Journal of Oceanography, Vol. 62, pp. 693 to 703, 2006

Diel Vertical Migration of pacifica (Crustacea, Euphausiacea) in Relation to Molt and Reproductive Processes, and Feeding Activity

† YOSHINARI ENDO* and FUHITO YAMANO

Laboratory of Aquatic Ecology, Graduate School of Agricultural Science, Tohoku University, Sendai 981-8555, Japan

(Received 27 January 2006; in revised form 1 June 2006; accepted 1 June 2006)

We investigated the diel vertical migration of Euphausia pacifica in relation to molt Keywords: and reproductive processes and feeding activity in April and September 2001 at fixed ⋅ DVM, stations off northeastern Japan. The vertical distribution of this species was shal- ⋅ molt cycle, ⋅ lower in April than in September during both day and night, which was partly ex- maturity, ⋅ plained by a high surface temperature (19°C) and the existence of a subsurface chlo- feeding, ⋅ Euphausia rophyll maximum in September. It has been demonstrated for the first time that diel pacifica. vertical migration of this species is influenced by molt processes because upward migration of molting individuals was restricted compared with non-molting ones. Feeding activity of molting individuals was reduced throughout the day, being lower than or similar to the daytime feeding activity of non-molting ones. The percentage of molting individuals was least (2Ð4%) among the gravid females, which suggests that gravid females molt less frequently than other stages of females and males. Molt and reproductive processes therefore seemed to be coupled in this species.

1. Introduction 1966; Iguchi and Ikeda, 1995). Assuming that they molt Euphausia pacifica performs diel vertical migration independently, or asynchronously, about 20% of them are (DVM) through a range of several hundred meters in Japa- molting each day. Molting individuals may be different nese waters (e.g., Iguchi, 1995). Various factors are known from non-molting ones in both physiology and behavior. to affect DVM of zooplankton in general (e.g., Longhurst, For example, extended intermolts in breeding females 1976; Ohman, 1990). For E. pacifica, water temperature have been recorded for a considerable number of crusta- (Iguchi et al., 1993; Taki, 1998), abundance of ceans (Hartnoll, 1985). Molting may affect other forms zooplanktivorous (Bollens et al., 1992), and matu- of behavior of euphausiids, such as feeding and diel ver- rity stage (Terazaki et al., 1986) have been investigated tical migration, but little information is available on the in relation to its DVM. effect of molting activity on DVM. The relationship be- Euphausiids display an indeterminate growth format, tween molt and reproductive processes has been investi- i.e., molting continues indefinitely past maturity, as op- gated among Antartctic , Euphausia superba and posed to a determinate growth format in which molting North Atlantic krill, Meganyctiphanes norvegica (Cuzin- ceases definitively at some point, with a clear terminal Roudy and Buchholtz, 1999; Cuzin-Roudy, 2000). Eggs ecdysis (Hartnoll, 1985). Other that show proved to be released in the premolt stages in both spe- indeterminate growth format include Branchiopoda, cies. Vertical migration of M. norvegica was investigated Tanaidacea, Cumacea and some species of Mysidacea, by Tarling et al. (1999) in relation to molt and reproductive Amphipoda and Decapoda (Hartnoll, 1985). Euphausia processes. They showed that molting occurred in the deep pacifica molt approximately every 5 days, each time shed- in the nighttime, away from the main part of the non- ding about 10% of body weight as molt (Jerde and Lasker, molting population. This is viewed as a mechanism to avoid cannibalism. They also showed that spawning fe- * Corresponding author. E-mail: [email protected] males were most evident in the warm uppermost layer, which accelerates reproductive processes and may also † Present address: Environment and Quality Control Group, The Maruha reduce the depth to which the eggs sink before hatching. Group Inc., 1-1-2, Otemachi, Chiyoda-ku, Tokyo 100-0004, Japan. The purpose of the present study was to clarify 1. if Copyright©The Oceanographic Society of Japan/TERRAPUB/Springer molting affects DVM, 2. if maturity, especially in females,

693 43oN Temperature (°C)Salinity Chlorophyll a (µg l -1 ) a) 0 10 20 33 34 35 0 2 4 0 0 0

