Occurrence of Planktonic Lancelets from Louisiana's Continental Shelf, with a Review of Pelagic Branchiostoma (Order Amphioxi)

Three forms of lancelets of the genus Branchiostoma (Family Branchiostomatidae) were collected on the continental shelf of western Louisiana. They were identified as larval and adult Branchiostoma longirostrum and adult Branchiostoma species. These findings represent the first collection of pelagic adults and larvae of B. longirostrum, the first large-scale collection of pelagic adults of the genus Branchiostoma in the Western Atlantic, and the first records of sexually immature adult specimens of a Branchiostoma species which is not unequivocally a described species. A total of 453 larval and metamorphosed (but sexually immature) adults were collected on five cruises during December 1981 to April 1982. The 24 larval specimens collected ranged from 6.2–8.0 mm TL. These larvae were “giant” in that they exceeded 5 mm TL. Neighboring B. floridae undergo metamorphosis at about 2–5 mm TL. Collectively lancelet density peaked in January with a mean density of 4.6 specimens/100 m³. Adult specimens were collected only at night while the larvae were taken equally during night or day. The Branchiostoma adults appeared to undergo diel vertical migration. We review literature and discuss research results on pelagic lancelets, a protracted spawning season, a prolonged larval stage, and diel vertical migration relative to various morphometric and ecological factors, and aspects of life history.