Scaphirhynchus Conference: Alabama, Pallid, and Shovelnose Sturgeon

St. Louis, Missouri, 11-13 January 2005

48.  PALLID AND SHOVELNOSE STURGEON LARVAE MORPHOLOGICAL DESCRIPTION AND IDENTIFICATION.

Darrel E. Snyder, Larval Fish Laboratory, Department of Fishery and Wildlife Biology, Colorado State University, 1474 Campus Delivery, Ft. Collins, CO 80523-1474; Phone 970-91-5295; Fax 970-91-5091; E-mail: desnyder@cnr.colostate.edu

Pallid sturgeon Scaphirhynchus albus and shovelnose sturgeon S. platorynchus are nearly identical as larvae, but many specimens greater than 10 mm total length can be at least tentatively distinguished using morphological criteria.  Both species hatch at 8-9 mm and by 10-11 mm have well-pigmented eyes, a forming mouth, and barbel buds.  Barbel-bud length is diagnostic for protolarvae prior to pelvic bud formation at about 12-13 mm but not for later yolk-bearing larvae.  Instead, these larvae are best distinguished by eye diameter.  Yolk is absorbed by 18-19 mm.  For protolarvae without yolk, eye diameter remains diagnostic and inner-barbel length relative to outer-barbel length again becomes diagnostic and remains so through the adult.  Ventral pigmentation over the heart and in the lower lip lobes becomes diagnostic during this phase and remains so in mesolarvae.  The first median fin rays appear in the dorsal fin between 21and 26 mm, marking transition to the mesolarval phase.  First anal-fin rays appear between 25 and 30 mm in shovelnose sturgeon but not until 30-31 mm in pallid sturgeon.  Beginning with mesolarvae, deltoid or semi-circular shape of the ventral aspect of the lateral rostral plates behind the outer barbels becomes diagnostic.  Appearance of the last caudal fin rays at 56-60 mm in shovelnose sturgeon and greater than 81 mm in pallid sturgeon completes acquisition of median-fin rays and marks transition to the metalarval phase.  Caudal-fin-ray counts are diagnostic.  Both sturgeon retain some preanal finfold, and thereby remain metalarvae, through at least 200 mm total length.