100 100 42o 50

200 200 100 300 300 41o Depth (m) KT-01-14 150 400 400

40o 500 500 200

b) 0 10 20 33 34 35 2 0.5 1 39o 0 0 0 KT-01-03 100 100 50

38o 200 200 Pacific Ocean 100 300 300 Depth (m)

o 150 37 400 400 140o 141o 142o 143o 144o 145oE

500 500 200 Fig. 1. Sampling stations in this study. Fig. 2. Vertical profiles of temperature (°C), salinity and chlo- rophyll a concentration (µg lÐ1) at fixed stations in April (a) and September (b) 2001. affects molting, and 3. if molting affects the feeding ac- tivity of E. pacifica in the sea area off northeastern Ja- pan. utes for shallow tows and from 10 to 30 minutes for deep 2. Materials and Methods ones, with longer duration for daytime or deep tows and Euphausiid surveys were carried out during two shorter towing time for nighttime or shallow tows in or- cruises of R.V. Tansei Maru in Sanriku waters, the sea der to obtain sufficient individuals for examination (Ta- area off northeastern Japan: cruise KT-01-03 (April 9Ð ble 1). The local sunrise and sunset occurred at 4:50 and 17, 2001) and cruise KT-01-14 (September 12Ð17, 2001). 18:00, respectively. The sea depth at this station was 5,600 Samples were taken at fixed stations: 39.00°N, 144.00°E m. In September, samples were obtained 5 times on 14 in April and 40.50°N, 143.00°E in September (Fig. 1). and 15 September using the same nets at 10 depth layers Temperature and salinity were determined with a CTD down to 400 m including deep tows (100, 150, 200, 300 system down to 500 m depth. The CTD cast was done at and 400 m) and shallow ones (10, 20, 30, 50 and 75 m). 22:30 on 14 April and at 08:00, 14:00 and 20:30 on 15 The towing duration ranged from 15 to 20 minutes for April during cruise KT-01-03, and at 06:20, 12:00, 18:00 shallow tows and from 20 minutes for deep ones. The on 14 September and at 00:00 on 15 September during times of local sunrise and sunset were 5:07 and 17:40, cruise KT-01-14. Water samples were collected from 9 respectively. The sea depth was 1,570 m. The volume of depths (0, 10, 20, 30, 50, 75, 100, 150 and 200 m) for water filtered was calculated using flowmeters attached chlorophyll a determination. A water sample of 200 ml to the mouth of the nets. The volume was generally larger from each depth was filtered through Whatman GF/F glass for deeper nets with a mean value of 120 m3 for 10 m fiber filter. Filters were frozen at Ð20°C. In the land labo- depth and 280 m3 for 400 m depth. E. pacifica were sorted ratory, filters were extracted in 90% acetone and the fluo- on board and preserved in 10% buffered formalin rescence was determined with a Turner Designs 10-AU- seawater. fluorometer. Juvenile and adult E. pacifica were counted under a In April, euphausiids were collected four times on dissecting microscope, but only adults were examined for 15 April by simultaneous horizontal tows with MTD nets the following items. Total length was taken from the an- (Motoda, 1971; 0.33 mm mesh aperture) at 10 depths terior tip of the rostrum to the distal end of telson using a down to 400 m, including deep tows (150, 200, 250, 300 micrometer with an accuracy of 0.1 mm. Maturity stage and 400 m), and shallow ones (10, 25, 50, 75 and 100 m) was determined according to Makarov and Denys (1981): (Table 1). The towing duration ranged from 10 to 20 min- Males

694 Y. Endo and F. Yamano Table 1. Positions and times of samplings with MTD nets during KT-01-03 and KT-01-14 cruises of R.V. Tansei Maru.

Depth Start Finish Date Time Latitude (°N) Longitude (°E) Date Time Latitude (°N) Longitude (°E) KT-01-03 1 deep 15 Apr. 0:25 38-59.7 144-00.0 15 0:35 38-59.4 144-00.1 shallow 15 1:12 38-59.1 143-59.7 15 1:22 38-59.0 143-59.3 2 deep 15 6:28 38-59.5 143-59.7 15 6:48 38-58.9 143-59.2 shallow 15 7:23 38-58.1 143-59.1 15 7:38 38-57.6 143-58.9 3 deep 15 12:20 38-59.6 143-59.9 15 12:50 38-58.8 143-58.8 shallow 15 13:21 38-58.4 143-58.5 15 13:41 38-58.0 143-57.9 4 deep 15 18:23 38-59.6 144-00.4 15 18:43 38-58.9 144-00.2 shallow 15 19:16 38-58.4 144-00.0 15 19:36 38-57.8 143-59.7

