The Temperature Effect on the Development of Calanoid Copepod, Acartia tsuensis, with Some Comments to Morphogenesis

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1 Journal of Oceanography Vol. 52, pp. 125 to The Temperature Effect on the Development of Calanoid Copepod, Acartia tsuensis, with Some Comments to Morphogenesis TAKASHI TAKAHASHI 1 and ATSUSHI OHNO 2 1 Joetsu Environmental Science Center, 231-2, Shimomonzen, Joetsu-shi 942, Japan 2 Tokyo University of Fisheries, Minato-ku, Konan, 4-5-7, Tokyo 108, Japan (Received 18 May 1995; in revised form 1 September 1995; accepted 4 September 1995) The temperature effect on the development of Calanoid copepod, Acartia tsuensis, was investigated in the laboratory. A. tsuensis could develop from eggs to adults within the water temperature range of C. The relationship between the duration required to attain a developmental stage i (Di; days) and water temperature (T; C) was expressed as Di = Ai (T 1) 2.05, where Ai is a coefficient obtained for the stage i. A. tsuensis developed isochronally at C, while the instar duration of advanced nauplii and copepodids tended to be longer at 17.5 C and 30.0 C. Quadratic equations could be applied to express the relationship between stage-specific mortality coefficients and water temperature with the minimum mortality observed at 22.5 C from egg to nauplius stage 3 (N-3) and at 25.0 C from N-3 to copepodide stage 6. Fecundity at C was larger ( eggs/ female/day) than that at C (0 2.6 eggs/female/day). These observations indicate that A. tsuensis adapts well physiologically in a warmwater environment with an optimal water temperature of about 25 C. The study includes morphological description of A. tsuensis, because the species seems to have been often confused with other Acartia species, such as A. omorii occurring in a similar locality, due largely to lack of the information on the morphological characters of the nauplii and early copepodids. 1. Introduction Marine copepods of the genus Acartia are commonly distributed in coastal waters. Since the Acartia species can be propagated in tanks and ponds, the possibility of their use as food organisms for fish larvae has been proposed by several authors (Ikeda, 1973; Omori, 1973). Acartia tsuensis was first identified by Ito (1956) from the brackish fish culture ponds in Mie Prefecture, in the central part of Japan. This species is common at unused salt production ponds in the Inland Sea of Japan. A series of studies on the propagation and population dynamics of this species has been carried out with the aim of elucidating on its ecological importance as an early food organism for the red sea bream, Pagrus major, which is reared extensively in coastal ponds (Ohno and Okamura, 1988; Ohno et al., 1990). Ohno (1992) proposed the feasibility of extensive mass seed production of Pagrus major using A. tsuensis as main food organism. The effect of water temperature on hatching, growth, and fecundity, among other things, have been well investigated in some Acartia species such as A. clausi (Landry, 1978), A. tonsa (Heinle, 1969) and A. calforniensis (Johnson, 1980). However, optimum conditions for the development of A. tsuensis are not well known (Ohno and Okamura, 1988; Ohno et al., 1990). Considering the lack of morphological knowledge on each developmental stage of this species is thought, to form a great barrier against the study on the developmental modes in different

2 126 T. Takahashi and A. Ohno environmental characters. We describe the morphological definition of each stage from nauplii to adult first. The major purpose of this study is to describe the developmental morphology of each stage and to investigate the effects of water temperature on the development of A. tsuensis based on rearing experiments in the laboratory, with special attention to the relationship between water temperature and mortality, fecundity and developmental rates of the species. 2. Materials and Methods 2.1 Morphological observation Acartia tsuensis and A. omorii were collected from a 0.75-ha coastal saltwater pond located at the Momoshima Station of the Japan Sea Farming Association (JASFA), at Hiroshima Prefecture in July 1984 and again in April The specimens for morphological observation were preserved in 5% formalin seawater. Total length (TL) and body width were measured in the nauplii and the length and width of the caudal ramus for the copepodids. At least 20 individuals were measured for each developmental stage. 2.2 Rearing experiments Adult Acartia tsuensis were collected in the pond at Momoshima Station of the Japan Sea Farming Association (JASFA), at night when they were aggregated at water surface under a torch light on 23 June They were then transfered into a 70-l polycarbonate tank for spawning. The eggs spawned within three hours after incubation were collected from the tank bottom and kept in filtered cold seawater at 6 10 C. On 2 July 1985, the eggs were transferred to the Tokyo University of Fisheries, Tokyo, Japan, for rearing experiments, as described below. The eggs were incubated in 1-l beakers with the density of 150 eggs per beaker containing filtered seawater (salinity, 16.6 ppt). Twenty beakers with 150 eggs were then placed in each temperature controlled water bath. Six water baths were prepared at 17.5, 20.0, 22.5, 25.0, 27.5 and 30.0 C, respectively. Variations in temperature of each water bath was within ±0.5 C of the set temperature during the experiment. The unicellar green algae, Nannochloropsis oculata, was offered to nauplii and copepodids at the density of cells ml 1 daily. This density is an excessive food condition for A. tsuensis (Ohno et al., 1990). About two thirds of the water in the beakers was replaced with filtered seawater every five days through a 40-µm mesh using a pipette. Light condition during the experiment was µe m 2 s 1 at surface of water and 12:12 light-dark photoperiod. One beaker was randomly chosen daily from each water bath and all individuals therein were preserved in 5% seawater formalin. The number of individuals were then counted at each developmental stage. The eggs used to measure the hatching time were obtained from the adults reared at 25 C. The eggs were then incubated at 150 eggs/10-ml bowl. Bowls were used in six different water temperatures, respectively. The number of hatched nauplii were counted at 2 4 hour intervals. Incubation time was estimated from 50% individual hatching time. 3. Results 3.1 Morphological development of Acartia tsuensis Acartia tsuensis were observed to develop through six nauplius stages (nauplius stage 1 to

