The lowly adipose fin: the only fin in ray-finned fishes without rays. Or so one might think... Adipose fins are found in more than 6000 species of living teleosts, the major radiation of ray-finned (actinopterygian) fishes. Based on a recent study by Tom Stewart, Leo Smith, and Michael Coates (Proc. R. Soc. B: 2014), adipose fins have evolved multiple times (perhaps as many three). Their study found support for one origin in the group that includes Siluriformes (catfishes) and Characiformes (pacus, piranhas, tetras, etc.). They also found that not all adipose fins are internally alike, and have evolved several kinds of skeletons (i.e., endoskeleton and dermal skeleton). In loricarioids (e.g., astroblepids, callichthyids, loricariids), the adipose fin has an anterior spine of dermal bone and is likely derived from a modified scute. In other catfishes (e.g., Clarotes, Mochokus, Phractocephalus hemioliopterus) and at least one characiform (the tambaqui, Colossoma macropomum), the adipose fin develops bony rays similar to other fins. In those taxa, the adipose-fin rays are composed of segmented and bilaterally paired hemitrichia, with each pair called a lepidotrichium ( = bony fin ray). In the redtail catfish, lepidotrichia segments appear to fuse to form non-segmented dermal rods, though segmentation remains visible in the smallest fin rays. Tom Stewart is currently investigating the development of bony rays in the adipose fin of Phractocephalus, based in part on x-rays prepared by Kyle Luckenbill and Academy WINS students Tamira Bell and Gere Johnson. Two Xingu specimens provide important data points because they are large (755 & 785 mm SL) and wild caught (other large specimens in the study having been kept in public aquaria). Why has the adipose fin evolved in some fishes, but not others? The answer is a bit elusive. In studies of juvenile trout, Reimchen & Temple (2004) found that adipose fin removal produces an average 8% (range –3% to 23%) increase in caudal fin amplitude (i.e., work) relative to unclipped fish. They speculated that the adipose fin might improve swimming efficiency by controlling vortices that envelop the caudal fin (i.e., vortex dampener), or by operating as a precaudal flow sensor that allows adjustments of caudal fin motion in turbulent waters. In a follow-up study, Temple & Reimchen (2008) surveyed 1906 species of catfishes for relationships between presence/absence of adipose fin and flow regime. Catfishes living in habitats with flow (rivers) exhibited an adipose fin significantly more often than expected relative to no-flow habitats (lakes, marine, parasitic). Their results are concordant with the hydrodynamic function of the adipose fin. Buckland-Nicks et al. (2011) examined the ultrastructure of the adipose fin in brown trout, and discovered evidence for extensive nervous tissue, consistent with the hypothesis that the adipose fin also serves as a precaudal flow sensor. Finally, Stewart & Hale (2013) discovered a musculoskeletal linkage in the adipose fin of the sun catfish, Horabagrus brachysoma. They considered that linkage to be an evolutionary innovation, a novel mechanism for controlling adipose-fin movement. Prior to their discovery, it was generally assumed that the adipose fin is a passive structure with no associated musculature. And why do some adipose fins have rays? I would speculate that in Phractocephalus, perhaps the hard bony rays discourage piranhas and other fin nippers from biting off a mouthful of adipose fin.
Posted on: Tue, 09 Sep 2014 14:36:29 +0000
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