Journal of Undergraduate Research


morphological variation, alfaro cultratus, stream flow regimes


Life Sciences




Alfaro cultratus is a freshwater fish native to Costa Rica.1 The common name for Alfaro cultratus is ‘the knife edge livebearer’. It is named after its sharply keeled ventral surface which allows for movement through river currents with speed and agility. Yet this species is a live-­bearer, meaning it gives birth to free-­swimming young rather than laying eggs. Its shape does not allow for the normal distended abdomen during pregnancy like other live-­bearers. This morphology likely provides advantages in high flow regimes, but produces costs for reproductive investment. To begin looking at this paradoxical evolutionary trait, I looked at the possibility of morphological variation in other stream flow regimes.

Body shape is known to differ in various flow regimes for many fish.2 One study done on a mosquito fish (Gambusia species-­ related to Alfaro cultratus) found physical variation between the same species in different flow regimes.2 They found a higher amount of fusiform, stream-­lined body shape, in high velocity environments. I performed a similar analysis on Alfaro cultratus hypothesizing that a stream-­lined body shape is more advantageous in high stream gradients than in low stream gradients. Therefore, I expect to see a greater variety in body morphology in lower stream gradients than higher;; I also expect to see stricter stream-­lined body morphology found in high stream gradients. The results of this experiment will provide a basis for further life history analysis on this species.

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