by Reimchen & Temple (2004): no organ would be retained by diverse fish taxa for a time period of over 70 million years if it was absolutely without any purpose. This would be mainly due to energetic reasons. The authors also mention a number of indications for the decryption of potential functions of the adipose fin. One of these is the finding that the size of the adipose fin can vary intraspecifically. This variability becomes manifest in a form of sexual dimorphism in salmons and trouts, in a way that males do not only have larger adipose fins than females (Beachem & Murray 1986, Næsje  et al. 1988), but also that dominant males have larger adipose fins then their subdominant consexuals (Haugland et al. 2011).  

Moreover, there seems to be a correlation between the presence or absence of an adipose fin and the position of the rayed dorsal fin in closely related taxa. That is, in species possessing an adipose fin, the dorsal fin will be placed centrally and also the caudal fin will be forked in most of the cases. In close relatives without an adipose fin, the dorsal fin will most likely be displaced caudally und the caudal fin will be rounded. Reimchen & Temple (2004) consider this as an aspect of hydrodynamic function. The same authors tested the impact of adipose fin removal on the swimming performance of rainbow trouts of various sizes and in different flow velocities and recorded a significant increase of tail beat amplitude for specimen ranging from 7 to 12 cm body length. These findings are looked at as a fortification of earlier proposed hypotheses, according to which adipose fins might exert a similar function as the finlets of tuna and mackerels. These finlets are known to influence vortices and flow regime to the caudal fin and also reduce cross-flow and boundary layer separation around the caudal peduncle. This way the fish are able to increase tail beat effectivity with regard to thrust generation in forward swimming. Possibly, also the second dorsal fin of Lamnids – fast swimming sharks of the open sea – which closely resembles an adipose fin in position and size compared to the first dorsal fin, has a similar function.

In addition to that, the adipose fin is considered to act as a precaudal flow sensor, at least in salmonids. As a consequence, this hypothesis made by the authors mentioned above inevitably requires an innervation of the fin as an anatomical and physiological prerequisite. Indeed, such an innervation was recently discovered by Buckland-Nicks et al. (2011) in terms of a neural network in the adipose fin of Salmo trutta. The nerve tissue detected is located in intervening spaces of the loose connective tissue and frequently has direct contact to the collagen cables bridging the connective tissue and spanning the two sides of the fin. According to the authors, their findings could especially support the hypothesis of the adipose fin as a precaudal flow sensor. Further studies on stimulus transmission and processing might be helpful to further elucidate this aspect.

With regard to a possible role of the adipose fin as a flow sensor, however, it might appear inconclusive at first glance that also fish species exhibiting a rather inactive, sedentary way of life or those of standing waters can also have adipose fins. Again, Reimchen and Temple (2008) compared 1906 catfish species from various freshwater and marine habitats worldwide and have shown that at least in this group of fish, those species that inhabit lotic environments significantly more often possess an adipose fin than species from lentic or standing waters. These findings are interpreted by the authors as another confirmation of the hypothesis on the function of the adipose fin as a flow sensor.

Despite the widespread lack of knowledge about the origin and function of the adipose fin, it is used by humans for various purposes. Its significance for taxonomy, for example, has already been mentioned in the beginning. In addition to that, finclips of adipose fins are used for marking and tagging individuals in fish stocks, as well as in systematic zoology for clarification of relationships on a molecular level. In the 1980s, proposals for practical uses went even further beyond. Considerations were to make use of the adipose fin as an indicator of potential contaminant concentrations in the muscle mass of edible fish, in order not to sacrifice complete live animals for this purpose. This was based on the findings of a study of Skurdal et al. (1986), who described a high correlation in the concentration of mercury in adipose fin tissue and in the musculature of Salmo trutta. 

In summary, it should be noted that the mystery about the function of the adipose fin remains unsolved. Although there are indications and tendencies, which become more and more coherent regarding certain aspects, a final, satisfactory physiological explanation for this organ has yet to be found. Also, the question of why the adipose fin apparently represents a beneficial anatomical feature that is still worthwhile to be maintained for some non-acanthopterygian teleosts, while on the other hand being entirely absent in modern bony fishes, still awaits an answer. Therefore, we may continue to be curious…