Goldfish Breeding Tubercles

Goldfish Breeding Tubercles (Carassius auratus auratus) 

What are they and what are they for?
Jennie Connelly 2012 
Breeding tubercles are keratin-based skin nodules found on male goldfish.  They most often occur on the leading rays of the pectoral fins and the opercula (gill covers), but they can also be found on all rays of the pectoral fins, on the head and around the eyes, on the leading rays of the pelvic fins, and even on the body of the fish (not to be confused with the parasite Ichthyophthirius multifiliis).  When breeding tubercles occur on the body of the fish, they are arranged on the scales in a neat pattern matching the contour of the scales.  In contrast, the ich parasite occurs randomly all over the body.  Breeding tubercles are induced, in part, by testosterone, and there is great variation in their appearance.  There may be just a few tubercles on the pectoral fins, many tubercles all over the fish, or anything in-between. 
This red broadtail has many tubercles all over the rays of his pectoral fin, all over his operculum, and all over his face.
This black butterfly telescope has many tubercles on the leading ray of his pectoral fin and some tubercles on his operculum.
This red veiltail telescope has tubercles above his eye, on the leading ray of his pectoral fin, and on his operculum.
This calico butterfly telescope has tubercles on his operculum and on the leading ray of his pectoral fin.
This image illustrates the difference between breeding tubercles on the body and the ich parasite on the body.
There is no clear consensus as to what the breeding tubercles are used for, but there are many possibilities.  They may be used for male/female differentiation, protection against injury, weapons in intense pre-spawning male behavior, as a means to stay close to their mate during spawning, stimulators during spawning, an indicator of male health, or an indicator of male dominance.  The actual purpose of breeding tubercles could be some, or all, of the above things. 

In a study by Kortet et al, it was found that the breeding tubercles of the Roach (a fish related to the goldfish) serve as a status badge, with more dominant males possessing more tubercles than less dominant males.  They also found that the tubercles serve as an indicator of male quality, meaning that males with more tubercles have greater reproductive success and are healthier (have fewer parasites).  This means that the breeding tubercles serve as a marker to help the female fish pick mates that are healthier and have a better chance at fertilizing their eggs during a spawn.

Work Cited:
Kortet R., J. Taskinen, A. Vainikka, and H. Ylonen. 2004. Breeding Tubercles, Papillomatosis and Dominance Behavior of Male Roach (Rutilus rutilus) During the Spawning Period. Ethology 110:591-601.

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