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.