KT-01-14 1 deep 14 Sep. 7:06 40-30.0 142-59.8 14 7:26 40-30.0 143-00.4 shallow 14 7:57 40-30.0 143-00.5 14 8:17 40-30.0 143-00.9 2 deep 14 12:49 40-30.5 142-59.7 14 13:09 40-30.9 143-00.2 shallow 14 13:38 40-31.2 143-00.4 14 13:58 40-31.2 143-01.0 3 deep 14 18:52 40-30.0 143-00.0 14 19:12 40-29.8 143-00.6 shallow 14 19:42 40-29.7 143-00.9 14 19:57 40-29.5 143-01.4 4 deep 14 21:10 40-29.9 143-00.0 14 21:30 40-29.7 143-00.6 shallow 14 22:00 40-29.6 143-00.4 14 22:15 40-29.5 143-00.8 5 deep 15 0:50 40-30.1 142-59.6 15 1:10 40-30.2 142-59.9 shallow 15 1:35 40-30.2 142-59.9 15 1:50 40-30.2 143-00.2

IIA1: petasma single-lobed, 0 = 0; class 1 = 12.5; class 2 = 37.5; class 3 = 62.5; class IIA2: petasma two-lobed, but without wing, 4 = 87.5). IIA3: petasma with wing, Finally, the molt stage of each individual was deter- IIIA: petasma fully developed but without fully mined by examining either antennal scale or uropod un- formed spermatophores, der a light microscope. We followed Buchholz (1982, IIIB: fully formed spermatophores present. 1991) in determining the molt stage and classified the Females following five stages: IIB: developing thelycum present, A: Early postmolt stage, body soft, tissue granular or IIIA: thelycum fully developed but no few structures, spermatophores attached, BC: Late postmolt stage, body hard, stripe pattern present IIIB, C: spermatophores present, carapace not swol- in tissue, no epidermal pockets, len, D1: Early premolt stage, body hard, stripe pattern and IIID: spermatophores present, carapace swollen, invaginations present in tissue, no second cuticle, IIIE: spermatophores present, carapace with hol- D2: Late premolt stage, body hard, second cuticle present, low space owing to recent spawning. D3: Immediate premolt stage, body soft, all cuticle struc- Stomach fullness was determined according to tures doubled. Ponomareva (1963): class 0, empty stomach; class 1, up The system is based on macroscopic integumental changes to 25% full; class 2, up to 50% full; class 3, up to 75% and the development of setae originally proposed by full; and class 4, up to 100% full. A stomach fullness in- Drach (1939). Just after the actual molt, the new chitinous dex (SFI) was calculated as the mean stomach fullness at cuticle has no further layers (early postmolt), after which each station: the cuticle hardens and several layers are added. The in- tegument is then completed and the epidermis is no longer active (late postmolt). The epidermis becomes active again SFI=×∑∑∑() Fn Fm100 ∑ Fn ij j ij and secretes a new cuticle (premolt). As we used preserved i j i j samples, we classed D0 stage, the earliest premolt stage, in the BC stage according to Tarling et al. (1999) because where Fn is the abundance of individuals of stomach ij of the disturbance caused by formalin to euphausiid tis- fullness class j (number/1000 m3) at depth i, and Fm is j sues. Specimens were examined within 5 months after the median of each stomach fullness class j (i.e., class

DVM of Euphausia pacifica in Relation to Molt and Reproductive Processes 695 a) 1:00 7:00 13:00 19:00 10 25 50 75 100 150 200 Depth (m) 250 300 400

0 1000 2000 0 20 40 0 20 40 0 2000 4000

b) 7:30 13:30 19:30 22:00 1:30

10 20 30 50 75 100 Depth (m) 150 200 300 400

0 50 100 0 100 200 0 200 400 0 200 400 0 100 200 Individual number (No. 1000 m-3)

Fig. 3. Vertical distribution of Euphausia pacifica at each sampling in April (a) and September (b) 2001. Sampling times shown are approximate midpoint of deep and shallow tows at each sampling.

