Studies on chaetognaths off Ubatuba region, Brazil. II. Feeding habits

Bolm Inl;1. oceano~., S Paulo, 43(1):27-40, 1C)()S Studles on chaetognaths off Ubatuba reglon, Brazll. II. 'Feedlng habita Tsui Hua LIANG & Luz Amelia VEGA-PÉREZ Instituto Oceanográfico da Universidade de São Paulo (Caixa Postal 66149, 05389-970 São Paulo, SP, Brasil) • AbstTact: The diet of chaetognath species were studied by examining the gut contents of 9466 specimens coUected off Ubatuba regioo, São Paulo State. The greatest proportion of chaetognaths (7119 individuaIs) showed their gut contents empty. Copepods, moUusc eggs, appendicularians, cladocerans and annelidswere the most common food items in tbe gut contents of juvenilesand mature stages. Cannibalism occurred in low frequency. ln Summer the copepods Temonl stylifenl and Paraca/anus spp were more abundant, whereas Oncaea spp and mollusc eggs were heavi1y preyed in Winter. There was a clear trend of increasing prey size with the developmental stage. • Resumo: O estudo dos hábitos alimentares das espécies de Chaetognatha foi reali7.ado a partir da análise do trato digestivo de 9466 indivíduos dos estágios O IV. Os quet6gnatos foram coletados ao largo da região de Ubatuba, Estado de São Paulo, com o auxilio da rede Bongo (Malha 0,200 mm e 0,303 mm), nos verões de 1985 - 1987 e invernos de 1986 e 1987. Dos 9466 tratos digestivos analisados, 7119 estavam vazios e 2347 apresentaram de 1 a 3 presas. Grande quantidade de material amorfo e semi-digerido também foram detectados. A dieta esteve constituída basicamente de copépodos (Calanoida e Poeci1ostomatoida), clad6ceros, ovos de moluscos, náuplios de crustáceos, apendiculárias e poliquetos, entre outros. O canibalismo foi observado a partir do estágio I, porém com baixa frequência. Os estágios jovens (O-I) mostraram preferência por presas de tamanho pequeno como náuplios e copépodos do gênero Oncaea, enquanto que os estágios maduros por presas maiores como Temora stylifera, Corycaeus sp e Eucalanus pileatus. • Descriptors: Feeding, Predation, Zooplankton, Chaetognatha, S. enflata, S. friderici, S. hispida, Ubatuba, Brazil, South Atlantic. • Descritores: Alimentação, Predação, Zooplâncton, Chaetognatha, S. enflata, S. friderici, S. hispida, Ubatuba, Brasi~ Atlântico Sul. Introduction The interest in the feeding of zooplanktoo has increased considerably during recent years. Behavioral responses to food species, concentration and their tropbic interactions bave been studied for many years in an attempt to elucidate fundamental patterns structuring plankton communities (Gallager, 1988). Zooplankton plays a criticai role in the ecology of marine systems by serving as a link between phytoplankton, bacterioplankton and higher trophic leveis in tbe nekton and benthos. Ao understanding of marine planktonic food webs requires informatioo on the abuodaoce, biomass and rate of ioteractioos betweeo ali trophic leveis (Buskey, 1993). Studies 00 feediog have beeo focuscd maioly 00 the herbivorous organisms like copepods wbile carnivorous Contr. no. 775 do Inst. oceanogr. da Usp. species bave been neglected. According to Gallager (1988) tbe importance of studyiog predator feeding mecbanisms is that we can predict how carnivorous zooplankton will affed community structure and fundion by indirectly measuring feeding behavior, e. g., gut content analyses or counts of available prey items. Chaetognaths are known as one of the most important and adive predators of zooplankton, preying upon a wide variety of organisms (Raymont, 1983; Alvariiío, 1985). They may affect the distributioo, density and occurence of their preys (Cheney, 1985). ln Brazilian waters, many studies on chaetognaths have examined their systematics, distribution and occurrence (Vannucci & Hosoe, 1952; Almeida-Prado, 1961a, b; 1968; Ferreira da Costa, 1970; Coelho, 1993). The knowledge of their feeding habits has received little attention, being limited to the study of the chaetognath S. friderici off Ubatuba (Vega-Pérez & Liang, 1992). Bolm Inst. oceanogr., S Paulo, 43(1), 1995 28 The aim ofthe present study is to analyse the qualitative and quantitative food composition of the chaetognath species collected off Ubatuba region. Wcll-digested food was called unidentified. Copepods were idcntified hased on Bjõmherg (Boltovskoy, 1981). Percentage numher was used as an index of the importance of eaeh prey item in the dict. Similarities of the diet were calculated using the Schoener index (1968) wich measures the overlap hetween two categories of comparison and gives values from O (no overlap) to 1 (complete overlap): Material and methods This study was part of a program of multidisciplinary study of tropical marine ecosystems: "Brazilian Coastal Ecosystems", off Ubatuba region, São Paulo State, from 1985 to 1989. Zooplankton sam pies studied io this work were collected 00 five oceanographic cruises during Oecember 1985, 1986, 1987 and July 1986, 1987. A survey grid of 30 standard planktoo statioos distributed along six traosects was established in the sea frontal to the Ubatuba regiDo (24°1O'S-44"30'W and 23"30'S-45"20'W). Oblique hauls were made with a Bongo net fitted with a fioer (0,200 mm) and a coaser meshed net (0,303 mm), and provided with flowmeters. Samples were preserved in a seawater solution of 10% buffered formalin. ln this study, only samples obtained by regular mesh Si7.e were analysed. Temperature and salinity data of the water column were obtained simultaneously with the use of Nansen bottles. lo the laboratory, plankton was subsampled with the cylindrical Motoda Splitter (Omori & Ikeda, 1984). Ali the chaetognath specimens were removed from the subsamples and identified accordingly to Almeida-Prado (1961a) and Alvarifio (1969). The maturity stages were determined based on Reeve (1970): (O-II) young stages; (III-IV) adult stages. Gut content of 9466 specimens were examined by body transparency (Pearre, 1980) and the prey items were identified to species or to the lowest taxonomic levei possible using a stereoscopic microscope (Wild M7). o D = 1 - 0,5 r~ (Pxi- Pyi)) i= 1 were Pxi is the proportioo of food category in lhe diet of species x. Pyi is lhe proportioo of food category io lhe diet of species y, and o is the oumber of food categories. Biologically, overlap is considered signi6cant wheo lhe value exceeds 0,60 (Zaret & Rand, 1971; Wallace, I981). Results The analysis of the gut contents of 9466 chaetognaths 00" Ubatuba region showed a total of 7119 specimens with empty guts and 2347 containing 1 to 3 preys (Table 1). The bulk of chaetognath specimens contained only one prey. Multiple preys (2 - 3) were found in S. enflata (129), S. hispida (24), S. friderici (16) and S. tenrtis (4). Chaetognath species preyed upon a widc variety of organisms. The food organisms most frequently found in their guts were crustaceans, mollusc eggs, appendicularians and polychaetes. ln ali cases, however, the most important prey within the global diet were the copepods, specially Calanoida and Poecilosto (...truncated)