Burrhead and Silverstripe Shiner Reproductive Biology

I'm finally starting to wrap up a paper on a research project that I started over two years ago. Many common fish species in the southeastern United States are still poorly known in terms of their reproductive ecology. I became interested in two such common species, the burrhead shiner, Notropis asperifrons, and the silverstripe shiner, N. stilbius. Both species are found throughout the Mobile basin in Alabama, especially above the fall line.

My question was the timing and relative effort of their reproduction. Shiners are typically reproductively active in Alabama from April through July, more or less. Individuals of these two species had been observed either tuberculated or gravid in this time frame, and that's been the extent of knowledge. Both species are found in the Sipsey Fork system in and near the Bankhead National Forest in north Alabama, so I arranged for permission to collect these fish in Borden Creek in the Sipsey Wilderness of the Bankhead. We (me and two students) made monthly collections of both species from late March until September. Our primary interest was examination of the ovaries of females of the two species, involving the histological preparation of stained tissue samples to quantify oocyte development. I made four random microscopic photos from individual females so prepared, to count and measure these developing eggs at different stages. Two such images are below:

This image is of ovarian tissue from a burrhead shiner collected in early July, using standard histological stains, embedding in parafin, and sliced 4 micrometers thick. The large red cells are in a late stage of maturation called late exogenous vitellogenesis, and are about half a millimeter across. The smaller purple cells are in earlier stages of development. This is evidence that burrheads in Borden Creek are still reproductively competent in early July.
The above image is from a silverstripe shiner in May. The edges of the ovary are clearly visible looking like a relaxed bag, probably evidence that this individual has recently spawned and the ovary isn't jammed with mature ova. But the red cells are maturing oocytes, not quite as mature as the big red oocytes in the burrhead image. And there are a lot of smaller oocytes in earlier stages of development, evidence of on-going ova production for the near future.

What we found was that burrheads are more reproductively active in April, and continue into late July, while the silverstripes don't really kick in until May and drop off more quickly than the burrheads but are still active in late July. Silverstripes are also bigger than burrheads, with females averaging about 56 mm long compared to burrhead females at about 46 mm. In a given time period burrhead oocytes are also usually larger than silverstripe oocytes, often statistically significantly so. Freshly spawned burrhead eggs are 2 mm in diameter, while mature silverstripe eggs found in sacrificed females were found to be 1.4 mm. It seems that relatively small burrheads produce fewer, larger eggs while larger silverstripes produce more small eggs.

Another interesting difference between the two species is that burrheads show sexual size dimorphism, with females being statistically significantly bigger than males (46 mm vs 42 mm) while female silverstripes are only slightly larger than males (56 mm vs. 55 mm). Neither species is territorial or nest-building, which usually results in larger males. But it seems that larger bodied female burrheads may be an adaptation for the production of larger eggs over a longer spawning season, compared to silverstripes.

This is a synopsis of our research, which also includes aquarium work done by Bob Muller. I'll be presenting this at the July meeting of the American Society of Ichthyologists and Herpetologists (ASIH) in New Orleans. If you're there, I'll be moderating the Fish Reproduction session at 8 a.m. on Saturday, July 15, as well as presenting the 15 minute version of this work. And I hope the editors at Ecology of Freshwater Fish like the full article for publication(!).