Eye Picking and Pebble Picking Behaviors of Cutlip Minnow, by Don Orth

Cutlip Minnow Exoglossum maxillingua is no ordinary minnow.  Two behaviors make it quite unique -- nest building and eye picking. Compared to other minnows, its movements are sluggish, staying near the bottom of clear, rocky streams. But during the spring breeding season, males become hard-working nest builders, selecting pebbles and bringing them to the nest site at a rate up to 6-10 per minute.  This eventually results in a pebble mound that can be 12 to 18 inches across and 5 to 6 inches high.   Wow!  Just consider the energy expended by nest building and tending – a 6-inch Cutlip Minnow can barely transfer a ¾ inch pebble.  Females are smaller and do not participate in the nest building.  The male stays at the nest day and night until breeding has ceased (Hankinson 1922; van Duzer 1939). 

Cutlip Minnow.  Photo by Matt Tillet

The distribution of the Cutlip Minnow ranges from Virginia to New York in streams of the mountains and piedmont provinces.   Here, the Cutlip Minnow co-occurs with many other fishes, including the Common Shiner Luxilus cornutus, Creek Chub Semotilus atromaculatus, Rosyface Shiner Notropis rubellus,  Tesselated Darter Etheostoma olmstedi, White Sucker Catostomus commersoni, and Blacknose Dace Rhinichthys atratulus.   Common Shiner and Rosyface Shiner breed on the nests built by Cutlip Minnows and their constant swimming and darting is in contrast to the behavior of the Cutlip Minnow (van Duzer 1939; Maraukis et al. 1991).  

Distribution of the Cutlip Minnow from NatureServe.

The eye-picking behavior of the Cutlip Minnows has frustrated many field biologists when collecting these fishes.  All types of fishes collected are typically placed in a large bucket until enough are collected to identify and count them all.  Collected fishes held in the bucket with the Cutlip Minnows often have missing or damaged eyes.  Antonios Pappantoniou and George Dale  (1986) discovered that the Cutlip Minnow would immediately pick at the eyes of a goldfish added to an aquarium with many Cutlip minnows.   Furthermore, the Cutlip Minnows were not fooled by the camouflage of  false eyespots or eye lines on fishes (Dale and Pappantoniou 1986).  When in crowded situations, the Cutlip Minnows like fish eyes!

Close-up, ventral view of the mouth of the Cutlip Minnow.  Photo by Brian Zimmerman.

The mouth of the Cutlip Minnow is unique in that the lower jaw consists of a central bony plate flanked by two fleshy lobes.  Only one other fish, the Tonguetied Minnow Exoglossum laurae, has this unique mouth morphology   The ventral mouth would seem to be specialized adaptation for benthic feeding on snails, insect larvae, and diatoms.  Eye-picking does not appear to be an adaptation for feeding on the eyes of other fishes.  The mouth morphology also facilitates the transport of pebbles of a particular size as seen in other nest building cyprinids (Bolton et al. 2015).

In a recent study, Bramburger et al. (2018) observed that nests of Cutlip Minnow were composed of mainly dark pigmented pebbles.  They speculated that the colorful, dark pebble might enhance mate selection by female Cutlip Minnows. Male Cutlip Minnows get darker during breeding but they do not possess secondary sexual characteristics that would serve as cues for sexual selection.   However, Bramburger et al. discovered that the substrate from nests were significantly darker and more saturated than random samples of stream substrata.  No other examples of nest substratum color selectivity has been reported in fishes.  At this stage, all one can do is speculate.   Perhaps darker substrate absorbs/conducts more heat energy (Brown 1969; Johnson 2004) that speeds embryo development.

Our not so ordinary little minnow may possess secrets that are yet to be explained.  

References

Bolton, C., B.K. Peoples, and E.A. Frimpong. 2015. Recognizing gape limitation and interannual variability in bluehead chub nesting microhabitat use in a small Virginia stream. Journal of Freshwater Ecology 30: 503-511.  

Bramburger, A. J., K.E. Moir, and M.B.C. Hickey. 2018. Preferential incorporation of dark, coloured materials into nests by a mound-nesting stream cyprinid. Journal of Fish Biology

Brown, G. W. 1969. Predicting temperatures of small streams. Water Resources Research 5:68-75. 

Dale, G. and A. Pappantoniou. 1986.  Eye picking behavior of the cutlips minnow, Exoglossum maxillingua:  Applications to studies of eye spot mimicry.  Annals of the New York Academy of Science 463:177-178.

Hankinson, T.L. 1922.  Nest of cut-lips minnow, Exoglossum maxillingua (LeSueur). Copeia 102:1-3.

Johnson, S. L. 2004. Factors influencing stream temperatures in small streams: substrate effects and a shading experiment. Canadian Journal of Fisheries and Aquatic Sciences 61(6):913-923.

Maurakis, E.G., W.S. Woolcott, and M.H. Sabaj. 1991. Reproductive behavior of Exoglossum species. Bulletin of the Alabama Museum of Natural History 10:11-16.

Pappantoniou, A., and G. Dale.  1986.  Eye-picking behavior of the cutlips minnow Exoglossum maxillingua: density relationships.   Annals of the New York Academy of Sciences. 463:206-208.

van Duzer, E.M. (1939) Observations on the Breeding Habits of the Cut-Lips Minnow, Exoglossum maxillingua. Copeia  1939:65-75.  

