Quantification of Mnemiopsis leidyi (Ctenophora, Lobata) from formalin-preserved plankton samples

Abstract

Lobate ctenophores were thought to be impossible to quantify using standard plankton collection and preservation techniques. I show that the tentacle bulbs of the lobate species Mnemiopsis leidyi provide a direct measure of numbers and sizes of the ctenophores in formalin-preserved plankton samples. The tentacle bulbs persist intact after preservation in 5 % formaldehyde solution for at least 9 mo. Bulb lengths of Live and preserved specimens were closely correlated with ctenophore wet weights. This method is more accurate and convenient than measures of abundance previously used for M. leidyi, and it may be applicable to other species of ctenophores. Ecological studies of ctenophores, other than a few species of cydippids (i.e. Pleurobrachia spp.), have been hampered because of the difficulty in sampling them quantitatively. Standard plankton collection and preservation methods cause the destruction of lobate ctenophores in the class Tentaculata. This has led to special techniques of collection (Harbison et al. 1978) and preservation (Adams et al. 1976) that are very labor intensive. Lobate ctenophores in the genus Mnemiopsis form conspicuous population blooms during spring and summer in estuaries of the eastern United States. These ctenophores are so abundant that methods have been developed to exclude them from plankton nets and to dissolve them if collected (Heinle 1965 and Burrell & Van Engel 1970, respectively). The most common method of measuring the abundance of Mnemiopsis spp. ctenophores has been to measure their total Live volume from plankton tows (Table 1). Total volumes have been converted to approximate numbers from size measurements of the ctenophores (Table 1). Alternatively, live ctenophores have been counted and measured immediately following plankton tows (Table 1). This method is extremely tedious, and potentially inaccurate for small ctenophores. Le Blanc & Straw (1972) and Kremer & Nixon (1976) used luminescent flashes of the ctenophores to measure abundance. O Inter-Research/Printed in F. R. Germany In the present study, I show that tentacle bulb length increases directly with size (wet weight) in Mnemiopsis leidyi. Metamorphosis from the cydippid larva to the adult form in lobate ctenophores involves disappearance of the tentacle sheaths, reduction of the tentacles, and elongation of the tentacular canals and bulbs (bases) as the tentacles shift orally in position (Hyman 1940). These structures remain after preservation in 5 O/ O formaldehyde solution, and can be used to quantify the abundance and biomass of this lobate ctenophore species. Table 1. Studies in which measures of biomass, size, and abundance of A4nern1opsis spp. ctenophores were made. All data were from live specimens from net tows Volume Length Number (m11 (mm)