collection. The results are displayed using a simplified They occurred only at 10 m depth at 19:00 hours with a system proposed by Tarling et al. (1999), as follows: density of 2342 indiv. 1000 mÐ3. The range of their DVM molting; D3 and A, postmolt; BC, and premolt; D1 and was only 100Ð150 m in April. D2. Postmolt and premolt were sometimes treated collec- The vertical distribution was generally deeper in tively as non-molting. September. The creatures occurred only at 300 and 400 m depths at 07:30 and 13:30 hours with a peak occur- 3. Results rence at 400 m. It is possible that part of their population was distributed deeper than the deepest sampling depth, 3.1 Hydrography 400 m. At 19:30 and 22:00 hours they occurred through- In April the surface water temperature ranged from out a very wide depth range from 10 to 400 m, with a 10 to 12°C with a seasonal thermocline at 20Ð40 m depth peak occurrence at 50 m depth. Their density at the peak (Fig. 2). In September a well developed seasonal was 220 and 346 indiv. 1000 mÐ3 at 19:30 and 22:00 hours, thermocline was present at 20Ð50 m depth. The surface respectively. At 01:30 hours they distributed from 10 to 20 m was well mixed because of fall cooling with a tem- 200 m depth with peaks at 10 m (140 indiv. 1000 mÐ3) perature of 19°C. and 75 m depth (145 indiv. 1000 mÐ3). The range of their Chlorophyll a concentration was higher in April than DVM was larger than in April, up to 400 m. in September with maximum values of 2.1Ð3.3 µg lÐ1 in E. pacifica was more abundant in the nighttime than the top 25 m (Fig. 2). In September the chlorophyll a in daytime: when summed over the sampled depth layers maximum was in the subsurface layer, 30Ð60 m, with a the difference was 10.6 times in April and 1.3 times in value of 0.57Ð0.8 µg lÐ1. September. It is probable that the difference was caused by net avoidance by large individuals during the day in 3.2 Vertical migration and molt stage April, when larger individuals predominated. In April the In April the vertical distribution of E. pacifica was total length of females has a large peak at 18 mm and a generally shallower throughout the day than in Septem- small one at 12 mm. Male lengths peaked at 17 mm and ber (Fig. 3). At 01:00 hours their occurrence peaked at 10 the length-frequency distribution was skewed to the left. m depth with a density of 1287 indiv. 1000 mÐ3. Occur- In September, the total length of both sexes showed a bi- rence was restricted to 100 and 150 m depths at 07:00 modal distribution: a large peak at 11 mm and a small hours with densities of 20 and 30 indiv. 1000 mÐ3, re- one at 17Ð18 mm. spectively. At 13:00 hours they occurred at a similar den- Molting and non-molting E. pacifica showed differ- sity from 100 to 200 m depth with a peak at 100 m depth. ent diel vertical migration patterns. In April most of the

696 Y. Endo and F. Yamano 1:00 7:00 13:00 19:00

a) 10 25 50 75 100 Non-molting 150 200 250 300 400

0 1000 2000 0 20 40 0 20 40 0 2000 4000

Depth (m) 10 25 50 75 100 150 Molting 200 250 300 400

0 20 40 0 5 10 0 5

7:30 13:30 19:30 22:00 1:30

b) 10 20 30 50 75 Non-molting 100 150 200 300 400

0 50 100 0 100 200 0 200 400 0 200 400 0 100 200 Depth (m) 10 20 30 50 75 Molting 100 150 200 300 400

0 10 20 0 5 10 0 20 40 0 10 20 0 20 40 Individual number (No. 1000 m-3)

Fig. 4. Vertical distribution of molting and non-molting Euphausia pacifica at each sampling in April (a) and September (b) 2001.

non-molting E. pacifica migrated to the upper 25 m at 3.3 Vertical migration and maturity stage 01:00 and 19:00 hours (Fig. 4(a)). However, molting in- The mean depth of occurrence for each maturity stage dividuals showed rather uniform occurrence from the sur- was calculated as the sum of the product of abundance face to 200 m depth at 01:00 hours. Molting euphausiids (indiv. 1000 mÐ3) and depth (m) divided by the sum of were not found at 19:00 hours. Molting and non-molting abundances in all the sampling layers. In April all the euphausiids resided at a similar depth in the daytime maturity stages were in the upper 30 m except for IIIA (07:00 and 13:00 hours), 100Ð200 m. females at night (Fig. 5(a)). During the day, mature indi- In September, molting and non-molting E. pacifica viduals were generally found in deeper layers: the mean occurred at 300 and 400 m depths in the daytime (07:30 depth of the most mature individuals, IIIB males and IIID and 13:30 hours, Fig. 4(b)). Non-molting individuals mi- females, was 180 m. A similar vertical distribution pat- grated to depths as shallow as 10 m at night with their tern was found in September, but their distribution depth peak occurrence at 50 m depth at 19:30 and 22:00 hours was apparently deeper both day and night, especially dur- and at 10 m at 01:30 hours. However, no molting indi- ing the daytime: 20Ð80 m at night and 300Ð400 m during viduals were found at depths shallower than 50 m. the day (Fig. 5(b)). The DVM range was large for mature

DVM of Euphausia pacifica in Relation to Molt and Reproductive Processes 697 Male Female 1:00 7:00 13:00 19:00 a) 0.51 0.23 0.27 0.53 IIA1 IIA2 IIA3 IIIA IIIB IIB IIIA IIIBC IIID 0

