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. 2A). No separation of data for male and female was made because there (r=0.992, N=65), (1) were no appreciable differences between the two.

log10 WW=-1.517+2.832 log10 BL From this scatter diagram it is seen that the IP (r=0.993, N=59), (2) is a function of BL of the specimens and temper- and ature. The greater IP is associated with the larger specimens and specimens maintained at log10 DW=-2.314+2.957 log10BL lower temperatures. The shortest IP of 3 days (r=0.994, N=65). (3) was recorded for the specimen with 2.7mm

No appreciable differences due to sex, maturity, pre-moult BL at 12•Ž, and the longest IP of 62 264 Ikeda

A

Fig.2. A: The relationship between IP and pre-moult BL of Themisto japonica.

Regression model is log10 IP= b+vBL. For details of the regression statistics, see Table 1. B: The relationship between intercepts (b) and

experimental temperature (T•Ž); the regression equation is log10 b = 0.0417 -0 .0277T(r=-0.991,N=4).

Table 1. Regression analyses of IP on pre-moult BL for Themisto japonica at four different

temperatures. Regression model adopted is: log10 IP=b+v BL. Since 95% CIs of v overlap each other, a common v was calculated (v for 1•Ž was excluded from this calcu-

lation because of few data sets). N=,data number.

* Mean for v values from 5 , 8, and 12•Ž experiments.

days for the specimen with 12.3 mm pre-moult moult BL (BL, mm) and temperature (T•Ž): BL at 1•Ž. Regression analyses of log10 IP on (4) pre-moult BL revealed that the ranges of 95% CI of the slope obtained at each temperature 3.4. Number of segments of moult pleopod overlap each other (Table 1). Excluding the rami data for the 1•Ž experiment because of fewer From consecutive moults collected from five data sets, a common slope of 0.0709 was calcu- specimens (coded as 12S-8, L, J, 14, and 12) lated from the data of 5, 8, and 12•Ž. The successive increases of the segment number of regression lines thus obtained were superimposed the third pleopod exopodite were followed (Fig. in Fig. 1. Intercepts of the regression lines from 3). On the whole, one new segment is added the 1, 5, 8, and 12•Ž experiments are closely at each moulting. However, variations of this related to the temperature (Fig. 2B), therefore pattern were seen in the consecutive data of the IP (days) is expressed as a function of pre- specimens J and L: one moulting which failed Growth Model for Themisto japonica 265

Consecutive moults

Fig. 3. The sequential increase of the segment number of the third pleopod exopodites of Themisto japonica, recorded on consecutive moults of five specimens coded as 12S-8, L, J, 14, and 12. The specimens with 1 and 3 segments are those hatched and just released from a female's marsupium.

Table 2. Variance analyses of ĢBL obtained at three (or two) different temperature

categories for five pre-moult BL groups of Themisto japonica.

to add a new segment was supplemented by later experimental temperature (data of 1 and 5•Ž moulting, in which two new segments were experiments were combined because of few data, added at one time. No addition of segments Table 2). Among these five pre-moult E. was observed twice for the largest specimen 12. groups, the temperature effect onƒ¢BL was signifi- 3.5. ƒ¢BL cant for the 4.0-5.9mm and 6.0-7.9mm groups,

The readings of moult UL were converted to but this was not the case for the other groups,

BL from the relationship between UL and BL 2.0-3.9mm, 8.0-9.9mm and 10.0-14.0mm. In

(Fig. 1). ƒ¢BLs calculated from consecutive the 4.0-5.9mm and 6.0-7.9mm groups, the moults were divided into 5 pre-moult BL groups, difference was significant only for the pair of 8 in which the data were subdivided into each and 1+5•Ž means in the former group, and for 266 Ikeda

the 12•Ž mean as compared with the other two marsupium. Specimens with one segment are means in the latter group (LSD-test at P=0.05 those just hatched out in the female's marsupium level). These results indicate no consistent and it was difficult to measure BL because of temperature effect on ƒ¢BL over the 2.0-14.0 mm their heavily curled body. No separation of sex pre-moult BL range. The relationship between was made as no appreciable differences were the pooled mean ƒ¢BL for each pre-moult BL seen between males and females. A comparison group and the pre-moult BL (the mid-range of of the ƒ¢BL obtained for the wild specimens with each group) is expressed as: those from the moulting experiment (Fig. 4)

yields a good agreement for small specimens, but the results from these two methods diverged

