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Copepoda, Calanoida) in Relation to Their Trophic Habits

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Copepoda, Calanoida) in Relation to Their Trophic Habits INTEGUMENTAL STRUCTURES IN THE ORAL FIELD OF EURYTEMORA AFFINIS AND ACARTIA TONSA (COPEPODA, CALANOIDA) IN RELATION TO THEIR TROPHIC HABITS BY M. S. HOFFMEYER1) and M. PRADO FIGUEROA2) 1) Instituto Argentino de Oceanografia, CONICET/ Universidad Nacional Sur, Av. Alem 53, 8000 Bahía Blanca, Argentina 2) Instituto de Investigaciones Bioquimicas, CONICET/ Universidad Nacional Sur, C.C. 857, 8000 Bahía Blanca, Argentina ABSTRACT Scanning electron microscopy (SEM) was used to study the morphology of the integumental structures in the oral field of the copepods Eurytemora affinis and Acartia tonsa. Structures observed in these two copepods were different in type, number, and location. E. affinis presented only structures of a filiform type, distributed all over the oral field, whereas in A. tonsa three different types of structures were observed: filiform, papilliform, and conical, distributed according to a complex pattern. The results obtained on the chaetotaxy of the oral field demonstrate a strong morphological adaptation of each copepod to its trophic habits. The ecological implications of these results on the coexistence of populations of both species in the Bahía Blanca estuary are discussed. RESUMEN Se estudió la morfología de las estructuras tegumentarias del campo oral de los copepodos Eurytemora affinis y Acartia tonsa utilizando Microscopía Electrónica de Barrido. Las estructuras observadas en ambos copepodos difirieron en tipo, número y localización. E. affinis presentó un patrón morfológico simple constituído por estructuras de tipo filiforme distribuídas en todo el campo oral, mientras en A. tonsa se observó un patrón morfológico complejo con tres tipos de estructuras: filiforme, papiliforme y cónico distribuídas en áreas determinadas del campo oral. Los resultados obtenidos sobre la quetotaxia del campo oral, demuestran una fuerte adaptación morfológica en cada uno de los copepodos relacionada con sus hábitos tróficos. Se discute la importancia ecológica de estos resultados, con respecto al fenómeno de coexistencia de las poblaciones de ambas especies, en el estuario de Bahía Blanca. INTRODUCTION . Eurytemora affinis (Poppe, 1880) and Acartia tonsa Dana, 1849 are planktonic, filter-feeding copepods which are found in coastal and estuarine areas on the 258 northern and southern hemispheres and which coexist in the plankton during a variable period of the year (Jeffries, 1962; Heinle, 1966; Hoffmeyer, 1983, 1994; Castel & Fuertet, 1989; Wellershaus & Soltanpour-Gargari, 1991). At present, E. affinis and A. tonsa are both numerically and in terms of biomass the most important copepods in the zooplankton of the Bahia Blanca estuary and they are coexisting in the period June to October (Hoffmeyer, 1994). Both copepods are omnivorous and the morphology of their cephalic ap- pendages has been described (e.g., Anraku & Omori, 1963; Itoh, 1970; Schnack, 1982; Hoffmeyer, 1987). On the contrary, the oral field of the Copepoda is an area that has been scarcely studied, although it is a valuable source of mor- phological information on, i.a., species feeding habits. To date, we only know reports on the oral fields of E. affinis and A. tonsa, viz., by Friedman (1982) and of Euchirella messinensis (Claus, 1863) by Von Vaupel Klein (1982a, 1982b) and Koomen (1991). This study was performed in order to obtain comparative morphological data on the integumental structures in the oral field of Eurytemora affinis and Acartia tonsa, hoping to find new elements that might give clues to help explain the coexistence of these species in the Bahia Blanca estuary. MATERIAL AND METHODS Zooplankton samples used in this study were collected from Ingeniero White and Cuatreros ports at the innermost zone of the Bahia Blanca estuary, using a 0.30 m mouth conical net, of 200 pm mesh pore, and operated in vertical hauls. Adult females of both species were isolated from the samples and fixed in 2% glutaraldehyde in filtered sea water for 1 h at 4°C, following a modification of Bresciani's (1988) technique. They were cut into two halves and the cephalic appendages, either from both sides of the prosome or from one side only, were removed. Then, these portions were placed in ad hoc cages made from Beem capsules and 30 ttm net mesh at both ends (Hayunga, 1977), washed with dis- tillated water, dehydrated in a graded acetonic series and incubated for 1 h with Freon 113 diluted 1 : 3, 2 : 3 and 3 : 1 in acetone (15 min in each solution), in order to dehydrate the samples in the critical point chamber (Polaron, England). The samples were subsequently mounted on stubs with double-sided adhesive tape and coated with gold (200 A) in a sputter coater model 3 (Pelco, Ted Pella Inc., U.S.A.). They were finally observed with a scanning electron microscope Jeol 35 CF at 10-25 KV. Micrographs were taken using Kodak V P 120 electron image film with a magnification range of 300 x to 5000 x . .
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