50

100

150

200

1:00 7:00 Depth (m) 250 13:00 19:00 300 4 3 2 350 1 0 400 7:30 13:30 19:30 22:00 1:30 IIA1 IIA2 IIA3 IIIA IIIB IIB IIIA IIIBC IIID 0.22 0.24 0.56 0.64 0.71 b) 0

50

100

150 7:30 13:30 19:30 200 22:00 01:30

Depth (m) 250

300

350

400

Fig. 5. Mean depth of occurrence of each maturity stage of male and female Euphausia pacifica at each sampling time Fig. 6. Composition of stomach fullness classes at each depth in April (a) and September (b) 2001. at each sampling time in April (a) and September (b) 2001. Italic figures under sampling times show stomach fullness index (SFI). individuals in both seasons. The timing of upward mi- gration seemed to differ among different maturity stages. Mature individuals began upward migration later than less in the daytime (7:30 and 13:30 hours) but high at night mature ones because at 19:30 IIA1ÐIIIA males and IIB (19:30, 22:00 and 1:30 hours) (Fig. 6(b)). At 19:30 hours, and IIIA females were in the upper 100 m layer while class 4 individuals occurred in the upper 75 m, attaining IIIB males and IIIBC and IIID females were still in the 86% at 10 m depth but were 50% or less at 20, 50 and 75 deeper layers (Fig. 5(b)). m. At 22:00 hours class 4 individuals were also found in the deeper layers, 100 m and 400 m, and at 1:30 hours the 3.4 Vertical migration and stomach fullness contribution of class 4 individuals generally increased in Stomach fullness was generally low in the daytime each depth layer, reaching 100% at 20 and 30 m depth. in April with no class 4 individuals (Fig. 6(a)). Class 4 SFI was 0.22Ð0.24 in the daytime and increased from 0.56 individuals occurred at 19:00, increasing to 54% at 25 m at 19:30 to 0.71 at 1:30 hours. depth at 1:00. Deeper than 75 m stomach fullness was low even at 1:00 hours. The stomach fullness index (SFI) 3.5 Molt stage and maturity stage was 0.23Ð0.27 in the daytime and 0.51Ð0.53 in the Post-molt individuals dominated among both males nighttime. In September, stomach fullness was low again and females in April, except for IIA3 males and IIIA fe-

698 Y. Endo and F. Yamano Male Female 0.8 a) a) b) 0.7 IIIB Non-molt IIID Molt 0.6 IIIA IIIB,C 0.5

IIA3 SFI 0.4

0.3 molt IIIA IIA2 post-molt pre-molt 0.2

IIB IIA1 0.1

0 20 40 60 80 100 0 20 40 60 80 100 0 1 7 13 19 7 13 19 22 1 b) Time of day (hours)

IIIB IIID Fig. 8. Stomach fullness index (SFI) of molting and non-molting Euphausia pacifica at each sampling time in April (a) and IIIA September (b) 2001. IIIB,C

IIA3

IIIA significant in April (ANOVA, p = 0.0554) but significant IIA2 in September (ANOVA, p = 0.0209). For non-molting individuals, the mean SFI was higher at night than in the IIB daytime: nighttime SFI was about 0.5 in April and 0.6Ð IIA1 0.76 in September and daytime SFI was about 0.3 in April 0 20 40 60 80 100 0 20 40 60 80 100 and 0.24 in September. However, the mean SFI was low Percentage throughout the day in molting individuals, ranging from 0.06 to 0.22 in April and from 0.1 to 0.29 in September. Fig. 7. Composition of molt stages in each maturity stage of male and female Euphausia pacifica in April (a) and Sep- 4. Discussion tember (b) 2001. The vertical distribution of E. pacifica was gener- ally deeper during both day and night in September than in April (Fig. 3). At nighttime, they were most abundant males (Fig. 7(a)). Pre-molt individuals dominated in the at 10Ð25 m in April while in September they were segre- latter cases. Among males, the percentage of molting in- gated between 10 m and 50Ð75 m. This difference can be dividuals decreased with increasing maturity, with the explained in terms of water temperature and the depth of smallest contribution (2.8%) in the most mature individu- the chlorophyll maximum. The upward migration of this als, IIIB. The contribution was also lowest (1.9%) in most species is reported to be restricted by a warmer water tem- mature females, IIID. In September post-molt individu- perature of about 20°C (Iguchi et al., 1993). It is there- als predominated again among both males and females. fore probable that E. pacifica could not come up to or The exception was IIIB,C females, with pre-molt indi- could not stay longer at the surface in September. On the viduals dominating. Among females, the contribution of other hand, chlorophyll a concentration showed a sub- molting individuals was also lowest (4.3%) in IIID stage. surface maximum at 30Ð50 m depth. Part of the E. pacifica Among males, the contribution was lower in IIA3 and population may have stayed in this layer and did not as- IIIA than in other stages. cend to the upper layer in September. The daytime distribution of E. pacifica in April was 3.6 Molt stage and stomach fullness also shallower than in September. A similar seasonal dif- Figure 8 shows the temporal change in the stomach ference was reported by Taki et al. (1996) who showed fullness index (SFI) of molting and non-molting individu- that E. pacifica was concentrated in the shallow areas als. SFI of molting individuals was generally lower than (<150 m) and pelagic layer during both day and night that of non-molting individuals. The difference in SFI and their DVM was not active in spring, while in the other between molting and non-molting individuals was not seasons they move out to the upper continental slope re-

