Black Darter Discussion, From Unpublished Manuscript

This is a little odd, but I want to "publish" the Discussion from my manuscript on observing black darters, Etheostoma duryi, from two creeks near Huntsville, AL, several years ago. This is the manuscript that was accepted by Southeastern Naturalist but I had to withdraw it when the co-author disappeared with all of the original data so that I couldn't make modifications on our Results asked for by the editor. We collected and examined both male and female darters from two populations, preparing stained slides of cross-sections through their gonads to estimate size and describe reproductive status. Rachel, the other author, also counted eggs and described their developmental stage from female fish. Town Creek is in Athens, AL, and is an urban, somewhat eutrophic creek. Limestone Creek is in the NW corner of Madison County, AL, in a rural area with lower nutrient inputs. Nanostoma is the subgenus to which black darters belong. I'll omit listing References at the end of this blurb...

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Two populations of Black Darter from nearby creeks did not significantly differ in reproductive investment or in reproductive output. There were, however, significant differences in the total oocyte complement between the sites, best attributable to significant body size differences. Town Creek females were larger, and had attained this larger size going into the breeding season. The month of capture had a significant effect, the site of capture had a highly significant effect and the sex of the fish had a highly significant effect on the mean standard length. Town Creek fish had a greater mean standard length than Limestone Creek fish, and males were longer than females. In darter species the males grow faster and larger than females, so this result was expected (Page 1983).

The observed increase in standard length of Limestone Creek females (but not of Town Creek females) may indicate that any significant growth at Town Creek had occurred prior to the beginning of the study and resulted in larger females. One key difference for Town Creek females was that as standard length increased, the GSI decreased. This indicates that larger females at Town Creek expend less energy toward reproductive investment and more toward physical growth than do smaller females. Fuller (1998) demonstrated that E. caeruleum females in Michigan have a similar relationship between body growth and reproductive development.

Heins et al. (1992) found significant differences between the mature (MA) oocyte weight and the ripening (MR) oocyte weight. If females from Limestone Creek were slow in maturing, they could just be beginning MR development, during which oocytes can have varying sizes. If most of the MR oocytes from Limestone Creek are in the early ripening stage and most of the MR oocytes from Town Creek are in the later ripening stage, the result could be a significant difference in total body weight. Furthermore, if more Town Creek females have ripe, hydrated eggs than do Limestone Creek females, then the water from hydration could increase the overall body weight.

As could have been predicted by the results regarding standard length and weight, there was a highly significant trend for a positive increase in the ratio of weight/standard length in females from Limestone Creek, but no such trend existed for Town Creek females. This suggests that some Town Creek females may have spawned some ripe eggs prior to capture, thereby conferring less of an increase in the ratio than seen in Limestone Creek females. For males, there were no significant trends in the mean ratio of weight/standard length over the three months studied. Males appear to be at their optimum condition in January, prior to any significant female development.

The two populations showed similar patterns in the development of reproductive phenotype, with Town Creek individuals tending to develop earlier. At Town Creek, females were gravid in the February capture. Male breeding coloration began to stand out during the same time period. Limestone Creek females were beginning to become gravid in February, and males began exhibiting the orange breeding colors typical of Black Darters. In March, the abdomens of Town Creek females were greatly extended, and thus gravid, and male breeding colors brightened, with blue-green additions around the opercular/cheek area and at the caudal peduncle, and along the nape. The fin rays were a brick red, and had multiple striations. Similarly, in early April, Limestone Creek females were gravid, and male colors brightened.

While it appears that physical growth and development may have occurred more slowly at Limestone Creek than at Town Creek, it may be the case that fish at Limestone Creek just grow to a different size than fish at Town Creek. Both sites had females with ripe eggs in January, although the numbers were very small. Although the Limestone Creek sample in January had a smaller proportion of ripe (RE) females than the Town Creek samples, both sites had at least 50% RE females in February. Town Creek females were exhibiting the occasional atretic follicle as early as January and throughout the study. No atretic follicles were found in females from Limestone Creek, but this could have been due to chance of sampling.

During all months of capture, male samples from Town Creek were at an earlier stage of development than samples at Limestone Creek. However, sampling error could have influenced these results. Male darters tend to be ahead of females in terms of growth, and, thus, maturation (Page 1983). It is possible that errors occurred in differentiating Stage IV from Stage V in males. Stage V in described by Leino et al. (2005) as having numerous spermatids in the lumen of the testes. If some males were captured just after releasing their milt they may have been classified erroneously as being in Stage IV development.

There could be several different causes for the size difference between the two populations. One cause could have been the differences in water temperatures between the two sites. While not extremely different, Town Creek had higher water temperatures during the first two months of the study. In the third month of the study, Limestone Creek had a higher water temperature than Town Creek. However, this should not have affected the size of the fish, since most should have been at their optimum size by their March/April capture.

Another possibility for the size difference between the sites could be the food availability for larval development in each habitat. Limestone Creek had a greater depth and swifter current with little vegetation. Town Creek, however, was of shallow depth and a low current speed with heavy vegetation in the capture areas. Heavy vegetation would offer protection for the offspring of many forage species, including insect larvae and crustaceans, a staple in the diet of congeners such as E. simoterum (Page and Mayden 1981).

The total dissolved solids (TDS) could have affected the overall size of the fish in this study. Fallo and Warren (1982) studied another Nanostoma species, E. atripinne, in Kentucky, finding that they were in better condition and more successful in streams with high levels of TDS. Town Creek consistently had higher TDS readings than Limestone Creek.

Many studies have tried to elucidate the spawning season for various darters of the subgenus Nanostoma (e.g., E. coosae, O'Neil (1981); E. baileyi, Clayton (unpublished 1984); E. zonale, Hubbs (1985); E. rafinesque, Weddle and Burr (1991)). Considering the trends in the current study in mean oocyte and mean RE egg size and mass and trends in GSI, the spawning season for Black Darters likely begins in early-to-mid March, with the peak possibly occurring in April and, perhaps, into May. We urge that further research into the reproductive biology of other Etheostoma species be done with histological methods similar to ours to better clarify the seasonal patterns of gonadal growth and function in this speciose and ecologically important North American genus.