EMBRYONIC DEVELOPMENT TIME OF Penilia avirostris DANA , 1852 IN A TROPICAL BAY IN BRAZIL

Development time of embryos in the brood pouch of the cladoceran Penilia avirostris Dana, 1852, was estimated by collecting zooplankton daily for 15 days in surface water of Guanabara Bay, Brazil. Each day the maturity stage of embryos of 90 parthenogenic females was noted. Total development time (egg to birth) varied from 2 to 3 days, the immature phase (stages I to IV) being generally longer (2 days) than intermediate and mature phases (1 day, stages V to XII). Similar results were obtained from Bottrell’s equation, which takes water temperature into account.


INTRODUCTION
In spite of its high density and important role in trophodynamics, the marine cladoceran Penilia avirostris has been little studied compared to the large number of works on other planktonic crustaceans.This species is cosmopolitan, typically coastal (Onbé, 1974), and may be an important component of the "microbial loop" between bacterioplankton and higher consumers because of its predation on bacterivorous microflagellates (Turner et al., 1988).
Available information on marine cladoceran dynamics mainly stems from observations in temperate waters (Onbé & Ikeda, 1995;Tang et al., 1995).But this study was carried out in Guanabara Bay (Brazil), a tropical and highly eutrophic bay (Marazzo & Valentin, 2001), in which recent studies of cladoceran distribution have shown that P. avirostris is one of the most abundant mesozooplanktonic components.However, in spite of favorable conditions for spectacular growth (high water temperature and food availability), maximum densities reached by the local P. avirostris population are much lower than maxima reported from other coastal waters (50.10 3 ind.m - in Jamaica, Grahame, 1976; 150.10 3 ind.m - in Japan, Onbé, 1974).Different factors may contribute separately or together to the low density of the P. avirostris population in Guanabara Bay.High densities of animals are attained as a result of high embryonic growth rates combined with parthenogenic reproduction.Della Croce & Bettanin (1965) arbitrarily divided the developmental process of embryos from parthenogenic eggs in the brood chamber into 12 stages, differentiated by changes in shape and progressive appearance of appendages.This division constitutes a useful guide for determining developmental stages in physiological and ecological studies.
This study aimed to estimate the development time of embryos in the brood chamber of parthenogenic females of P. avirostris in Guanabara Bay, as an important parameter for studying cladoceran population dynamics in tropical waters.
Zooplankton samples were taken daily from surface waters over a 15-day period (18 April through 2 May 2001).The sampling station (30 m depth) was located in a narrow part of the bay (Fig. 1).Zooplankton samples were taken in triplicate with a conical net (200 µ µ µ µ µm mesh, 0.5 m diameter, 2 m long) provided with a flowmeter.To prevent egg loss, samples were preserved in a chilled sugar (4%) formaldehyde solution.In the laboratory, young and adult individuals were counted, and 30 females were randomly taken from each sample (a total of 90 females on each day of collection) and cleared in glycerin droplets for 24 hours.Next, the maturity stage of embryos in the brood chamber was noted and classified as "premature" (stages I to IV), "intermediate" (stages V-VIII), and "mature" (stages IX-XII).The percentage of females at each maturity stage was calculated.
During the 15-day study period, temperature and salinity of the surface water were relatively stable, varying respectively from 24.7 to 26.0 o C and from 31.5 to 34.2.Densities of Penilia avirostris mostly ranged between 200 and 400 ind.m -3 , except on three occasions when there were peaks of 1928 ± 248 ind.m -3 (18 April), 1019 ± 180 ind.m -3 (23 April), and 638 ± 118 ind.m -3 (27 April).The percentage of young individuals in the total population varied from 22 to 45, and the density of gamogenic individuals was very low (< 2 ind.m -3 ).Table 1 shows the changes in maturity stage of embryos of the population.There was a time-lag of approximately 2 days between peaks of females bearing embryos in the premature (stages I-IV) and intermediate stages (V-VIII), and just 1 day between peaks of females bearing embryos in the intermediate and mature stages (IX-XII).

Guanabara Bay
Daily collections of Penilia avirostris in Guanabara Bay allowed us to visualize the successive maturity stages of embryos during a 15-day period.Under conditions of high temperature (24-25 o C) and typical coastal salinity (32-34), total development time (egg to birth) varied from 2 to 3 days, depending on duration of the premature phase, which could vary from 1 to 2 days.The development time of embryos of P. avirostris in Guanabara Bay is similar to that observed in results of Paffenhöfer & Orcutt (1986), who also recorded a 3-day period for complete embryo development of this species at a 21 o C temperature.There is a consensus in the literature that temperature is the most important factor governing embryo development time in the brood pouch of parthenogenic females of cladocerans.In some cases, late development stages may also be greatly prolonged because of stress or injury (Egloff et al., 1997).Bottrell (1975) related data for embryo development time (D) of Evadne normanni to water temperature (T) by a second-degree logarithmic polynomial equation: log D = 0.847 (log T) 2 -3.609 (log T) + 3.796 When we applied this equation to our data using the minimum (24.7 o C) and maximum (26.0 o C) temperatures observed during the study period in Guanabara Bay, the estimated development times were 2.66 days and 2.43 days respectively.These periods conform to the development times estimated from the field data.

Fig. 1 -
Fig. 1 -Location of the sampling station in Guanabara Bay.