Breeding tubercles are small whitish nodular structures often found on male goldfish, and many other types of fish, spanning across at least 15 different families, and 4 orders. While some of these nodules are a constant presence on male goldfish, others appear preceding spawning, and disappear shortly after. Breeding tubercles, sometimes also called breeding stars, are found mainly on male goldfish, although in some rare instances can be found on females. Because breeding stars are usually found predominantly on males, these structures are fairly reliable markers for gender identification.
There are two places where breeding tubercles are frequently found. First, they can be found on the front rays of the pectoral fins. Once the pectoral fins have developed (prior to sexual maturity), these structures remain in place for the duration of the fish's life. Pectoral stars can diminish in size after spawning, but will grow even larger during the next season.
Breeding stars/tubercles on the pectoral fins of male goldfish
Breeding stars are also often found on the gill opercula/covers of goldfish. Unlike the pectoral tubercles, these stars usually disappear after a season's spawning, and reappear in the next season.
Breeding stars/tubercles on the gill plates of male goldfish
Breeding stars/tubercles on both gill plates and pectoral fins of male goldfish
In some instances, breeding tubercles can be found distributed throughout the entire body of the goldfish.
II. Regulation & Function of Breeding Tubercles
Breeding tubercles are essentially overgrown skin cells covered by a cap of keratin. These structures are formed in response to hormones such as testosterone from the gonads and other hormones from the pituitary glands. Stimulated cells begin to divide much faster than normal. These epidermal cells are also somehow reprogrammed to switch from mucus production to that of keratin, resulting in the keratin cap. Vascular networks are also induced and put in place, to support the long term survival of these structures (see figure below). Experimental studies have shown that injection of testosterone in both male and female fish are sufficient to induce the formation of breeding stars, although the process is much more easily done in males than in females, demonstrating that there are other male hormones/factors at work to enhance the process. These studies also suggest a way whereby sometimes female goldfish might acquire breeding tubercles, such as the overproduction of a particular hormone.
(figure taken from Wiley & Collette)
So, do these tubercles have any functions? While not much definitive work have been shown, a number of functions have been inferred, or suggested:
- sexual display - to differentiate between male and females.
- tactile stimulation of females during spawning
- defense of territories and nests. It's been suggested that these hardened structures could be brandished as weapons of sorts.
- dominance display among males
- indicator of health (males free from parasitism will display more stars, while disease ridden ones may have few or none)
It's important to note that in different species of tubercle bearing fish, the function(s) may vary. In goldfish, it's not clear what roles the breeding stars play in the spawning process, although territorial defense and usage as weapons are not likely.
While not much is known about the function of breeding tubercles, these contact organs are very useful in sex determination in goldfish, as they predominantly appear in males.
1. Wiley, ML & Collette, BB. 1970. Breeding tubercles and contact organs in fishes: their occurrence, structure, and significance. Bulletin of the American Museum of Natural History 143:147.
2. Kortet, R. et al. 2004. Breeding tubercles, papillomatosis and dominance behaviour of male roach (Rutilus rutilus) during the spawning period. Ethology 110:591.
3. Ghadially, FN & Whiteley, HJ. 1952. Hormonally induced epithelial hyperplasia in the goldfish (Carrasius auratus). Br J Cancer 6:246.