3 The Temperature Effect on the Development of Acartia tsuensis 127 6; N-1 to N-6) and six copepodid stages (copepodid stage 1 to 6; C-1 to C-6) where in C-6 being adults Nauplius stages Nauplius Stage 1 (N-1) (Fig. 1a) Total length mm, average mm (n = 20). The antennules, antennae, and mandibles are rudimentary. In the antennule, there were three terminal setae on the distal segment. The caudal armature is represented by a pair of end setae existed at the posteroventral part of the body. Nauplius Stage 2 (N-2) (Fig. 1b) Total length mm, average mm (n = 21). There are four terminal setae in the antennule on the distal segment and setae is added at the middle of the proximal segment. The end setae are accompanied by a pair of small lateral hooks. Nauplius Stage 3 (N-3) (Fig. 1c) Total length mm, average mm (n = 25). The maxillule is present posterior to the mandible. A pair of slight end hooks are present at the posteroventral end of the body. Fig. 1. Acartia tsuensis, naupliar stages: a d, stage 1 4; e f, stage 5; g h, stage 6.

4 128 T. Takahashi and A. Ohno Nauplius Stage 4 (N-4) (Fig. 1d) Total length mm, average mm (n = 20). A pair of relatively large ventral hooks are present anteriorly to the end hooks. Nauplius Stage 5 (N-5) (Figs. 1e and f) Total length mm, average mm (n = 20). Caudal armature is similar to that of N-4. The rudiments of the maxilla and maxilliped are present. Nauplius Stage 6 (N-6) (Figs. 1g and h) Total length mm, average mm (n = 20). The body is slightly elongated compared to N-5. Two pairs of the first indication of swimming legs, warty in shape, are present on the posteroventral part of the body Copepodid stages When the nauplius metamorphose to the copepodid, swimming legs appear on the ventral side of the body. C-1 has two pairs of swimming legs. The number of the legs increases by one pair following the advancement of each stages up to C-4, which attains the adult complement of five pairs of swimming legs. The sex of copepodids becomes identifiable from C-4 by differences in the shape of the urosome and fifth legs. Fig. 2. Dorsal view of the urosome of copepodid and adult of Acartia tsuensis. a b, stage 1 3; d f, male of stage 4, 5 and 6; g i, female of stage 4, 5 and 6.

5 The Temperature Effect on the Development of Acartia tsuensis 129 In C-1, four setae are present on each caudal ramus (Fig. 2a). The number of setae increases with development, being five in C-2 and six in C-3~C-5 (Figs. 2b e and g h). In the adult female, two setules are present on the lateral side of the caudal ramus (Fig. 2i). The lateral setule is absent in the male (Fig. 2f). Of the four terminal setae of the caudal ramus, the medial two are nearly straight in the adult female while those are curved laterally in the male. Fifth legs in the female (Figs. 3a c) unsegmented in C-4, consist of the basal segment and exopodite in C-5 and adult. In C-5 and adult, exopod is long with a lateral seta and a terminal spine. The terminal spine of C-5 is short with finely-serrated spine, about 1.5 times longer than the exopod. The terminal spine of the adult were long and gently curved, about 4 times longer than the exopod. In the adult female, the basal portion of the terminal spine was swollen, reduced posteriorly as a small knob. Fifth legs in male (Figs. 3d f) unsegmented in C-4, consist of basal segment and segmented exopod in C-5 and adult. In the adult male, the left expod is composed of two segments, while the right one of three segments that coiled internally. There is a rectangular projection in the second segment of the right expod. 3.2 Comparison of morphological characters of A. tsuensis with A. omorii Nauplius stages The body width to total length ratio is significantly lower in A. tsuensis than in A. omorii for all the six stages (P < 0.05), although the differences between the maximum ratios of the former and the minimum ratios of the latter are not large (Fig. 4). Another diagnostic difference is the shape of the end hooks from stages N-3 to N-6, being relatively slender in A. tsuensis compared to A. omorii. Fig. 3. Fifth legs of Acartia tsuensis in posterior view. a c, Fifth feet of female of stage 4, 5 and 6; d f, Fifth feet of male of stage 4, 5 and 6

