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A Growth Model for a Hyperiid Amphipod Themisto Japonica (Bovallius) in the Japan Sea, Based on Its Intermoult Period and Moult Increment*

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A Growth Model for a Hyperiid Amphipod Themisto Japonica (Bovallius) in the Japan Sea, Based on Its Intermoult Period and Moult Increment* Journal of the Oceanographical Society of Japan Vol.46, pp.261 to 272 1990 A Growth Model for a Hyperiid Amphipod Themisto japonica (Bovallius) in the Japan Sea, Based on Its Intermoult Period and Moult Increment* Tsutomu Ikeda•õ Abstract: Themisto japonica was reared at 1, 5, 8, and 12•Ž in the laboratory to estimate its intermoult period (IP) and increase in body length (BL) at each moulting (ABL). IP was found to be a function of temperature and BL of the specimens, longer IPs being associated with lower temperature and larger specimens. ABL was not affected by temperature but increased with growth of the specimens. Observations on consecutive moults indicated that one new segment was added to pleopod rami at each moulting. ABLs obtained from the measurement of the seg- ment number of pleopod rami and BL of wild specimens were slightly larger than values obtained from laboratory-raised specimens. IP data obtained from laboratory- reared specimens are combined with ABL data from wild specimens to establish a growth model for T. japonica from its release from the marsupium (1.31mm BL) to the maximum size (17mm BL) as a function of temperature. This growth model predicts that a total of of 18 moultings is needed for T. japonica to reach the maximum size regardless of temperature, although the time needed to reach the maximum size is highly dependent on temperature. The life cycle, from the newly released larvae (1.31mm BL) to the spent females (10-17mm BL), was estimated as 333-593 days at 1•Ž, 195-347 days at 5•Ž, 118-210 days at 10•Ž and 82-146 days at 15•Ž; the last may be the upper temperature limit for T. japonica. Growth rates of T. japonica expressed on the basis of body mass are comparable to the rates of euphausiids of equivalent size when the effect of temperature is accounted for. Feeding conditions of T. japonica in the field are also discussed. 1. Introduction (Theragra chalcogramma), masu salmon (Oncor- The hyperiid amphipod Themisto japonica is hynchus masou) and pink salmon (O. gorbuscha) distributed in the Okhotsk Sea, Japan Sea, west- are considered to be major predators on T. ern North Pacific, off the east coasts of northern japonica in the Japan Sea (Okiyama, 1965; Japan and southern Kuriles (Bowman, 1960). Fukataki, 1967, 1969; K. Nashida, personal This species is known to be an important prey communication). T. japonica is a carnivore. for salmonid fishes in these waters (Zenkevitch, Appendicularians, copepods and other crusta- 1963; Takeuchi, 1972). In the Japan Sea, T. ceans, and fish larvae have been found in the japonica, occurring from near the surface through stomachs of T. japonica from waters of north- more than 1,000m depth, is the second to fourth eastern Honshu, Japan (Yamashita et al., 1985). dominant group of net zooplankton biomass Despite its importance in pelagic food webs, (Vinogradov and Sazhin, 1978; Hirakawa et al., little is known about the growth and life span 1990; Ikeda unpublished data). The common of T. japonica in the Japan Sea. squid (Todarodes pacificus), walleye pollack Growth of crustaceans is achieved through * Received 16 August 1990; in revised form 6 successive moutlings so that the intermoult period October 1990; accepted 28 November 1990. (IP) and moult increment (i.e. the increase in † Japan Sea National Fisheries Research Institute, body length at each moulting)(ABL) are two 1 Suido-cho, Niigata 951, Japan. major components in analyzing crustacean growth. 262 Ikeda Various internal and external conditions can USA, Japan Pet Drugs Co., Ltd) were used as a affect both IP and 4BL, and its effects are some- staple food. Frozen zooplankton (copepods, what species specific (Hartnoll, 1982). As a good euphausiids, chaetognaths. etc.) collected from the example of this method Mauchline (1980) same sampling sites as T. japonica was provided estimated the life span of the Antarctic krill occasionally as supplemental food. All these Euphausia superba from laboratory data of IP foods were given in excess during the experiments. and rather arbitrary, assumed 4BL data. To 2.3. Moulting. apply this approach, laboratory experiments on Males and females of various sizes, including live specimens are usually needed to gain accurate larvae released from females in the laboratory, data of IP and 4BL. However, ZIBL may be were isolated and placed individually into 20- obtained from field samples in some special 1,000ml glass containers filled with seawater. cases. A bathypelagic mysid Gnathophausia Experiments were run at 1, 5, 8, and 12•Ž in ingens has 13 instars, each