(5) as the BL of the specimens increased. The

relationship between ĢBL and pre-moult BL If one new segment is added to the pleopod from the wild specimens was: exopodite at each moulting and there is no temperature effect on ĢBL, ĢBL can be estimated directly from wild specimens of T. japonica by (6) counting the number of pleopod exopodite seg- ments and measuring their BL. The results of T. japonica just released from the female's these measurements on formalin-preserved wild marsupium are 1.310 mm and have completed specimens of various sizes are summarized in a second moult. From Eq.(6), ĢBL expected Table 3 (left column). Specimens with three for the third moult of this specimen is 0.425 mm. segments are those just released from the fe- Therefore, BL of specimens which have com- male's marsupium, and two are still in the pleted the third moult is 1.310+0.425=1.735 (mm). Equation (6) predicts the ĢBL of 0.451 Table 3. The segment number of the third pleopod mm for the specimen of 1.735 mm. By repeating exopodite and BL of wild specimens of Themisto the calculation in this way, a sequential stepwise japonica (left column), and predicted BL and ĢBL from Eq.(3) starting with 1.310 mm BL specimen growth pattern of T. japonica up to 16.811 mm BL was established (Table 3, right column). which completed the second moult after hatching

(right column). See text for details.

4. Discussion To date laboratory observations of moulting in Themisto amphipods are limited to T. paczfica and T. libellula from Alaskan coastal waters

(Wing, 1976) and T. gaudichaudii from the North Sea (Sheader, 1981). Wing's (1976) observations were not very successful because most specimens moulted only once and died. Sheader (1981) established the relationship be- tween IP and temperature as IP=46549/(T+8)2.675 for T.gaudichaudii

with BL of 8-12 mm. From this equation, the IP for T. gaudichaudii (BL=8-12 mm) is cal- culated as 49 days at 5•Ž, 28 days at 8•Ž, and 15 days at 12•Ž, which fall into 23-45 days, 17-33 days, and 12-23 days of IP computed from Eq.(4) obtained in this study for T. japonica with BL of 8-12 mm at respective temperatures. Sheader (1981) used the "growth factor"

(=ΔBL/pre-moult BL×100)to express the growth pattern of Themisto gaudichaudii fed an excess amount of food in the laboratory. According Growth Model for Themisto japonica 267

Fig. 4. The relationship between ĢBL and pre-moult BL of Themisto japonica.

Data for laboratory-raised specimens are from pooled means of ĢBL and the mid-range of pre-moult BL for each BL size group in Table 2 (regres-

sion equation : log10 ĢBL=-0.3592+0.0256 BL). Data for wild specimens are from Table 3 (left column)(regression equation: log10 ĢBL=-0.4503

+0.0602BL).Thefigure includes re-calculated ΔBL data on Tgemisto gaudichaudii in Sheader (1981).

to his results, the "growth factor" of T. gaudi- (1978) found also that the moult increment chaudii declined rapidly as BL increased. Shea- obtained from the laboratory-maintained bathy-

der's (1981) "growth factor" data on T. gaudi- pelagic mysid Gnathophausia ingens was chaudii were converted to ĢBL and are com- lower than that estimated from wild specimens.

pared with the present results on T. japonica Probably, experimental conditions including food.

in Fig. 4. Clearly, laboratory experiment data quality are responsible for the lower moult in- on the relationship between ĢBL and BL of T. crement of laboratory-raised specimens. For this

gaudichaudii is similar to that of T. japonica. reason, ĢBLs of wild specimens were used in The temperature effect on the "growth factor" the following calculations of the growth of T.

was not examined by Sheader (1981), but the japonica in the field.

present results on T. japonica indicate no appreci- The growth curve for Themisto japonica just able temperature effect on ĢBL. The lack of released from the female's marsupium can be

temperature effect on ĢBL has been reported on established by combining Eq.(4) with Eq.(6). mysids, Leptomysis lingvura, and Hemimysis Daily growth of BL (=dy/dt) is:

speluncola, within the temperature range of 10-

22•Ž (Gaudy and Guerin, 1979). However, the

relationship between temperature and ƒ¢BL is variable for crustaceans in general (cf. Hartnoll, (7) 1982). where a=2.3026(-0.0107), and ΔBLs of both laboratory-raised and wild b=2.3026(-0.4503-10-0.0277T+0.0417). Themisto japonica were observed to be similar Equation (7) leads to for small specimens, but the progressive increment dy/eaY=ebdt, or e-aYdy=ebdt. of ƒ¢BL with BL for the former was less than (8) that of the latter (Fig. 4). Childress and Price The integrated form of Eq.(8) is 268 Ikeda

