VARIATION IN PATTERNS OF INTERDIGITATION AMONG SUPRANEURALS, PTERYGIOPHORES, AND VERTEBRAL ELEMENTS DIAGNOSTIC FOR STRIPED BASS AND WHITE PERCH

It is frequently important to discriminate between larvae of striped b ass Morone saxatilis and white perch M. americana; however, these spec ies are not well differentiated at the postyolk-sac stage. Previous an alysis had indicated high discrimination potential (4-14% overlap) for three osteological characters, interdigitation patterns between (1) s upraneurals, pterygiophores, and interneural spaces 1 to 6 (IN 1-6); ( 2) pterygiophores and interneural spaces 10 to 13 (IN 10-13); and (3) pterygiophores and interhaemal spaces 12 to 15 (IH 12-15). However, th e earlier study was based on only 40 striped bass and 50 white perch o f a single year class from a limited number of tributaries to Chesapea ke Bay. We examined variation in pattern frequencies in these three ch aracters among an additional 463 striped bass and 517 white perch from three major estuaries where they are reproductively sympatric-the Hud son River, Delaware River, and Chesapeake Bay-and from selected allopa tric populations. Among samples from the three major estuaries, we fou nd high univariate discrimination potential for IN 10-13 (3-7% overlap ); moderate discrimination potential for IN 1-6 (14-24% overlap); and poor discrimination potential for IH 12-15 (24-69% overlap). When all three characters were used in conjunction to discriminate Hudson River samples, 88.6% of striped bass and 98.0% of white perch were positive ly identified. Much of the poorer univariate discrimination potential for IN 1-6 and IH 12-15 in comparison with the earlier study was due t o the many additional patterns we identified for these characters. Sig nificant differences (P < 0.05) in pattern frequencies within each spe cies were observed in numerous comparisons among samples from primary populations; subpopulations within Chesapeake Bay; and year classes fr om the Hudson River. Year class differences are almost certainly envir onmental in origin. We hypothesize that year class differences are the products of differential survival of subcohorts within and among year classes; subcohorts may experience different environmental conditions that influence the relationships among the timing of supraneural and pterygiophore development, total pterygiophore counts, and body morpho metry.