DVM of Euphausia pacifica in Relation to Molt and Reproductive Processes 699 gions and become benthopelagic during the day. Resi- Table 2. Percentage of molting individuals at each sampling dence in the shallow layers may be related to their re- depth at each sampling time in April and September. productive behavior, as suggested by Endo (1984), who showed in an LHPR survey that recently spawned eggs April were present in the surface layer. Sampling time Upward migration of molting individuals was re- Depth (m) 1:00 7:00 13:00 19:00 stricted compared with non-molting ones (Fig. 4). A simi- lar phenomenon was reported for Meganyctiphanes 10 1.67 ——0.00 norvegica by Tarling et al. (1999), who pointed out three 25 1.64 ——— possible reasons for this. First, there is not enough en- 50 ———— ergy to perform vertical migration during the period of 75 66.67 ——— increased basal metabolism needed for molting. Secondly, 100 — 0.00 16.67 — they cannot feed because the stomach lining is also exu- 150 75.00 28.57 33.33 — 200 50.00 — 12.50 — viated. Thirdly, molting individuals have a soft body and 250 100.00 ——— are vulnerable to predation, including cannibalism. These 300 ———— hypotheses may hold true for E. pacifica. Dexter (1978) 400 ———— compared the molt stage composition of E. pacifica be- tween two sampling depths off Oregon based on her stag- ing system (Dexter, 1981), which consists of only three September stages (premolt and ecdysis, postmolt, and intermolt), Sampling time which has not been adopted by later workers. She found no significant difference in molt stage composition, prob- Depth (m) 7:30 13:30 19:30 22:00 1:30 ably because she compared the composition only in the 10 ——0.00 0.00 0.00 shallow layers: 20 m vs. 50 m and 10 m vs. 50 m. 20 ——0.00 0.00 0.00 Tarling et al. (1999) showed that molting occurred 30 ———0.00 0.00 in the deep in the nighttime for M. norvegica. The present 50 ——7.50 3.51 0.00 study found molting individuals generally distributed in 75 ——15.38 4.76 18.75 deeper layers and the percentage of molting individuals 100 ——40.00 16.67 25.00 (molt %) was generally high in deeper layers at night: a 150 ——0.00 50.00 0.00 higher than 50% molt % occurred at 75 m or deeper in 200 ——50.00 — 0.00 April, with more than 17% at 100 m or deeper in Septem- 300 26.67 7.14 100.00 —— ber (Table 2). Therefore, molting of E. pacifica may also 400 11.11 7.41 0.00 0.00 — occur in deeper layers. However, it was not certain that they molt in the nighttime because the mean molt % at each sampling time (summed molting individuals over mesozooplankton. This suggestion of rapid sinking of sampling layers divided by summed total individuals over satiated is supported for this species by Nakagawa sampling layers × 100) was larger in the daytime than in et al. (2003), for Meganyctiphanes norvegica by Simmard the nighttime in both seasons. The net avoidance of ac- et al. (1986), for E. lucens by Gibbons (1993) and for tive non-molting individuals in the daytime may have copepods by Simmard et al. (1985) and Atkinson (1992). affected the results. Surveys with larger nets are neces- The feeding activity of molting individuals was re- sary to resolve this question. duced, with similar SFI in the daytime and nighttime (Fig. Non-molting E. pacifica showed high feeding activ- 8), which suggests that molting substantially reduces feed- ity at night and low activity in the daytime, as shown by ing, irrespective of time of the day. However, the feeding Nakagawa et al. (2003). In September, when samples were activity may be influenced by food abundance at the resi- obtained more frequently than in April, satiated individu- dence depth. Therefore, the mean stomach fullness of als (class 4) appeared first in the surface layer at 19:30 molting and non-molting individuals at the same depth(s) hours (Fig. 6). Satiated individuals also appeared in deeper was compared (Table 3). Statistical analysis (ANOVA) layers at 22:00 hours, and their percentage increased in was done on pooled data at 150 m depth at 7:00 hours nearly all the sampling layers at 1:30 hours. This sug- and at 100, 150 and 200 m depth at 13:00 hours for day- gests that satiated animals moved down to deeper layers time in April, at 10 and 25 m depth at 1:00 hours for after feeding in the surface layer, although we only know nighttime in April, at 300 and 400 m at 7:30 hours and that the chlorophyll peak was at 30Ð60 m depth but we 13:30 hours for daytime in September, and at 50 and 75 have no data on the vertical distribution of other food m depth at 19:30 hours and 22:00 hours, and at 75 and organisms such as microzooplankton and 100 m depth at 1:30 hours for nighttime in September.