with non- the dark. Seawater of each container was overlapped distinct size range (Childress and changed every 5-10 days. The containers were Price, 1978). This is not the case for Themisto examined every morning for moults. Moults, japonica of which each instar has a well over- when found, were preserved in buffered formalin- lapped size distribution. Instead, the segment seawater for later measurements of the third number of pleopod rami of Themisto amphipods uropod length (excluding rami)(UL, mm) as a has been considered to indicate the number of measure of body length (BL, mm). The number instars (Kane, 1963; Evans, 1968; Sheader, 1977). of segments of third pleopod exopodite was Experimental evaluation of this hypothesis is counted, and morphological observations of the currently limited to T. gaudichaudii by Sheader second antenna and the first uropod rami were (1981). made for the determinations of sex and maturity In this study Themisto japonica was main- after Kane (1963) and Sheader (1981). tained in the laboratory to obtain IP and ABL 2.4. Body allometry data of this species. As the number of segments The UL and BL (the maximum distance be- of pleopod rami was revealed to mark the number tween the anterior tip of the head and the distal of instars, 413L was determined also for field end of the uropods of a straightened body) of samples of T. japonica. These laboratory data live specimens anesthetized with MS-222 (ethyl- and field data were combined together to establish m-aminobenzoate) were measured under a Wild the potential growth rate and to estimate the dissecting microscope to the nearest 0.02mm. life span of this species in the Japan Sea. Observations of the second antenna and the first uropod were made at the same time to determine 2. Materials and methods sex and maturity conditions. Kane (1964) has 2.1. Animals noted that an ellobiopsid parasite Thalassomyces Themisto japonica was collected from waters marsupii is a potential source of error for the around Yamato Rise (central Japan Sea) in May determination of sex and maturity of Themisto and September 1988, and off Toyama Bay in gaudichaudii because its structure resembles an egg July and October 1989. Specimens were sampled mass and it occurs characterically in the position with oblique hauls of a fish larva net (1mm of a female's marsupium. T. marsupii was mesh openings) from 200m depth to the surface found also in T. japonica in this study, there- at night. Live specimens of T. japonica in the fore a special effort was paid to avoid this source catches were sorted out quickly on shipboard, of error. The specimens were then rinsed briefly maintained at 8•Ž, and transported to a land with distilled water, blotted on filter papers, and laboratory for moulting experiments. At the stored frozen. Frozen specimens were weighed same sampling sites, seawater was collected with (wet weight)(WW, mg) and freeze-dried to Niskin bottles from 100m depth and kept in 20 obtain dry weight (DW, mg). In addition to litre plastic containers for use in the laboratory fresh specimens, supplemental measurements were experiments. also made on the species preserved in buffered 2.2. Food formalin-seawater. Newly hatched Artemia nauplii (Salt Lake in 2.5. Statistical treatment of data Growth Model for Themisto japonica 263 Fresh ○Female △Male □Juvenile Preserved Fig. 1. The relationship between BL and UL of fresh Themisto japonica (Fresh) and those preserved in formalin-seawater (Preserved). Regression equation for pooled data is BL=0.1893+9.033 UL Means•}one standard deviation (SD) are given and conditions of the speciments (fresh or pre- in the tables. For comparison, means•}a 95% served in buffered formalin) were seen in these confidence interval (CI) are also calculated. For relationships so that all data were pooled for the the analysis of interrelationships between two calculation of the regression equations. parameters AM or GM regressions were com- 3.2. Survival puted, depending on regresssion situations as The number of specimens that moulted more listed in Ricker (1973). The Student t-test and than twice consecutively was 4 at 1•Ž, 9 at 4•Ž, F-test together with the LSD-test were used to 22 at 8•Ž, and 7 at 12•Ž. The maximum evaluate differences between means at the 5% number of consecutive moults recorded was 5 at level of significance unless otherwise specified 1•Ž, 7 at 4•Ž, 12 at 8•Ž, and 7 at 12•Ž. (Snedecor and Cochran, 1967). Among a total of 43 specimens used in this study (4 specimens at 1•Ž, 9 at 4•Ž, 23 at 8•Ž, and 3. Results 8 at 12•Ž) unsuccessful moulting was the major 3.1. Body allometry cause of their death (38%). The relationships between BL and UL (Fig. 3.3. IP 1), between WW and BL, and between DW The IPs recorded at each experimental temper- and BL were established as: ature were plotted against the pre-moult BL of BL=0.1893+9.033 UL the specimens (Fig.
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