Fig. 5. Predicted growth curves of Themisto japonica living at 1, 5, 10, and 15•Ž (hatched

lines), and actual stepwise growth through moultings (solid lines). The moult number since hatching is in circles, which is the same for all four temperatures.

japonica to reach maturity was 14 for females where C is an integral constant. Re-arranging and 10 for males (Table 3, right column). The

the equation for y, we obtain time required to grow to the minimum maturity

size is highly dependent on temperature and is when t=0, y=1.31 (the BL of specimens just calculated from Eq. (9) for females as 282 days released from the female's marsupium), then C at 1•Ž, 165 days at 5•Ž, 100 days at 10•Ž, =1.033. Finally, we obtain and 69 days at 15•Ž, and for male s 145 days at 1•Ž, 85 days at 5•Ž, 59 days at 10•Ž,

(9) and 36 days at 15•Ž (calculations are based on

Estimated growth curves of T. japonica growing the specimen with the nearest BL, i.e. 9.424 mm

at 4 selected temperatures (1, 5, 10, and 15•Ž) for females and 5.677 mm for males in Table 3,

are shown. in Fig. 5. Among these four temper- right column).

atures, 1 and 15•Ž are near the limits of the The BL of spent females (marsupium is fully

thermal range of this species in the Japan Sea. developed but no or few larvae are left in it) of

Actual stepwise growth by moulting can also be Themisto japonica is 10-17 mm, and the time

estimated by combining these smooth growth for the newly released larvae to grow to spent

curves with ĢBL between instars (Table 3, right females (= single life cycle) is estimated as 333-

column), and the results are also shown in Fig. 5. 593 days at 1•Ž, 195-347 days at 5•Ž, 118-210

Examinations of approximately 2,000 specimens days at 10•Ž, and 82-146 days at 15•Ž (calcu-

of Themisto japonica collected from Toyama Bay lations are based on the specimens of 10.733 mm . in June and September (see Hirakawa et al., and 16.811mm BL in Table 3, right column). The

1990) indicated that the size of mature females total number of moults per life cycle is independent was 9-17 mm BL (with fully developed marsupium of temperature and is 15-18. The time for a which carries developing embryos or larvae in single life cycle can be estimated more accurately it), and the size of mature males was 6-12 mm by knowing the embryonic development time (from

BL (with extended second antenna and well deposited egg to its hatching) and the time for the developed "excavate" organ between the rami development of the larvae in the marsupium (from of the first pair of uropods). The minimum hatched larvae to those ready to release). Un- number of moults needed for newly hatched T. fortunately, no information is available for T. Growth Model for Themisto japonica 269

Table 4. Summary of life history parameters of Themisto amphipods.

japonica. For T. gaudichaudii the time for for T. japonica, the larger maturity size of this

embryonic development is reported as 6 days at species may be related to cooler habitat temper-

13-14•Ž and 30 days at 5.6•Ž, and additional ature, which is down to 0-1•Ž in the Japan Sea.

3-4 days are required for the larval development The lower temperature limit of T. gaudichaudii

in the marsupium (Sheader, 1977). is reported to be about 4•Ž (Sheader, 1981).

Presently available information about the life Despite the fact that the maximum size of T.

history of Themisto amphipods is summarized in gaudichaudii is similar to that of T. japonica, Table 4. Within the same species seasonal and fecundity of the former (maximum ca. 200 eggs

local differences in minimum adult size, life span per female; Sheader, 1977) is considerably less and maximum size are evident in T. libellula, than that of the latter (maximum ca. 500 eggs

T. paczfica, and T. gaudichaudii. Among the per female; Ikeda, unpublisned data). three Themisto species listed, T. gaudichaudii For the production estimates of a given popu-

studied by Sheader (1977, 1981) is similar to T. lation of Themisto japonica in which BL com- japonica of this study in terms of maximum size position is known, information about the in-

(16-18 mm). However, the time required for the crease in biomass (DW) rather than BL may be single life cycle of T. gaudichaudii in the North more useful. The antilogarithmic form of Eq.

