The diets of the chaetognaths Sagitta enflata, S. serratodentata atlantica and S. bipunctata at different seasons in Eastern Mediterranean coastal waters

ICES Journal of Marine Science, 53: 837–846. 1996 The diets of the chaetognaths Sagitta enflata, S. serratodentata atlantica and S. bipunctata at different seasons in Eastern Mediterranean coastal waters G. Kehayias, J. Lykakis, and N. Fragopoulu Kehayias, G., Lykakis, J., and Fragopoulu, N. 1996. The diets of the chaetognaths Sagitta enflata, S. serratodentata atlantica and S. bipunctata at different seasons in Eastern Mediterranean coastal waters. – ICES Journal of Marine Science, 53: 837–846. ? 1996 International Council for the Exploration of the Sea Key words: feeding habits, diets, Sagitta enflata, Sagitta serratodentata atlantica, Sagitta bipunctata, Eastern Mediterranean. Received 26 October 1995; accepted 29 February 1996. G. Kehayias, J. Lykakis, and N. Fragopoulu: Section of Animal Biology, Department of Biology, University of Patra, 26500 Patra, Greece. Introduction Chaetognaths are dominant zooplankton predators and are generally believed to have a considerable influence on their prey populations, especially under conditions of low environmental productivity (Kimmerer, 1984; Øresland, 1990). Copepods seem to be the most important prey (Øresland, 1987). Reeve (1970) speculated that most of the energy converted to animal biomass by copepods is transferred to higher trophic levels via these predators. Chaetognaths are food for a wide variety of larger organisms, and therefore occupy a central position in the planktonic food webs (Feigenbaum and Maris, 1984). The literature concerning chaetognath feeding has been reviewed by Feigenbaum and Maris (1984) and Feigenbaum (1991). However, data on chaetognaths feeding in Mediterranean waters are available only for the three species Sagitta enflata, S. minima and S. friderici, common in the coastal waters of Spain and the Western Mediterranean (Pearre, 1974, 1976). Sagitta enflata, S. serratodentata atlantica and S. bipunctata are the three more numerically important species in Cretan Sea coastal waters and the Eastern 1054–3139/96/050837+10 $18.00/0 Mediterranean, comprising almost 90% of the total chaetognaths (Kehayias et al., 1992). In the present study, the diel diet and food selection of these three species were estimated during six different sampling periods. We consider prey species abundance in the diet as well as in the environment, and size selection between prey. Materials and methods Five sampling stations were located in the Kisamos Gulf (23)40*E, 34)35*N) (Cretan Sea, Eastern Mediterranean) (Fig. 1). Zooplankton was sampled from the upper 50 m on 25 September and 20 November 1988, and 22 February, 8 April, 23 May and 29 July 1989. During the first three sampling dates, day and night samples were collected while on the other dates only day samples were taken. On each occasion double oblique hauls were conducted with a Bongo net (500 ìm mesh size) and a WP-2 net (200 ìm mesh size) for the collection of the samples, which were preserved in 4% formaldehyde buffered solution immediately after collection. WP-2 net samples were not taken in September 1988. The samples ? 1996 International Council for the Exploration of the Sea The chaetognaths Sagitta enflata, S. serratodentata atlantica and S. bipunctata were caught in six sampling periods from September 1988 until July 1989 during both day and night in Kisamos Gulf (Cretan Sea, Eastern Mediterranean) in the upper layers (0–50 m). Gut content analysis showed that copepods were the predominant food organisms in the diet of these three predators. Positive electivity indices were found for Corycaeus spp. and Oncaea spp. but negative for Clausocalanus spp. copepodites and Clausocalanus furcatus females, which were the most numerous among copepods. Cannibalism was observed mainly in S. bipunctata while S. serratodentata atlantica was found to be less cannibalistic. The size of the predator was reflected in the size of prey. Feeding intensity indicated by Food containing ratio (FCR) and Number of prey per chaetognath (NPC) increased just before the reproductive period of each chaetognath species. Differences between day and night feeding were only found in S. enflata. 838 G. Kehayias et al. 22°30' 25°30' 24° 27° 28°30' E 23°35' 23°40' 23°45' E 41°N Kisamos Gulf 35°40' N eg A 39° 3 4 n ea a Se 2 35°35' 200 1 5 Cretan Sea 35°30' 35° 22°30' 24° 25°30' 27° 28°30' E 23°35' 23°40' 23°45' E Figure 1. The Gulf of Kisamos (Cretan Sea, Eastern Mediterranean) indicating the five stations of sampling. collected with the finer mesh were used to estimate abundances of chaetognath prey in the field. The samples taken with both nets were used for gut analysis in order to examine a wider range of sizes of chaetognaths. In the laboratory, all chaetognaths were extracted from the samples, and they were further sorted by species. The chaetognaths were then classified under a dissecting microscope by maturity stage based on the development of the ovary and the seminal vesicles, using a modification of Ghirardelli’s (1961) system, as follows: stage I, young without visible ovaries; stage II, immature with visible ovaries but no visible seminal vesicles; stage III, seminal vesicles present, ova visible, a few large; stage IV, filled seminal vesicles and large ova. All chaetognaths apparently containing food particles were examined to identify the food material. Prey was visible through the body wall. Chaetognaths which had not completely swallowed their prey, or in which the prey was found in the forward third of their gut were not included in the further analysis. Prey was identified to zooplankton taxa while copepods were identified to species or only to genus when there was difficulty in identification owing to partial digestion. Copepods which were in an advanced state of digestion were considered as unidentified copepods. Head width of chaetognaths is considered to be more closely related to prey size than body length (Pearre, 1980). Head widths of all chaetognath specimens were therefore measured with the chaeta in the closed position as well as the maximum body width of the food specimens, and a regression equation of head width vs. food width was established for each chaetognath species. Total lengths of chaetognaths and food items were also measured. Food containing ratio (FCR) and number of prey per chaetognath (NPC) (Feigenbaum and Maris, 1984) were estimated. FCR is expressed as the frequency of chaetognaths containing food while NPC is expressed as the frequency of prey in the chaetognaths examined. Selectivity calculations for every species were based on all identifiable prey for a given sampling date, and mean abundances from all plankton samples from that date. Therefore Ivlev’s (1961) index of electivity (E) was used: Ei =ri "ni/ri +ni where Ei =the selectivity for prey i, ri =the frequency of prey i in the diet, and ni =frequency of prey i in the 200 ìm mesh size net plankton samples. Results Of the 12 000 chaetognaths examine (...truncated)