Journal of the Oceanographical Society of Japan Vol.28, pp.181 to 186, October, 1972 Feeding of a Pelagic Chaetognath, Sagitta nagae ALVARINO in Suruga Bay, Central Japan* Sachiko NAGASAWA**and Ryuzo MARUMO** Abstract: The feeding of Sagitta nagae, the most predominant chaetognath species in Suruga Bay, was studied. Specimens were collected with ORI-100 net (1.0 mm in mesh aperture) and MTD net (0.35 mm in mesh aperture) and were fixed with neutralized formalin. The main food organisms of Sagitta nagae were the copepods, Candacia bipinnata, Calanus pacificus and Pareuchaeta russelli. The food-containing ratio (PIT, %) was higher at night and lower in the daytime. Here, P is the number of Sagitta containing food organisms in the gut and T is the total number of Sagitta examined. P/T was almost constant vertically in the upper 100 m layer. P/T was generally influenced neither by food density nor by the copepod numbers per Sagitta in this study. Sagitta nagae took food throughout life. Food ingested by Sagitta per day was calculated as 37.6 % of Sagitta in dry weight. 1. Introduction Suruga Bay is located about 35•‹N, 138.5•‹E in It has been frequently observed that chaetog- the central area of Honshu, Japan, along the naths feed on zooplankton such as copepods, Pacific coast. The biomass of chaetognaths is amphipods, euphausiids, medusae, siphono- second only to copepods among zooplankton in phores, chaetognaths, Tomopteris, tunicates Sagami Bay which is located near to Suruga and fish larvae (LEBOUR,1922, 1923; BIGELOW, Bay, the two bays having similar characters 1926; THOMSON,1947; SUAREZ-CAABRO,1955; of hydrography and plankton community MURAKAMI, 1957, 1959; ALVARINO, 1962). (AIZAWA and MARUMO, 1967). Epipelagic Minute plankton organisms, diatoms and cili- chaetognaths reach up to 15 species in Suruga ates, were also found from the gut of chaetog- Bay, but only one species, Sagitta nagae, pre- naths (BURFIELD, 1927; ALVARINO, 1965). dominates, accounting for 20-90 % of numbers Green-colored matter, possibly arising from of total chaetognaths throughout the year diatoms, was observed in the gut of young (MARUMO and NAGASAWA, 1972). This specimens of Sagitta setosa (WIMPENNY,1937). species is usually distributed in the upper 100 m Studies on seizing and digestion of food orga- layer in this bay. It also is widely distributed nisms were carried out by GREY(1930), PARRY in the Pacific coast of Japan, the Yellow Sea, (1944), DAVID(1955) and TAKANO (1971). the East China Sea and the South China Sea As for predators on chaetognaths, Anomalo- (TOKIOKA, 1940; KAWARADA et al., 1966; cera and Labidocera (copepods), Gigantocypris ALVARINO, 1967). Sagitta nagae had been miilleri (ostracod), Meganyctiphanes (euphau- reported as Sagitta bedoti by TOKIOKA (1939, siid), Palaemonetes (decapod), medusae and 1940, 1942) and other Japanese workers, until ctenophores were reported (LEBOUR, 1922, ALVARINO established this species as Sagitta 1923; CANNON,1940; BROAD,1957; MAUCH- nagae in 1967. In general, chaetognaths are LINE, 1960). Sagitta nagae was dominantly carnivorous and consequently Sagitta nagae is contained in the stomach of an anglerfish, considered to play a very important role as Lophius litulon, which was ingested by a the secondary consumer in the food chain in lancetfish, Alepisaurus ferox (KUBOTA, 1971). Suruga Bay. * Received July 3 , 1972 The present paper reports a study on the ** Ocean Research Institute , University of Tokyo, feeding of Sagitta nagae, a key species in the Nakano-ku, Tokyo 164, Japan pelagic ecosystem of Suruga Bay. First, the (1) 182 Sachiko NAGASAWA and Ryuzo MARUMO results are given concerning the food-containing 23-25, 1971, in order to study P/T at different ratio, P/T (%), in the daytime and at night depths and at different maturity stages. These (P is the number of Sagitta containing food materials were fixed with neutralized formalin. organisms in the gut, and T, the total number The body lengths of Sagitta specimens examined of Sagitta examined), P/T at different food were 8-23 mm in the first collections and 9-22 concentration in the environment, P/T at mm in the second collections. In this study different depths and P/T at different maturity specimens never contained more than two food stages. Secondly, calculation is made for the organisms. weight of food organisms ingested by Sagitta. Animals which contained food organisms The authors would like to express their were divided into three groups according to hearty thanks to Dr. M. R. REEVE for his the position of food in the gut and the degree valuable advice and criticism on this manu- of digestion as follows: script. Thanks are also extended to Mr. LEE