Notes on feeding of Chaetognatha in Guanabara Bay, Brazil

Journal of Plankton Research Vol.19 no.7 pp.819-828, 1997 Notes on feeding of Chaetognatha in Guanabara Bay, Brazil A.Marazzo, C.F.Machado and C.S.R.Nogueira Laboratdrio de Zoopl&ncton, Departamento de Biologia Marinha, Instituto de Biologia da Universidade Federal do Rio de Janeiro, Av. Brigadeiro Trompowsky, s/n, Cidade Universitdria, Ilha do Fundao, CCS, Bloco A, Rio de Janeiro, RJ, Brasil CEP: 21949-900 Abstract A preliminary analysis, by means of the gut content, of the diet of Sagitta friderid and Sagitta enflata occurring in Guanabara Bay was carried out. The samples were collected at different times over a 4 day period in September 1995, during vertical tows with a conical net of 200 um mesh size at a fixed station (30 m depth). In total, 1000 individuals were examined. Copepoda were the numerically dominant group of prey of S.friderici and S.enflata (67.2 and 74.6%, respectively); both species behave as opportunistic carnivores, feeding mainly on the more abundant copepod species throughout the water column. Adult individuals of S.friderici and S.enflata (stages III and IV) seem to have food requirements different from juveniles: adults fed on other items (chaetognaths, crustacean larvae, hydromedusae and luciferidae), and the food containing ratio (FCR) was higher in individuals of stage III. In general, only one prey per gut was registered. Chaetognaths may have chosen their prey in relation to their size, since the size of the prey was proportional to the size of their predators. Feeding intensity (NPC) was higher in individuals collected during the night periods. Introduction In the plankton, the chaetognaths generally rank second in numerical abundance, with the copepods in first place. All species are marine or estuarine, and are considered great predators of the pelagic community. Their diet consists basically of copepods; these organisms are probably one of the main sources of predation pressure in the copepod community, sometimes having a considerable influence on the structure of the lower trophic levels (Pearre, 1980). On the other hand, the food item and the size of the prey may change in accordance with the maturity stage of the chaetognath (Reeve and Walter, 1972; Pearre, 1980; Feigenbaum and Mans, 1984). Although they have been considered strictly carnivorous, several authors have made observations about the presence of phytoplankton in the gut of these organisms and related it with active alimentation, and not with involuntary ingestion (Alvariflo, 1965; Boltovskoy, 1981). The chaetognaths are frequently described as an important link food between the high number of copepods existing in the plankton and the larger predators, including many species of fish of commercial importance (Reeve, 1970; Nagasawa and Marumo, 1981). Thus, these organisms may serve as good indicators of potentially important fishery areas (Boltovskoy, 1981). Data on the feeding habits of chaetognaths, through analysis of the gut content of individuals collected in the field, have revealed important information about the diet of these organisms (e.g. Pearre, 1974; Feigenbaum, 1979; 0resland, 1990). The gut content of chaetognaths may be examined in a relatively simple way, since these organisms, besides being transparent, swallow their prey whole (Pearre, 1980). O Oxford University Press 819 AJMarazzo, CRMachado and GSJLNogneini The objective of this study is to carry out a preliminary analysis of the diet of the more abundant species of chaetognaths occurring in Guanabara Bay, since no record of their feeding habits in this environment has been made in the past. Method The 13 samples of zooplankton studied here are part of the Trofo Project being developed by the Marine Biology Department of Rio de Janeiro Federal University. The samples were obtained at a station (30 m depth) in Guanabara Bay, State of Rio de Janeiro, Brazil (23°41'-23°56'S and 43°02'^3°18'W) (Figure 1). The collections were always made at different times between 12 and 15 September 1995 (Table I). Vertical tows were made using a conical net (0.6 m diameter and 2 m long) of 200 um mesh size. The collected material was preserved according to Griffiths et al. (1976). In the laboratory, each sample was subsampled using a Folsom plankton splitter (McEwen et al., 1954), due to the high zooplankton density. The subsamples were never less than an eighth of the sample. The copepods were identified following descriptions given by Bj6rnberg (1981). The chaetognaths, totalling exactly 1000 individuals, were removed from the subsamples (Table I), and identified to species according to the descriptions given by Alvarifio (1969), Moreno (1973) and Boltovskoy (1981). SOUTH AMERICA 15' Iff 43° W .1I" GDAMAfiAKA BAY g. - 45' - 35* Skm 23* S Fig. L Location of the sampling station in Guanabara Bay. 820 Diet of Chaetognatha in Gnanabara Bay Table L Total number of chaetognaths examined, number containing food and the food containing ratio (FCR) at different hours of the day Date 1995 September Tune Number examined Number with food FCR 12 12 13 13 13 14 14 14 14 15 15 15 15 Total 12.45 1720 09.00 12.00 17.00 00.20 05.10 12.40 17.40 01.30 06.00 14.00 18.00 38 71 175 57 42 148 62 57 24 122 62 78 64 1000 4 10 38 11 19 47 19 18 6 42 16 20 13 263 10.53 14.08 21.71 1930 4524 31.76 30.65 31.58 25.00 34.43 25.81 25.64 2031 2630 Table IL Total number of S.friderici and S.enflata examined, number containing food and food containing ratio (FCR) at different maturity stages Maturity stages S.friderici 0 I n m rv Total S.enflata n u I n in rv Total Number examined Number with food FCR 228 209 234 21 692 — 53 37 67 6 163 23.25 17.70 28.63 28.57 23.55 255 15 19 4 293 81 5 11 1 98 31.76 3333 57.89 25.00 33.45 Chaetognath specimens suspected of containing material in the gut were transferred to separate receptacles. Each individual was measured with the help of a segmented plate under a stereomicroscope, and classified at intervals of 1 mm. Body length was measured from the anterior tip of the head to the end of the tail, excluding the tailfin.Maturity stages were classified according to Reeve (1970). The food items were identified to species level where possible, independent of position in the gut. The food material was confirmed only after dissection of the gut with sharp needles, and observed under an optical microscope. The prey were measured only when shown in sufficient detail, and classified at intervals of 0.1 mm. The percentage of the population at each maturity stage containing food in the gut—the food containing ratio (FCR = number of chaetognaths containing food/total number of chaetognaths x 100)—was calculated, as well as the number 821 AJVfirazzo, CEMacbado and CSJUSognein Table in. Composition of the gut contents of S.friderid and S.enflata by percentage, in comparison with the composition of zooplankton in the water column during the study period Food item Water column Gut (...truncated)