Adaptive Radiation of Cichlid Fishes in Lake Tanganyika, by Kyle Taylor

Evolution is unique in that two similar scenarios can yield vastly different results. Natural selection does not work with future information, it simply selects on individuals that have traits that are currently or have previously selected them over others. An example of this is in the East African Rift Lakes, or more specifically, Lake Tanganyika. The oldest of the lakes (Brown ET. Al, 2010), Lake Tanganyika is famed for being the second largest freshwater lake in the world (N.A. 2016), with the countries of Zambia, Tanzania, DR Congo, and Burundi bordering its boundaries. Here, adaptive radiation has caused for the explosive speciation of the Cichlid Family of fishes, in which the multiple niche levels began to be filled by the family.  The causes of the increase in fish species diversity in Lake Tanganyika is likely due to external factors such as shifting lake levels, but could also be attributed to internal factors such as sexual selection and predation.

            Estimated at being nearly 9-12 million years old (Brown et. Al, 2010), nearly 60% of the animal species that inhabit the lake originated in the body of water (Sweke ET. Al, 2016). With such an extended time of geographic isolation, the lake has experienced multiple water level fluctuations throughout its history. Major lake level fluctuations could thereby explain intralacustrine allopatric speciation; low water levels in the lake would have left the Cichlid fishes geographically divided by different sub-basins (Koblmüller ET. Al, 2008). The fact that many Cichlid species are only found in small, separated sections of the lake also supports this idea (Salzburger and Meyer, 2004).  Many of the rock-dwelling Cichlids of the lake are divided due to habitat separation. Even minor lake level fluctuations can have significant impacts on fish lineages. Littoral or shoreline dwelling species could be significantly affected by minor fall in water level (Salzburger and Meyer, 2004) since their habitat would be the first to be effected by lake fluctuations.

Adaptive radiation of east African cichlid fish. 

            In this case, the extremely large size of the lake would have a greater effect on the speciation of the fish than the age of the lake would.  The emergence of multiple deep-water habitats could then act as a barrier to fish populations, while also separating the spawning sites of the Cichlid fishes, further influencing allopatric speciation (Salzburger and Meyer, 2004) . Other species have also experienced unique speciation in Lake Tanganyika as well, such as the Mastacembelid eels. While Mastacembelid eels can be found throughout Asia and Africa, a separate lineage has formed in the lake (Brown ET. Al, 2010). It is believed that low water levels in the lake nearly 7-8 million years ago began the diversification of the lineage.

            Other factors besides environmental have also played a role on the speciation of the Cichlid family as well. Female Cichlids are known for choosing their mates based on their coloration; “Fisherian runaway sexual selection”, as Meyer calls it, could describe how speciation of the lineages further occurred. Short separation of the fish could cause for gradual coloration shifts in males due to preferred selection by females. These changes, even small, could have dramatic effects on how females would select their mates, further seeding the separation of species flocks. While predation of Cichlid fishes has not be explained for their rapid speciation, it has been known to affect other organisms in the lake. For the freshwater snails of the lake, predation from the freshwater crab Potamonautes lirrangensis has affected both their shell size and shape, and has given rise to their thalassic shells (Weigand ET. Al, 2014).

Unique habitat fluctuations on the lake has led to the lacustrine diversification of fish species in Lake Tanganyika. While other factors may have also played a smaller role in shaping the diversity of aquatic species in Lake Tanganyika, evidence shows that lake levels were a major contributing factor. The lake is remarkable in that it has allowed scientists the opportunity of an isolated study environment in which the effects of allopatric speciation has been greatly induced. In more recent times, influences such as overfishing and climate change (N.A. 2016) have been affecting the lake, putting the unique diversification of species in question. Perhaps we can use these factors to study if overfishing can further induce the effects of allopatric speciation on the Cichlid fishes, but only time will tell how the formation of Lake Species will end. By studying this extraordinary ecosystem now, we can better understand the effects that allopatric speciation will have on future species and ecosystems to come.

Cichlid fishes of Lake Tanganyika.  Animal Press. Source 

Works Cited

Brown, K.J., L. Rüber, R. Bills, and J.J. Day. 2010. Mastacembelid eels support Lake Tanganyika as an evolutionary hotspot of diversification. BMC Evolutionary Biology 10:188

Koblmüller, S., K.M. Sefc, and C. Sturmbauer. 2008. The Lake Tanganyika cichlid species assemblage: recent advances in molecular phylogenetics. Hydrobiologia 615: 5

N.A., 2016. FISH: Lake Tanganyika. Africa research bulletin. Economic, financial and technical series 53: 21402A-21402B

Salzburger, W. and A. Meyer. 2004. The species flocks of East African cichlid fishes: recent advances in molecular phylogenetics and population genetics. Naturwissenschaften 91: 277-290

Sweke, E.A., J.M. Assam, A. I. Chande, A.S. Mbonde, and M. Magnus. 2016. Comparing the performance of protected and unprotected areas in conserving freshwater fish abundance and biodiversity in Lake Tanganyika, Tanzania. International Journal of Ecology 2016

Weigand, A.M., and M. Plath. 2014. Prey preferences in captivity of the freshwater crab Potamonautes lirrangensis from Lake Malawi with special emphasis on molluscivory. Hydrobiologia 739: 145-153