6 130 T. Takahashi and A. Ohno Copepodid stages There is a significant difference in the shape of the caudal ramus of copepodids between the two species. The length to width ratio of the caudal ramus is lower in A. tsuensis than in A. omorii (P < 0.05) (Fig. 5). Such differences, however, tend to be less marked minor in the C-5 and adult (Fig. 5). The adult male of A. tsuensis possesses apparently curved setae on the caudal ramus (Fig. 2f), while they are nearly straight in A. omorii. As for the morphology of the fifth legs in C-4, C-5 and adult, each exopod segments is longer Fig. 4. The median body width to total length ratio. Circle, nauplii of Acartia tsuensis; triangle, nauplii of Acartia omorii; solid horizontal line, means the range. Fig. 5. The median length to width ratio of the caudal ramus. Solid circle, male of Acartia tsuensis; open circle, female of Acartia tsuensis; solid triangle, male of Acartia omorii; open triangle, female of Acartia omorii; solid horizontal line, means the range.

7 The Temperature Effect on the Development of Acartia tsuensis 131 and larger in A. tsuensis than in A. omorii (Fig. 3). In the adult female of A. tsuensis, the terminal spine of the exopod of fifth legs is apparently more slender than in A. omorii. In the adult male of A. omorii, the inner projection developed in the second segment of the right exopod is bifurcated, while it is not bifurcated in A. tsuensis (Fig. 3f). 4. Development 4.1 Development time Acartia tsuensis was possible to develop from egg to adult at all water temperature examined from 17.5 to 30.0 C. The hatching success were approximately 62% at 30.0 C and more than 70% at C. The outline of the stage-specific developmental sequence is shown for individuals reared at 25.0 C in Fig. 6. Most of N-1 hatched within one day after the introduction of eggs (day 1) and developed to N-2 by day 4. Individuals which grew fast reached N-5 on day 5, C-1 on day 6 and adult on day 10. From day 11, immediately after the first appearance of females, the number of eggs in the beaker increased, indicating that the females began to spawn in the beaker. Development of nauplii from these eggs was also observed, but no nauplius was observed to grow to copepodid in this experiment. The elapsed time in days at which 50% individuals developed into the given stages (we called median development time ) was calculated for each water temperature examined (Fig. 7). In water temperatures under 27.5 C, the development time was shortered with temperatures. In all water temperatures examined, the instar duration of N-1 tended to be shorter than those of other stages, while the duration of N-2 was obviously longer than others (Fig. 7). The instar duration of N-3 to C-6 was more or less constant at water temperature from 20.0 C to 27.5 C, indicating that isochronal development was employed during these stages. The relationship between the cummulative number of stages attained (i), expressed as i = 1 (N-1) to 12 (C-6), and the mean development time for N-3 to C-6 at each water temperature was well regressed using linear equations, with regression coefficients ranging from (Fig. 7). Fig. 6. Stage-specific number of individuals of Acartia tsuensis reared at 25 C. Shaded part in the copepodid 4 6 stages denote females. The unit of scale at the top of the figure is 50 individuals in solid portion and 500 individuals in dotted portion.

8 132 T. Takahashi and A. Ohno Fig. 7. Relationship between the developmental stages and median development time (in days) of Acartia tsuensis reard at 17.5 C (solid square), 20.0 C (open triangle), 22.5 C (solid circle), 25.0 C (open square), 27.5 C (solid triangle) and 30.0 C (open circle). There were cases when the instar duration of certain stages was calculated to be exceptionally longer than others, i.e. N-5 at 17.5 C and C-1 at 30.0 C, indicating that isochronal development was somewhat disturbed at the lowest and highest water temperatures used in the experiment. Relationship between the mean development time of stage i (Di; days) and water temperature (T; C) was well regressed using Belehràdek s ˇ (1935) formula, as follows: Di = Ai (T 1.0) 2.05 where Ai is a coefficient obtained for stage i. Ai was calculated to be 612 for N-1, 1023 for N- 2, 2961 for N-3 and a surplus of 374 each following the advancement in developmental stage thereafter, being 6327 for C-6. The median development time for C-6 at 30.0 C obtained using the above equation was 6.4 days, shorter than the time obtained from the result of the rearing experiment (8.7 days) but was slightly longer than that obtained at 27.5 C (8.4 days) (Fig. 8). This is an indication that the development rate of this species was hindered at 30.0 C. 4.2 Mortality Survival rates of A. tsuensis calculated from initial number of eggs introduced decreased rapidly up to N-3 in all water temperatures tested, the rates being 47% at 30.0 C to about 63% at 22.5 C. Then from N-4, the mortality became comparatively low though the rates differed among the different water temperatures used (Fig. 9). Survival rates up to adult stage varied from 1.5% at 17.5 C to 57.0% at 25.0 C, which meant that only a few individuals died from N-3 to adult at a water temperature of 25.0 C. The number of individuals surviving at cummulative stage i (Ni) were expressed in the

