Reproductive Response of Calan Us Helgolandicus. 11
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MARINE ECOLOGY PROGRESS SERIES Vol. 129: 97-105,1995 Published December 14 Mar Ecol Prog Ser Reproductive response of Calan us helgolandicus. 11. In situ inhibition of embryonic development ' Station Biologique de Roscoff. CNRS et Universite Paris VI, F-28682 Roscoff, France Stazione Zoologica Anton Dohrn, Villa Comunale, 1-80121 Naples, Italy ABSTRACT: Seasonal variations in the viability of Calanus helgolandicus eggs are reported in coastal waters off Roscoff, in the western English Channel. In situ hatching success was highly variable with 230% amplitude between minimum and maximum values. Highest production of abnormal embryos and nauplii was recorded during spl-ing and midsummer. Examination of the faecal pellet contents pro- duced by wild females revealed that they were feedlng on extremely diversified prey, depending on the time of year. During spring-summer phytoplankton blooms, diatoms were always the dominant food, the remains of which were found in the faeces. During the rest of the year, non-diatom diets constituted the most important fraction of the prey. Inhibition of embryonic development was attributed to the ingestion of diatoms by wild females, based on laboratory experiments demonstrating similar low hatching success and anomal~eswhen females were fed dense diatom cultures. The failure to statisti- cally establish a straightforward correlation between the concentration of chlorophyll a and variations of hatching success In the field is discussed cvlth reference to experimental data KEY WORDS: Calanus helgoland~cus Diatom. lnhib~t~on. Egg. Viability INTRODUCTION Egg quality is linked to the recent past feeding his- tory of females in their natural environment. In fact, Until now, the problem of poor egg quality in copepods the lag time necessary for the conversion of food to and the causes inducing low hatching success have not eggs in several species is minimal (24 to 92 h; Tester been closely investigated in the field. In fact, naturally & Turner 1990) when compared to the total life of occurring, inviable eggs were usually considered as the females (>2 mo for Calanus helgolandicus) and dead, unfertilized or dormant (Marshal1 & Orr 1952, their reproductive activity (230 d in C. helgolandicus; Parrish & Wilson 1978, Watras & Haney 1980). Ianora Paffenhofer 1975). et al. (1992) showed that remating was probably not In the laboratory, diatoms have been shown to have the only factor controlling egg viability in copepods. a detrimental effect on egg viability (Ianora & Poulet Good egg quality, leading to vigorous nauplii, deter- 1993, Poulet et al. 1994, 1995b, Ianora et al. 1995a, mines variability in the recruitment rate of natural Miralto et al. 1995) either when algae were fed to populations (Kjsrsvik et al. 1990, Pepin & Myers 1991, adults at saturation levels, or when freshly spawned Ianora & Poulet 1993) and enhances the ability of eggs were exposed to diatom extracts. the first developmental stages to survive. Quantifying In the field, Ianora & Poulet (1993) reported that in viable eggs, which represent the net gonadic growth wild Temora stylifera a succession of low and high egg of adult females, is important since it determines the viability coincided with periods of high and low diatom fate of secondary production (Poulet et al. 1995a). It biomass, respectively. Anomalies in naturally occur- also indirectly influences fish recruitment, since the ring Centropages typicus eggs were reported earlier, first naupliar stages of copepods are important prey of although their causes were unknown (Ianora et al. fish larvae (Cushing 1975, Turner 1984a). 1992). In both cases, correlations between in situ chlorophyll a (chl a) concentration and hatching suc- cess were not significant, implying that a direct link between this food characteristic and variations in egg O Inter-Research 1995 Resale of full article not permitted Mar Ecol Prog Ser 129: 97-105, 1995 viability does not exist, or is not easy to establish in scale of abundance (Urban et al. 1992). When a group the field. represented >60% of the total pellet content, it was The objective of the present study was to verify if the recorded as 'abundant', whereas those comprised of major diatom blooms occurring in coastal waters off between 20 and 60% were recorded 'common', and Roscoff were being fed upon by Calanus helgolandicus 'rare' when <20%. The pellet content was estimated females and could subsequently induce either a sig- as follows: each image viewed on the SEM monitor nificant decrease in hatching success of eggs, or ab- screen corresponding to a fraction of an entire pellet normal development in embryos and nauplii. at the selected scale (i.e. x3500) was examined, and the proportion of each group of preys was quantified as equal to their relative surface on each frame. In each