700 Y. Endo and F. Yamano Table 3. Mean stomach fullness of non-molting and molting individuals in the daytime and nighttime for April and September.

Season day/night Molt condition n Stomach fullness p April daytime non-molting 21 2.48 0.1149 molting 6 1.83 nighttime non-molting 119 3.63 0.2648 molting 2 2.12

September daytime non-molting 65 2.40 0.2127 molting 9 2.00 nighttime non-molting 150 4.25 <0.0001 molting 14 2.57

The mean stomach fullness was always higher in non- lasted for only 10 to 20 s. (Buchholtz, 1991). A slightly molting individuals than in molting ones. However, a different time scale was reported for Meganyctiphanes significant difference was found only during nighttime norvegica (Cuzin-Roudy and Buchholtz, 1999): 5.1 d in September, when the number of molting individuals (39.8%) for postmolt, 5.5 d (42.9%) for premolt and 2.2 exceeded 10. Perhaps we need more molting individuals d (17.2%) for molting individuals with a total of 12.8 d. to give us a precise understanding of feeding activity. In the present study, the percentage of molting indi- The range of DVM was large for mature individuals viduals was least in IIID females in both April and Sep- of both sexes (Fig. 5). This may mean that larger indi- tember, which suggests that gravid females molt less fre- viduals perform long range DVM because mature indi- quently than other stages of females. The same trend was viduals are generally larger than less mature ones. Ma- seen in E. superba (Cuzin-Roudy, 1987) and M. norvegica ture females, IIID, migrated up to the shallower layer more (Cuzin-Roudy and Buchholtz, 1999), although a differ- frequently than any other stages of females both in April ent maturity staging system based on histological exami- and September, although mature males and females, in- nation of the ovary was adopted in the latter species. An cluding stage IIID, took longer to reach the shallowest extended intermolt period (60Ð80% longer) in breeding layer. This may imply that mature females come up to the females has been recorded for a considerable number of surface to spawn. As previously mentioned, Endo (1984) crustaceans (Hartnoll, 1985). More energy is poured into suggested that the spawning of E. pacifica occurs in the reproductive activity than molting in these females. surface layer during the swarming season of this species. Cuzin-Roudy (1987) also showed that in E. superba most Tarling et al. (1999) reported that ready-to-spawn (blue- of the spawning females were in the D0 stage, which is gray appearance due to the presence of mature oocytes in the first premolt stage just after apolysis, but which is the ovary) female M. norvegica are concentrated in the treated as postmolt stage in the present study, as explained uppermost layers at night. De Robertis et al. (2000) re- in the Materials and Methods section. In M. norvegica, ported that smaller E. pacifica began upward migration molting and spawning are reported to be coupled and the 30 minutes before larger individuals and began downward first spawning was synchronized with D0 and the second migration 45 minutes before large individuals, and they one with D1 stage within a molt cycle (Cuzin-Roudy and attributed this to the changes in trade-off with body size Buchholz, 1999). We used preserved individuals and between predator avoidance and feeding behavior: namely therefore could not identify ready-to-spawn individuals, smaller individuals are less conspicuous and stay longer so it is not known at which molt stage E. pacifica spawns. in the surface food-rich layer, while larger conspicuous Examination of fresh specimens is needed. The relation- individuals have a shorter stay there. ship between spawning and molting is not known for other The percentages of individuals in each molt stage species of euphausiids. have been reported for female E. superba (Cuzin-Roudy, Nelson (1991) reviewed the relationship between 1987): 56% were postmolt individuals, 30.5% were reproduction and molting in malacostracan crustaceans. premolt ones, and 13.5% were molting ones. These val- In brooding malacostracans, except in the stomatopods, ues are similar to those found in the present study for E. mechanisms are developed so as not to lose the clutch pacifica. The time scale of molt stages has been reported with exuvia. In most peracaridans, carideans and for subadult E. superba with body length of 27Ð37 mm anomurans, the solution to this problem is to position (Buchholtz, 1991): 7.1 d (43.3%) for postmolt, 6.2 d extrusion of the clutch as early in the molt cycle as possi- (37.8%) for premolt, and 3.1 d (18.9%) for molting indi- ble, to molt stage A or B. A sufficiently long molt cycle is viduals with a total of 16.4 d. Ecdysis, the actual molt, another means of ensuring that the clutch is not lost with

