Journal of Oceanography, Vol. 62, pp. 693 to 703, 2006 Diel Vertical Migration of Euphausia 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 (2–4%) 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 fish (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 krill, 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 crustaceans 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 l–1) 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 9– ble 1). The local sunrise and sunset occurred at 4:50 and 17, 2001) and cruise KT-01-14 (September 12–17, 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 –20°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).