MATERIALS AND METHODS sample, 3 to 10 faecal pellets were screened. Estimates of the relative abundance of each group of food were Zooplankton was collected weekly at a fixed station made in triplicate on each sample. Black and white in coastal waters off Roscoff (western English Channel) photographs (Kodak polaroid 200 ASA film) were during 1993, by gently towing a 500 pm mesh net taken of at least 2 to 5 different frames observed on obliquely from 10 m to the surface. Samples were kept the SEM screen, representative of each faecal pellet in an insulated box until they reached the laboratory, sample, depending on the diversity of food in pellets. usually 1 or 2 h after collection. Laboratory feedingkpawning experiments consisted Following a standardized 'egg incubation method' of monitoring for 1 to 2 wk the daily hatching success described by Laabir et al. (1995),5 ripe Calanus helgo- of eggs spawned by 10 females placed in crystallizing landicus females were placed in each of 6 incubators dishes containing 200 m1 of 0.22 pm filtered sea water containing 1 1 unfiltered sea water and equipped with enriched with 50 m1 of dense phytoplankton cultures. a 300 pm sieve partition to prevent cannibalism; eggs In 2 parallel series of experiments, 3 replicate batches were collected at the end of a 24 h incubation period. of 10 females each were fed the diatom Phaeodactylum Hatching success was determined by incubating tricornutum given in excess concentrations, ranging freshly spawned eggs in crystallizing dishes contain- from 15 to 30 X 104cells ml-' in one series and from 3 to ing 2 to 4 m1 of 0.22 pm filtered sea water for a period 6 X 105cells ml-' in the other series. In another experi- <72 h, depending on the temperature. Laboratory ment, triplicate batches of 10 females each were first experiments were run at 11 to 16 + 0.5"C and on a fed P tricornutum (4 to 7 X 105 cells ml-l) for 7 d and natural Light cycle, corresponding to those occurring then the dinoflagellate Prorocentrum minimum (0.7 to in situ at the time of capture. Abnormal embryonic 1.6 X 104cells ml-l) for another 11 d. Food was resus- development was observed microscopically. Details are reported by Poulet et al. (1995b). Samples of chl a (used as a food index) were col- lected weekly on GFF filters in duplicates and analyzed using a 'Turner design' fluorimeter and Lorenzen's (1966) equations. These samples were collected at the same time and site as zooplankton, at the 5 m depth. At the end of the incubations, faecal pellets (>20 to 50 per sample) were pipetted and placed on a 0.2 pm Nuclepore filter (13 mm in diameter). Duplicate sam- ples of faecal pellets were stored at -30°C prior to scanning electron microscope (SEM) analysis. Before SEM, the pellets were gently squashed and put through an ethanol dehydration series (50, 70, 85, 95, and loo%), critical-point dried, and coated with gold (Turner 1984b). Faecal pellet contents were examined with a Jeol JSM-5200 SEM with the upper detector set at 20 kV. To simplify the description of the faecal pellet con- Fig. 1. Calanus helgolandicus. Variations of the mean monthly tents, only 3 major groups of particles were monitored, chl a concentration at 5 m depth in the western English Chan- including diatoms, non-diatoms (dinoflagellates, cocco- nel off the coast of Roscoff, France, in 1993. Each mean corre- sponds with 4 to 5 weekly observations. (A) Mean monthly lithophonds, silicoflagellates and crustacean remains), values of the hatching success of eggs, show~ngthe mismatch and unidentified debris and bacteria, on a relative with chl a concentration Laabir et al.. Reproductive response of Calanus helgolandicus. 11. Inhibition of development pended by stirring 5 times d-l. Every 24 h, copepods were removed and placed in new incubators with fresh media. Eggs were transferred to dishes containing 2 to 4 m1 filtered sea water, and incubated for 172 h to estimate hatching rate (Laabir et al. 1995). l? tricornutum was grown in f/2 medium and l? minimum in K medium, both at 17°C and on a 12 L (200 pE m-2):12 D cycle, and pro- vided to copepods in the exponential phase of growth. 20 RESULTS FMAMJJASON 1993 Seasonal variations of chlorophyll a and egg viability Fig. 2. Calanus helgolandicus. Weekly estimates of hatching success of C. helgolandicus eggs, observed in the coastal waters off Roscoff. M: annual mean Chl a showed strong seasonal fluctuations with maximum mean abundance (1.5to 2 1-19 I-') in late spring. Mean concentrations remained high ilar anomalies were observed all year round, but were until August, then decreased sharply in autumn to less frequent during fall and winter, compared to reach minimum values (<0.5 pg I-') in winter (Fig. 1). spring and summer, when diatom remains were much Chl a was measured in samples collected at 5 m depth, more abundant in faecal pellets which was representative of the concentration in the water column (i.e.