DVM of Euphausia pacifica in Relation to Molt and Reproductive Processes 701 the exuvia, which is adopted by many reptant crustaceans. Euphausia lucens at two 72 h stations in the southern In the non-brooding Eucarida, including most euphausiid Benguela upwelling region. Mar. Biol., 116, 257Ð268. species, eggs are shed freely. In this group there may be Hartnoll, R. G. (1985): Growth. p. 111Ð196. In The Biology of more than one spawning within a molt cycle (Nelson, Crustacea, Vol. 2, ed. by L. G. Abele, Academic Press, New 1991), and that proved to be true for E. superba (Cuzin- York. Iguchi, N. (1995): Spring diel migration of a euphausiid Roudy, 2000) and M. norvegica (Cuzin-Roudy and Euphausia pacifica in Toyama Bay, southern Japan Sea. Buchholtz, 1999), although a complete cycle of egg pro- Bull. Japan Sea Natl. Fish. Res. Inst., 45, 59Ð68 (in Japa- duction is likely to last more than one molt cycle. How- nese with English abstract). ever, it is not known whether E. pacifica shows the same Iguchi, N. and T. Ikeda (1995): Growth, metabolism and growth reproductive and molting relationship. Detailed surveys efficiency of a euphausiid Euphausia pacifica of reproductive and molting activities are needed. in the southern Japan Sea, as influenced by temperature. J. Plankton Res., 17, 1757Ð1769. Acknowledgements Iguchi, N., T. Ikeda and A. Imamura (1993): Growth and life We would like to express our thanks to Prof. A. cycle of a euphausiid crustacean (Euphausia pacifica Taniguchi for encouragement throughout this study. Criti- Hansen) in Toyama Bay, southern Japan Sea. Bull. Japan cal comments of two anonymous referees were quite help- Sea Natl. Fish. Res. Inst., 43, 69Ð81. Jerde, C. W. and R. Lasker (1966): Moulting of euphausiid ful in improving the manuscript. Thanks are also given shrimps: shipboard observations. Limnol. Oceanogr., 11, to the captain and crew of R.V. Tansei Maru. 120Ð124. Longhurst, A. R. (1976): Vertical migration. p. 116Ð137. In The References Ecology of the Seas, ed. by D. H. Cushing, J. J. Walsh and Atkinson, A. (1992): Diel vertical migration and feeding of W. B. Saunders, Philadelphia, Pennsylvania, U.S.A. copepods at an oceanic site near South Georgia. Mar. Biol., Makarov, R. R. and C. J. Denys (1981): Stages of sexual matu- 113, 583Ð593. rity of Euphausia superba DANA. BIOMASS Handbook, Bollens, S. M., B. W. Frost and T. S. Lin (1992): Recruitment, 11, 1Ð11. growth, and diel vertical migration of Euphausia pacifica Motoda, S. (1971): Devices of simple plankton apparatus, V. in a temperate fjord. Mar. Biol., 114, 219Ð228. Bull. Fac. Fish., Hokkaido Univ., 22, 101Ð106. Buchholz, F. (1982): Drach’s molt staging system adapted for Nakagawa, Y., Y. Endo and H. Sugisaki (2003): Feeding rhythm euphausiids. Mar. Biol., 66, 301Ð305. and vertical migration of the euphausiid Euphausia pacifica Buchholz, F. 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702 Y. Endo and F. Yamano Tohoku Natl. Fish. Res. Inst., 58, 89Ð104 (in Japanese with Terazaki, M., D. Kitagawa and Y. Yamashita (1986): Occur- English abstract). rence of Euphausia pacifica Hansen (Crustacea: Tarling, G. A., J. Cuzin-Roudy and F. Buchholz (1999): Verti- Euphausiacea) with spermatophore in the vicinity of cal migration behaviour in the northern krill Otsuchi, northeastern Japan. Bull. Jpn. Soc. Scient. Fish., Meganyctiphanes norvegica is influenced by moult and re- 52, 1355Ð1358. productive processes. Mar. Ecol. Prog. Ser., 190, 253Ð262.

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