Filter Feeding in Lancelets (Amphioxus), Branchiostoma lanceolatum

Abstract

Little work has been done to describe lancelets as filter feeders, and hitherto no data have existed on either filtration rates or the efficiency with which food particles are retained in the filters. The aim of the present work was to describe and characterize Branchiostoma lanceolatum as a true filter-feeding species. The clearance-rate capacity in B. lanceolatum was measured at naturally low algal concentrations, and used as a reference state representing the normal filtration rate in nat ure and for comparisons with video recordings of the feeding currents. Simultaneous measurements of clearance different-sized particles, prepared as a mixture of flagellates (Isochrysis, Rhinomonas, and Tetraselmis) showed that particles ?4 pLm are retained by the mucus filter with 100% efficiency. The wavelength and speed of the metachronal wave of the lateral cilia, which generates the power of the branchial pump, were measured to estimate the ciliary beat frequency. Furthermore, transport rate, production, exposure time, and flow speed of water through the mucus filter was estimated. The loss of body dry-weight during a long term starvation period was used as an indirect measure of the respiration rate. The results are used to characterize filter feeding in B. lanceolatum in a way that may be compared to a number of other macro-invertebrate filter feeders that have been studied during recent years. Additional key words: water pumping, clearance, video recordings, retention efficiency, pump design Filter feeding lancelets may superficially resemble small primitive fishes with their dorsal fin and gill slits. They constitute a small phylum (or subphylum) (Cephalochordata), comprising only -25 marine species (Poss & Boschung 1994; Nielsen 1995). One of the most common of these transitional species, which share characteristics with both vertebrates and invertebrates, is Branchiostoma lanceolatum, generally known as amphioxus. This animal has been the focus of evolutionary studies for more than a century, and the bibliography of lancelets includes some 2700 references (Gans 1996). The cephalochordates and vertebrates are monophyletic taxa of the Chordata, but the two separate phyla should be regarded as sister groups (Nielsen 1995). Since the early contributions of Orton (1913) and Weel (1937), a number of papers have described lancelets as filter feeders (Barrington 1958; Olsson 1963; Welsch 1975; Baskin & Detmers 1976; Rahr 1982) but no data exists on either filtration rates or the efficiency with which food particles are retained in the filters (for a review of the early literature, see J0rgensen 1966). a Corresponding author. E-mail: hur@biology.ou.dk The aim of the present work is to further describe and characterize members of B. lanceolatum as true filter feeders which may be compared to the many macroinvertebrate filter feeders. A great variation in the design of biological filter-pumps has evolved to solve the same basic problem encountered by these animals, namely the necessity of extracting sufficiently large amounts of microscopic food particles from highly dilute suspensions (Riisgfard & Larsen 1995). But this has also led to a great number of similarities between widely different species of suspension feeders. The present work aims to accentuate such recognition. As obligate filter feeder, individuals of B. lanceolatum obtain food by straining nutritive particles, mainly phytoplankton, from the surrounding water. When feeding, the lancelet lies completely or partly buried in the bottom gravel with the ventral side turned upward and with the mouth opening free of the bottom (Fig. 1A,B). A feeding current enters at the anterior end of the animal between the buccal tentacles, which act as a coarse sieve, and flows through the vestibule (buccal cavity). The water then flows successively through the mouth, surrounded by a circular velum and attached velar tentacles, the branchial basket (pharThis content downloaded from 157.55.39.137 on Fri, 27 May 2016 05:34:38 UTC All use subject to http://about.jstor.org/terms