Sea is 6-16 weeks, which is much shorter than (3) is 12-85 weeks (82-593 days) estimated for T. (3') japonica in this study at the temperature range Then, of 1-15•Ž (cf. Table 4). As mentioned earlier, Dividing this equation by IP, we obtain, the relationship between IP and BL at a given

temperature is identical in these two species. The relationship between ĢBL and BL (Fig. 4) (10)

indicates nearly similar (laboratory-raised speci- where (ƒ¢D WaP) is the daily increase in DW, mens) or slightly greater ƒ¢BL (wild specimens) and (ƒ¢BL/IP) is from Eq.(7). The percent at a given BL in T. japonica than in T. gaudi- increase in DW (G=ƒ¢DW/IP•~100/DW) can be chaudii. It is therefore considered that the major obtained from Eq.(10) divided by Eq.(3'): cause for this discrepancy in the length of life

cycle is due to the dissimilar maturity size be- (11) tween these two species. Female T. gaudichaudii G for Themisto japonica growing at 1, 5, 10, reaches full maturity at 3-3.5 mm BL in contrast and 15•Ž is shown in Fig. 6. G is greater at to 9mm of T. japonica. Although the seasonal higher temperatures and drops rapidly with the

investigation of maturity size is still incomplete growth of T. japonica. For comparison, labor- 270 Ikeda

Fig. 6. The relationship between G (percent increase in DW per day) and

BL (or DW) of Themisto japonica growing at 1, 5, 10, and 15•Ž. The figur eincludes laboratory data on Eupgau5ia pacifica grown at 8 and12℃

by Ross (1982) and those of Meganyctiphanes norvegica at 13.5•Ž by Fowler

et al.(1971). Both data of Ross (1982) and Fowler et al.(1971) are plotted against DW.

atory data on two euphausiids, Euphausia pacifica Acknowledgements

grown at 8 and 12•Ž (Ross, 1982) and Megany- I am grateful to T. Akamine for his help in ctiphanes norvegica at 13.5•Ž (Fowler et al., solving the mathematical problems encountered 1971), are plotted in Fig. 6 against DW. It in the course of this study. I thank S. Nicol of is clear that the general level of G estimated for the Australian Antarctic Division for reviewing T. japonica at 10 and 15•Ž in this study is the early draft of this paper. This is contribution approximately comparable to these euphausiids in No. B-8908 from Japan Sea National Fisheries terms of equivalent DW. In this regard, growth Research Institute. of T. japonica evaluated in this study is not

atypical for a planktonic crustacean. References Finally, it is noted that the growth model of Bovallius, C.(1889): Contributions to a monograph Themisto japonica proposed in this study repre- of the Amphipoda Hyperiidea, Part I: 2, The sents the potential growth of this species when families Cyllopodidae, Paraphronimidae, Thauma- its feeding is unlimited. It is unknown whether topsidae, Mimonectidae, Hyperiidae, Phronimidae, T. japonica is food limited or not in the Japan and Anchylomeridae. Kongliga Svensk Vet. Sea. However, the greater ĢBL of wild speci- Akad. Handl., 22, 1-434. mens than that of laboratory-raised specimens Bowman, T. E.(1960): The pelagic amphipod genus Parathemisto (Hyperiidea, Hyperiidae) in the fed an excess amount of food as seen in this North Pacific and adjacent Arctic Ocean. Proc. study (Fig. 4) suggests that the food limited U. S. Nat. Mus., 112, 343-392. growth of T. japonica in the field is quite un- Childress, J. J. and M. H. Price (1978): Growth rate likely. According to Sheader (1981) T. gaudi- of the bathypelagic crustacean Gnathophausia chaudii breeds throughout the year in coastal ingens (Mysidacea: Lophogastridae). I. Dimen- waters of the North Sea, with peaks in spring sional growth and population structure. Mar. and autumn. Biol., 50, 47-62. Growth Model for Themisto japonica 271

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assimilation in a Mediterranean euphausiid. perature on measured and predicted production.- Thalassia Jugosl., 7, 35-47. Mar. Biol., 68, 1-13. Fukataki, H.(1967): Stomach contents of the pink Sheader, M.(1977): Breeding and marsupial develop- salmon, Oncorhynchus gorbuscha (Walbaum), in ment in laboratory-maintained Parathemisto

the Japan Sea during the spring season of 1965. gaudichaudi (Amphipoda). J. mar. biol. Ass. Bull. Japan Sea Reg. Fish. Res. Lab., 17, 49-66. U. K., 57, 943-954. Fukataki, H.(1969): Stomach contents of the masu Sheader, M.(1981): Development and growth in salmon, Oncorhynchus masou (Brevoort), in the laboratory-maintained and field populations of offshore regions of the Japan Sea. Bull. Japan Parathemisto gaudichaudi (Hyperiidea: Amphi-