I. The food organism is kept near the SU Bu who kindly made identification on mouth, just after being captured. copepod specimens in connection with the II. The food organism is not completely present study. digested and the shape is clearly seen. III. The food organism is situated in the 2. Methods and materials posterior patt of the gut or near the Specimens were taken by two series of hauls. anus. It is difficult to identify the kind First, oblique hauls were made with ORI-100 of the food organism because it has net (160 cm in mouth diameter and 1.0 mm in already lost its shape. mesh aperture) (OMORI, 1965 ; OMORI, MARU- The individual numbers are divided by this MO and AIZAWA, 1965) in the central part classification, expressed as PI, PH and Pm and (34•‹51'N, 138•‹38'E) of Suruga Bay on May consequently 11-15, 1969, in order to examine P/T in the P=PI +PII +Pm daytime and at night. Secondly, two series of horizontal hauls were made with MTD net 3. Results and discussion (56 cm in mouth diameter and 0.35 mm in (1) Food organisms of Sagitta nagae mesh aperture) (MOTODA, 1971) near the Sagitta fed mainly on copepods, and a lesser above station (34•‹56'N, 138•‹41'E) on January number of specimens contained euphausiids, Table 1. Food organisms ingested by S. nagae and total number of copepods in the sea water. * Copepods include species such as Calanus pacificus , Eucalanus subtenuis, Clausocalanus arcuicornis, Paracalanus parvus, Euchaeta marina, Pareuchaeta russelli, Aetideus sp., Euchirella sp., Scolecithrix danae, Candacia bipinnata, Corycaeus japonicus and Cor,yogeussp. (2) Feeding of a Pelagic Chaetognath, Sagitta nagae ALVAREZIO in Suruga Bay, Central Japan 183 chaetognaths and Lucifer (Table 1). natural environment. The food organism most frequently found in According to the laboratory experiments, the gut was Candacia bipinnata, Calanus Sagitta setosa feeds under low light intensity paczificus and Pareuchaeta russelli. These (PARRY, 1944), and Sagitta hispida also takes copepods were dominant also in the environ- food most actively during the night, showing mental water. a marked diurnal rhythm in the feeding rate (2) P/T in the daytime and at night (REEVE, 1964). These trends are identical P/T was examined for 12 samples collected with those in the natural environment obtained throughout a day. Pm/P is very high, 86-96 by us. Spadella cephaloptera, on the other %, indicating that food organisms which have hand, a bottom-living chaetognath, is reported been ingested go down quickly to the posterior to feed well under bright illumination (PARRY, part of the gut and that they are mainly 1944). digested there. The food seizure may take Most of copepods lied in the gut of Sagitta, place under the crowded condition of plankton directing their head to its anus. This seems organisms in the cod-end of the net during to show that a chaetognath attacks and swal- sampling, causing the overestimation of the lows a small organism from its head when it number of Sagitta containing food organisms. senses the organism in front of it. PARRY In this study, it will be clear that Sagitta of (1944) and TAKANO (1971) also showed that Pm, in the later stage of digestion, ingested a chaetognath never chases prey going away food organisms only before sampled. On the from it. other hand, it is not clear whether Sagitta of The unimpaired feeding ability of a chaetog- PI and PH fed on copepods in the cod-end or nath in the dark may be due to its numerous not. Fortunately, PI/P and PH/P are very sensory appendages generally considered to be small, ranging from 0 to 6 % and from 4 to tangoreceptor (HYMAN, 1959) instead of eyes. 11 %, respectively. Consequently such errors (3) P/T at different food concentration in will be almost disregarded in this study. P/T the environment is high, ranging from 16 to 22 %, with mean P/T was almost constant in the daytime and of 20.2 % at night (19-05 o'clock), while it is at night, respectively, independent of the food low, ranging from 5 to 14 %, with the mean density (9-33 copepods/m3), except a small of 10.3 536 in the daytime (06-18 o'clock) value of P/T, 5 %, for 6 copepods/m3 at 11 (Figure 1). P/T at night is about two times o'clock (Figure 2). At 11 o'clock it is con- as high as that in the daytime. The values of sidered that food organisms were too sparsely P/T at dawn and dusk lie between those at distributed for Sagitta to catch them enough. night and in the daytime. Pi +PH/T, the food- However, P/T at 11 o'clock may have been containing ratio in the early stage of digestion, reduced by the less active feeding owing to at night shows also double the number of that strong radiation in the daytime. in the daytime.