We sometimes encounter two very similar fish species in similar places. When the two species are difficult for the novice to differentiate, we further wonder “How are the species reproductively isolated?” Ernst Mayr, a leading evolutionary biologist of the 20th century, classified reproductive isolating mechanisms into pre-zygotic (act before fertilization) and post-zygotic (act after fertilization). In the case of sympatric species, behavioral isolation may involve distinctive behaviors designed to increase contact between breeding individuals of the same species. Post-zygotic mechanism are less efficient for reproductive isolation. Although, we don’t often observe these isolating behaviors, there must be some subtle cues present during breeding. Fish use pheromones (smell), vision, and sound for species recognition and reproductive isolation. Yes, fish make sounds that travel well underwater. We’ve known this for a long time (Myrberg et al. 1965; Gerald 1971). Listen to a few fish making sounds here.
|Reported maximum detection or propagation distances of mating sounds for (a) goby Gobiidae, (b) Oyster Toadfish Opsanus tau, (c) Bicolor Damselfish Stegastes partitus, and (d) Black Drum Pogonias cromis (Amorim et al. 2015).|
The Common Goby and the Sand Goby have sympatric distributions. During breeding they are found in close proximity, often competing over the same nest resources. To study the role of vision and sound in reproductive isolation, Blom and coauthors simultaneously recorded sounds with a hydrophone and visuals with a camcorder of breeding individuals under controlled lab conditions. What they learned was that females like males that sing. In addition, there were visual cues that differed between species so that a female Common Goby would not be fooled by the behaviors of a male Sand Goby.
|Common Goby (left, by Michel Barrabes) and Sand Goby (right, by Mazzun Tar-Ramel)|
Differences in courtship behaviors and cues also help explain why the two morphologically and ecologically similar species select different habitats for breeding. The courtship sound of the male Common Goby has shorter duration than the one of the Sand Goby. Male Common Goby swim faster than male Sand Goby during courtship displays. Finally, only the female Sand Goby display black eyes during courtship. Nests of Common Gobies are more prevalent in shallow, soft bottom areas whereas Sand Goby nests use Mya clams in deeper, sandy beaches. The habitat differences influence sound transmission.
Oscillograms of representative sounds illustrate the distinctness of sounds of (A) male Common Goby and (B) male Sand Goby. (Blom et al. 2016)
During breeding, visual cues are typically expressed by males as breeding coloration. In the Sand Goby, the female display of black eyes acts as a declaration of intent that the female is ready to mate. Female Sand Gobies are able to change eye coloration, sometimes gradually, other times quickly, even within seconds. The black eyes are not conspicuous in males. In experimental aquaria, Olsson et al. (2017) observed that occurrence of female displaying dark eyes are more likely in those females close to spawning readiness, as measured by body roundness.
|Bar chart illustrating the relationship between female roundness and frequency of dark eyes (black line). Frequency of dark eyes are illustrated in dark grey bars. (Olsson et al. 2017).|
As the studies of gobies illustrate, females prefer males that sing and can distinguish sounds for closely related species in order to isolate species reproductively. The study raises questions about how males learn to sing and whether and how noise from human activities affect signals during breeding. But now you know why the male goby sings.
Amorin, M.C.P., R.O. Vasconcelos, and P.J. Fonsesa. 2015. Pages 1-33 in F. Ladich, editor. Sound Communication in Fishes. Springer.
Blom,E-L., I. Mück, K. Heubel and O. Svensson. 2016. Courtship sound and associated behaviours of two sympatric marine Gobiidae species – Pomatoschistus microps and Pomatoschistus minutus. Environmental Biology of Fish 99: 999–1007.
Gerald, JW.1971. Sound production during courtship in six species of sunfish (Centrarchidae). Evolution 25:75–87.
Malavasi S, S. Collatuzzo, and P. Torricelli. 2008. Interspecific variation of acoustic signals in Mediterranean gobies (Perciformes, Gobiidae): comparative analysis and evolutionary outlook. Biological Journal of the Linnean Society 93:763–778.
Myrberg AA Jr, E. Kramer E, and P. Heinecke. 1965. Sound production by cichlid fishes. Science 149:555–558.
Olsson, K.H., S. Johansson, E-L. Blom, K. Lindström, O. Svensson, H. Nilsson Sköld and C. Kvarnemo. 2017. Dark eyes in female sand gobies indicate readiness to spawn. PLoS One 12: e0177714 http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0177714