Sea Reg. Fish. Res. Lab., 21, 17-34. poda). J. mar. biol. Ass. U. K., 61, 769-787. Gaudy, R. and J. P. Guerin (1979): Ecophysiologie Snedecor, G. W. and W. G.(Cochran, (1967): Statistical comparee des mysidaces Hemimysis speluncola methods. Iowa State University Press, Ames, Ledoyer (cavernicole) et Leptomysis linguura G. 593 pp. O.Sars (non cavericole). Action de la tempera- Takeuchi, I.(1972): Food animals collected from the ture sur la croissance en elevage. J. Exp. Mar. stomachs of three salmonid fishes (Oncorhynchus) Biol. Ecol., 38, 101-119. and their distribution in the natural environments Hartnoll, R. G.(1982): Growth. p.111-196. In: The in the northern North Pacific. Bull. Hokkaido biology of Crustacea, vol. 2, ed. by L. G. Abele, Reg. Fish. Res. Lab., 38, 1-119. Academic Press, New York and London. Vinogradov, M.E. and A.F. Sazhin (1978): Vertical Hirakawa, K., T. Ikeda and N. Kajihara (1990): distribution of the major groups of zooplankton Vertical distribution of zooplankton in Toyama in the northern part of the Sea of Japan. Ocean- Bay, Southern Japan Sea, with special reference ology, 18, 205-209. to Copepoda . Bull. Plankton Soc. Japan (in press). Wing, B. L.(1976): Ecology of Parathemisto libellula Kane, J. E.(1963): Stages in the early development and P. pacifica (Amphipoda: Hyperiidea) in of Parathemisto gaudichaudii (Geur.)(Crustacea Alaskan coastal waters. Ph. D thesis, University Amphipoda: Hyperiidea), the development of of Rhode Island, 266 pp. secondary sexual characters and of the ovary. Yamashita, Y., D. Kitagawa and T. Aoyama (1985): Trans. Royal Soc. New Zealand, 3, 35-45. A field study of the predation of the hyperiid Kane, J. E.(1964): Thalassomyces marsupii, a new amphipod Parathemisto japonica on larvae of the species of ellobiopsid parasite on the hyperiid Japanese sand eel Ammodytes personatus. Bull. amphipod Parathemisto gaudichaudii (Guer.). Japan. Soc. Sci. Fish., 51, 1599-1607. New Zealand J. Sci., 7, 289-303. Zenkevitch, L.(1963): Biology of the seas of U. S. Kane, J. E.(1966): The distribution of Parathemisto S. R. George Allen and Unwin Ltd, London, gaudichaudii (Guer.), with observations on it 955 pp. 272 Ikeda

脱皮間隔 と脱皮毎の体長増加に基づ く日本海産端脚類 Themisto japonica(ニ ホ ン ウ ミノ ミ)の 成 長 モ デ ル

池 田 勉*

要 旨:ニ ホ ン ウ ミノ ミを1°,5°,8°,12℃ で飼 育 し, 実 験 に よ るIPと,野 外 標 本 に よる ΔBLか ら水 温 を 変 数 脱 皮 間 隔(IP)と 脱 皮 毎 の体 長 増 加(ΔBL)を 測 定 した. とす る ニ ホ ン ウ ミノ ミの成 長 モ デ ル を 提 出 した.こ の 成 そ の結 果,IPは ニ ホ ンウ ミノ ミの体 長 が大 き くな る ほ 長 モ デル か ら,雌 の 育 児 嚢 よ り放 出 され た 仔 虫(体 長3 ど,水 温 が 低 い ほ ど長 くな った.ΔBLは 体 長 に よ って 1.31mm)が 仔 虫 を 放 出 す る雌(体 長:10-17mm)に 成 変 化 した が 水 温 に よ る影 響 は 見 られ な か った.飼 育 実 験 長 す る の に 要 す る 日数 は1℃ で333-593日,5℃ で で 得 られ た 同 一 個 体 に つ い て の連 続 した 脱 皮 殻 の観 察 か 195-347日,10℃ で118-210日,15℃ で82446日 と ら,脱 皮 に よ って 腹 肢 内 ・外葉 の 節 数 が 脱 皮 に よ って1 な る.ま た この 成 長 モ デル か ら計算 され る体 重 ベ ー ス の 節 ず つ 増 加 す る こ とが 分 か り,従 っ て野 外 標 本 に も とつ 成 長 速度 は 同 水 温 に おい て 同体 重 を有 す る オ キ ア ミのそ く ΔBLの 推定 が 可 能 とな った.こ の よ うに して,飼 育 れ に 匹 敵 す る.日 本 海 に お け る ニ ホ ンウ ミ ノ ミの摂 餌 状

〒951新 潟市 水 道 町1丁 目5939-22 態 に つ い て 若 干 の 論 議 を し た. 日本 海 区 水 産 研 究 所