Developmental defects in damselfish (Abudefduf sordidus: Pomacentridae) embryos from metal artificial reefs.
Like all pomacentrids, males of A. sordidus establish a nest site by cleaning an area of hard substrate, often biting off encrusting material (2). The males attract females to spawn in the nests and then guard the embryos until they hatch C5 days). A. sordidus nests were observed in the field at Johnston Atoll, Central Pacific. Photographs of nests on metal substrates were used to determine the area covered by corrosion that contained dead (opaque) embryos. Corroded or oxidized areas of the metal were easily distinguished by a bright orange precipitate (iron oxide) and by varying degrees of unevenness and deterioration. Non-corroded metal was intact, often covered with microturf algae or fine particles of sediment and organic matter. Samples of embryos were collected directly from areas with corrosion. Due to the patchy nature of the corrosion and the large area of the nests, samples that were collected from non-corroding metal were taken from an area of a nest without corrosion or from a nest on metal that did not contain corroding areas. About 200 embryos, constituting a sample, were removed and examined under a dissecting microscope at 40x magnification. Samples of embryos from corroding metal and from concrete were screened for iron content by atomic absorption spectroscopy.
In a survey of 88 A. sordidus nests, 40% were on metal, 43% were on a natural substrate (coral and coral rock), and 17% were on some other substrate (wood, plastic, or concrete). No nests were observed on a natural substrate in areas where artificial substrates were available, suggesting that these fish prefer artificial substrates. Dead and abnormal embryos on metal occurred chiefly in actively corroding areas; they were bright orange in color, and were coated with what appeared to be a layer of iron oxide (Fig. 1). In a preliminary analysis of the metal content of the embryos, a sample from corroding metal contained 200 ppm iron, whereas a sample from concrete contained 0.78 ppm iron. The combined incidences of mortalities and abnormalities in nests on corroding metal (mean = 22.7, SD = 14.1), non-corroding metal (mean = 3.5, SD = 1.6), and coral rock (mean = 3.6, SD = 2.9) were significantly different (Kruskal-Wallis P < 0.001)(n = 30). The list of abnormalities found among early embryos includes enlarged segmentation cavity under the blastodisc, developmental retardation or arrest, and abnormal cell proliferations during gastrulation (Fig. 1B & C). These abnormalities often led to death, which generally occurred before the tail bud stage (24 h). Measurements from photographs of five nests showed that between 0.7 and 15.5% of the nest area was corroding. This corresponds to 3,000 to 65,000 affected embryos in a typical nest of 30 X 60 cm containing 418,000 embryos.
These lethal abnormalities in A. sordidus embryos were correlated with nesting on a corroding metal substrate and could be due to metal toxicity or oxygen deprivation. While several metals have been shown to be toxic to fish embryos (3), and most metal ions readily pass through the chorion (4), the exact composition of the metal substrate is not known at this time. Some metals may also affect embryogenesis indirectly; this includes cadmium, which exerts its toxic effects by altering egg membrane properties (5). These changes in the membrane cause decreased oxygen exchange, which has been shown to cause developmental arrest, retardation, and abnormalities in fish embryos (5). Although there is no evidence at this time for cadmium in the metal substrate, other metals present might be acting similarly. Iron hydroxide particles were found to lodge within the pores of the chorion in fathead minnow embryos, reducing oxygen exchange and overall hatchability (6). Iron oxide particles may also clog the chorion in damselfish embryos, or the corrosion process itself may locally reduce oxygen concentration. Iron concentration was highly associated with one embryo sample in this study, although it is not known whether the iron was on the outside or inside of the embryo. Developmental defects due to parental contamination or other genetic causes are not likely since the abnormalities and resulting mortalities would be distributed randomly throughout the nest and not concentrated in one area. Fungal infections were also not observed. Although increased abnormalities and mortalities were observed in damselfish embryos associated with corroding metal, the specific causes and associated mechanisms will be revealed by future research.
Some damselfishes prefer artificial substrates, and their use often increases reproductive success (7,8). In this case, the preferential use of metal substrates by A. sordidus reduces the number of viable larvae at hatching, which may ultimately affect its overall reproductive success.
Research supported by grants to P. S. Lobel from the U. S. Army Chemical Demilitarization Program, the Office of Naval Research N00014-91-J1591 and N00014-92-J1969, and the U. S. Army Legacy Resource Management Program, DAMD 17-93-J-3052.
[1.] Unger,l.1996. Artificial Reefs: A Review, E.C. Bolster, ed. Spec. Pubi. 4. American Littoral Society, New Jersey. 74 pp.
[2.] Stanton, F. G. 1985. Pp. 361-366 in Proceedings of the Fifth International Coral Reef Congress, Tahiti. Vol. 5.
[3.] Weis, J. S., and P. Weis. 1991. Pp. 145-169 in Metal Ecotoxicology: Concepts and Applications, M. C. Newmand and A. W. Mclntosh, eds. Lewis Publishers, Boca Raton, FL.
[4.] Rombaugh, P. J. 1985. Comp. Biochem. Physiol. 82C: 115-117.
[5.] von Westernhagen, H. 1988. Pp. 253-346 in Fish Physiology, W. S. Hoar and D. J. Randall, eds. Academic Press, New York.
[6.] Smith, E. J., J. L. Sykora, and M. A. Shapiro. 1973. J. Fish. Res. Bd. Can. 30: 1147-1153.
[7.] Itzkowitz, M. 1991. Environ. Biol. Fish. 30: 287-293.
[8.] Itzkowitz, M., D. E. Itzkowitz, and D. Shelley. 1995. Bull. Mar. Sci. 57: 653-662.
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|Author:||Kerr, Lisa M.|
|Publication:||The Biological Bulletin|
|Date:||Oct 1, 1996|
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