Reproduction of the Florida Lancelet (Branchiostoma floridae): Spawning Patterns and Fluctuations in Gonad Indexes and Nutritional Reserves

Abstract

The reproductive biology of the Florida lancelet (Branchiostoma floridae) is described from over 21,000 animals collected from early summer 1991 through late summer 1995. Collections were made daily during summer and less frequently during other seasons. Minimal length at sexual maturity was about 23 mm for both sexes. The ratio of males to females was 1:1 throughout the year, and no hermaphrodites were found. During the breeding season (late spring through late summer), the population spawned just after sunset at intervals of 1-15 days, with an average interval of roughly a week. The proportion of lancelets spawning on a given date ranged from a small fraction to about 90% of the adult population. Spawning dates were not significantly correlated with lunar phase or fluctuations in salinity or water temperature. Lancelets could be induced to spawn by electric shock in the laboratory, but only on dates when they spawned in the field. A daily determination of gonad indexes suggested that individual males and females could spawn and then ripen and spawn again in the same breeding season, possibly in as little as 1-2 weeks. Fin-ray indexes (assumed to be proportional to somatic nutritional reserves) were highest in winter, when the lancelets were apparently well nourished but not breeding, and consistently low during the summer breeding season, when the animals were repeatedly spawning and presumably draining their nutritional reserves. Additional key words: amphioxus, cephalochordate The subphylum Cephalochordata of phylum Chordata comprises about 25 species of lancelets (=amphioxus) living in shallow marine or estuarine habitats. Because of their phylogenetic ties to vertebrate chordates, lancelets have been extensively studied anatomically and embryologically for over a century. In contrast, much less is known of their ecology in general and reproductive biology in particular, partly due to the relative inaccessibility of large populations of lancelets. Although most lancelet species have a wide geographic distribution, they are typically rare through much of their range and occur at high densities in only a few scattered localities (e.g., Lagos Lagoon, Nigeria; Xiamen and Qingdao, China; Kingston Harbour, Jamaica; and Tampa Bay, Florida). Much of the work on lancelet reproduction (reviewed in Wickstead 1975) concerns temperate populations of Branchiostoma lanceolatum in Europe and B. belcheri in China. The reproduction of one tropical lancelet, B. nigeriense, has also been studied extenaCurrent address: Hopkins Marine Station, Stanford University, Pacific Grove CA 93950, USA. sively, although for an estuarine population exterminated once a year by fresh water runoff and replenished after each rainy season by animals from the open sea. These studies may give a distorted picture of reproduction in cephalochordates as a whole, because most lancelets have sub-tropical and tropical distributions and maximal lifetimes of several years. The purpose of the present study is to describe the reproductive biology of Branchiostoma floridae HUBBS 1922. In contrast to lancelets previously studied in detail, B. floridae is sub-tropical and has a maximal lifetime of several years (Stokes 1996); therefore, the reproductive biology of this species may more fairly represent that of cephalochordates generally. The population studied lives in Tampa Bay, Florida, and, from a previous description of larval settlement patterns (Stokes 1996), its breeding season is known to last from late spring through late summer. Our results also help clarify the role of lancelet finrays, which lack any homology with vertebrate finrays (Azariah 1965) and comprise retroperitoneal accumulations of glycoprotein and lipid (Holland & Holland 1991 a). Here we assume that fin-ray volumes This content downloaded from 157.55.39.223 on Wed, 24 Aug 2016 05:51:42 UTC All use subject to http://about.jstor.org/terms