Daniel and Graves: Morphometry and genetic identification of sciaenid eggs 



255 



al., 1991; Saucier and Baltz, 1992; Saucier et al., 

 1992). However, the misidentifications that can result 

 from overlapping egg-diameter distributions and the 

 time-consuming nature of culture experiments make 

 methods based on other characters desirable. 



The application of biochemical genetics has pro- 

 vided an alternative to culture for positive identifi- 

 cation of morphologically similar eggs. Electrophore- 

 sis of water-soluble proteins (allozyme analysis) has 

 been used to distinguish between larvae and juve- 

 niles of morphologically similar species of marine 

 fishes (eg. Morgan, 1975; Smith and Benson, 1980; 

 Graves et al., 1988). Similarly, restriction fragment 

 length polymorphism (RFLP) analysis of mitochon- 

 drial DNA has been employed to discriminate be- 

 tween the eggs of three congeneric serranids that 

 could not be unambiguously identified with a single 

 allozyme locus (Graves et al., 1990). More recently, 

 direct sequencing and RFLP analysis of DNA am- 

 plified by the polymerase chain reaction (PCR) have 

 been used to identify morphologically similar larvae 

 of invertebrates (Olson et al., 1991; Silberman and 

 Walsh, 1992). 



In this paper we report that identification of eggs 

 of sciaenids in lower Chesapeake Bay during spring 

 based on published morphological criteria, rearing 

 experiments, and genetic analysis are inconsistent. 

 These results indicate that it is not possible to iden- 

 tify sciaenid eggs accurately by using diameter as 

 the sole criteria. In addition, we present the results 

 of weekly plots of egg size-frequency distributions 

 and a RFLP analysis of mtDNA to determine the 



specific composition of eggs of sciaenids that may be 

 present in lower Chesapeake Bay during spring. 



Material and methods 



Weekly zooplankton surveys of the lower Chesa- 

 peake Bay were conducted during April and May 

 1990 and 1991 to determine the distribution and 

 abundance of eggs of black drum for an estimate of 

 seasonal egg production. Samples of eggs were ob- 

 tained with an in situ silhouette photography sys- 

 tem consisting of paired 60-cm diameter, 335-u nets 

 fitted to a rigid frame (see Olney and Houde, 1993, 

 for a detailed gear description). All deployments 

 were 5-minute, stepped-oblique tows and yielded a 

 standard plankton sample and a replicate film record. 

 Plankton samples were preserved in 5—8% buffered 

 formalin and sciaenid eggs were identified by using the 

 criteria of Lippson and Moran (1974) and measured 

 to the nearest 0.025 mm with a Zeiss Stemi SR stere- 

 omicroscope. Ten subsamples of eggs (rc=75-100) 

 sorted from preserved plankton samples were 

 remeasured to assess measurement error. 



During several cruises in May 1990 and 1991, eggs 

 were collected in an area off the city of Cape Charles, 

 Virginia, with a 0.5-m Hansen net fitted with 202-u 

 mesh to seed 1-liter Imhoff settling cones for hatch- 

 ing experiments. Eggs were originally separated as 

 Type I (<0.80 mm) and Type II (>0.85 mm) based on 

 the morphological criteria of Joseph et al. ( 1964). Rear- 

 ing chambers were returned to the laboratory and held 

 for 3 to 14 days. In these, larvae were periodically sac- 



