The fishes of the Pine Barrens are beautiful and distinctive.  They include the rare blackbanded sunfish (Enneacanthus chaetodon), the shy swamp darter (Etheostoma fusiforme), and the unusual pirate perch (Aphredoderus sayanus).  These species, which are so characteristic of blackwaters on the Atlantic Coastal Plain, rarely occur in nearby clearwaters.

Blackwaters have high concentrations of humic substances, which stain the water dark brown.  The humic substances come from the decomposition of lignin, the molecule that makes woody plants woody.  In the Pine Barrens, much of the humic substances come from lignin in pine needles.  Humic substances interfere with the transmission of light through water, and so floating and emergent macrophytes are the most important primary producers in blackwaters.

Most blackwaters are also acidic.  The Pine Barrens lies on the Cohansey Sand, a sedimentary deposit having very low buffering capacity.  Thus, Pine Barrens blackwaters range in pH from 3.7 (acidic) to 6.7 (nearly neutral).

                             Banded, bluespotted, and blackbanded sunfish.

Three small sunfishes of the genus Enneacanthus offer a glimpse into the adaptations of fishes to acidic blackwaters.  For example, the banded sunfish (E. obesus) is nearly restricted to acidic blackwaters.  In contrast, the very similar bluespotted sunfish (E. gloriosus) is widely distributed in both clearwaters and blackwaters.  When these two species co-occur in the same pond, they utilize different microhabitats.  The banded sunfish occurs in shallow water having dense cover, especially submerged sphagnum moss.  It spends much of its time picking invertebrates from the surfaces of plants.  The bluespotted sunfish occurs among aquatic vegetation in slightly deeper water.  It feeds on planktonic and benthic invertebrates.  In some ponds, the relatively clear main body of the lake will have bluespotted sunfish, while the darker backwaters will have banded sunfish.  The third species of Enneacanthus, the blackbanded sunfish (E. chaetodon), is somewhat intermediate in geographic distribution and habitat.  It is found in both blackwaters and clearwaters, and is more likely to be found swimming in open water than either the banded or bluespotted sunfish.

The banded and bluespotted sunfishes are morphologically more similar early in development, as larvae, than they are as adults.  This is Ernst von Baer’s Law of Progressive Deviation.  One might logically expect their niches to diverge as well.  Surprisingly, however, larval banded and bluespotted sunfish are more dissimilar in their diet than are the adults.  Larval bluespotted sunfish feed on open water invertberates, such as the cladoceran Bosmina longispina, while larval banded sunfish feed on attached invertebrates, such as the cladoceran Sida crystallina (Graham 1985).

Morphological measurements on Enneacanthus

Stomach contents of Enneacanthus obesus
Aquatic wasp and copepod

It is often claimed that species diversity of fishes is lower in acidic blackwaters.  But the effect of pH on species diversity of native fishes in New Jersey lakes and ponds is statistically insignificant (Graham 1995).  In contrast, pH has a highly significant effect on the species diversity of exotic species, such as bluegill and largemouth bass.  Exotic species have been unable to invade acidic blackwaters.

References

Graham, J. H. 1978. Factors affecting the distribution of sunfishes (Centrarchidae) in Southern New Jersey.  M.Sc. Thesis, Rutgers University, Camden, NJ.

Graham, J. H. 1986. Niche ontogeny and progressive deviation in two congeneric sunfishes, Enneacanthus obesus and E. gloriosus (Centrarchidae). Ph.D. Dissertation, Rutgers University, New Brunswick, NJ.

Graham, J. H. 1989. Foraging by sunfishes in a bog lake. Pages 517-527 In R. R. Sharitz and J. W. Gibbons (Eds.) Freshwater wetlands and wildlife. CONF-8603101, DOE Symposium Series No. 61, USDOE Office of Scientific and Technical Information, Oak Ridge, TN.

Graham, J. H. 1993.  Species diversity of fishes in naturally acidic lakes in New Jersey. Transactions of the American Fisheries Society 122: 1043-1057.

Graham, J. H. and J. D. Felley. 1983.  Genomic coadaptation and developmental stability within introgressed populations of Enneacanthus gloriosus and E. obesus (Pisces, Centrarchidae) Evolution 39: 104-114.

Graham, J. H. and R. W. Hastings. 1984. Distributional patterns of sunfishes (Centrarchidae) on the New Jersey Coastal Plain. Environmental Biology of Fishes 10: 137-148.

Graham, J. H. and R. C. Vrijenhoek. 1988.  Detrended correspondence analysis of dietary data. Transactions of the American Fisheries Society 117: 29-36.

Last Updated 6 August 2015