9 The Temperature Effect on the Development of Acartia tsuensis 133 Fig. 8. Relationship between median development time (days) from eggs to each stage and rearing temperature in Acartia tsuensis reared. Fig. 9. Survival rate at different developmental stages of Acartia tsuensis reared at 17.5 C (solid square), 20.0 C (open triangle), 22.5 C (solid circle), 25.0 C (open square), 27.5 C (solid triangle) and 30.0 C (open circle). following exponential equation, for developmental stages from egg to N-3 (i = 0 3) and from N- 3 to C-6 (i = 3 12): Ni = A exp[ Mt i] (r = ) where A is a constant and Mt means a temperature-dependent mortality coefficient. The temperature-dependent mortality coefficient calculated for the stages from egg to N-3 (Mt1) was lowest at 22.5 C at 0.16, increasing slightly when water temperatures became higher

10 134 T. Takahashi and A. Ohno or lower than 22.5 C, being 0.21 and 0.24 at 17.5 C and 30.0 C, respectively (Fig. 10). The relationship between Mt1 and water temperature (T; C) could be expressed as the following quadratic equation: Mt1 = (T 22.5) 2 (r = 0.87). Likewise, the mortality coefficient for stages from N-3 to C-6 (Mt2) was lowest at 25.0 C at 0.00, increasing substantially when water temperature was changed, being 0.24 at 17.5 C and 0.12 at 30.0 C (Fig. 11). Mt2 could be approximated in relation to water temperature, as follows: Mt2 = (T 25.0) 2 (r = 0.99). Within the water temperature range examined, it can be deduced that the mortality of A. tsuensis Fig. 10. Relationship between temperature and temperature-dependent mortality (Mt1) from egg to nauplius-3 stage of Acartia tsuensis. Fig. 11. Relationship between temperature and temperature-dependent mortality (Mt2) during nauplius- 3 to copepodid-6 stage of Acartia tsuensis.

11 The Temperature Effect on the Development of Acartia tsuensis 135 Table 1. Female sex ratio (=the ratio of females to females plus males) and fecundity (number of eggs laid per day per female) of Acartia tsuensis at various temperatures. Set temperature ( C) Sex ratio of female (%) Fecundity (No./day/female) Average did not largely vary in stages up to N-3 (Fig. 10) but was highly affected by water temperature at stages from N-3 to C-6, with the minimum mortality being observed at a water temperature of 25.0 C (Fig. 11). 4.3 Sex ratio and fecundity Sex ratio and fecundity was calculated for the period when abundance of adults was highest in each water temperature. In all water temperatures tested, the female ratio (=the ratio of females to females plus males) was ranged between 54.8% at 20.0 C and 93.3% at 17.5 C (mean, 75.2%) (Table 1). Fecundity, in terms of the number of eggs increased per day per female, was relatively high at higher water temperatures, being eggs/day/female at C. At lower water temperatures of 17.5 and 20.0 C, the fecundity was eggs/day/female. 5. Discussion Acartia tsuensis may be discriminated from A. omorii through the differences in body proportion for nauplii and proportion of caudal ramus for copepodids (Figs. 4 and 5), although the differences may not be large enough to be an identification character by itself. In advanced nauplius stages, N-3 to N-6, the size of the end hooks is comparatively more slender in A. tsuensis than in A. omorii. The size of the naupliar end hook seems to be another important character for the identification of the nauplii having similar body proportions as suggested by Conovor (1956) for A. clausi and A. tonsa. Likewise, in copepodids of C-4 to C-6, tangible differences in morphology are seen in the shape of the setae of the caudal ramus and the fifth legs. Discrimination between A. tsuensis and A. omorii seem to be possible when the above characters are examined. The water temperature range in which A. tsuensis showed more than 80% hatching rates was from 20.0 to 30.0 C, which is relatively higher than that of A. clausi (=A. omorii), 5 25 C (Uye, 1980a) but corresponds to that of A. steueri at C (Uye, 1980a). In Belehràdek s ˇ (1935) equation (Di = Ai(T α) B ), applied for the relationship between the mean development time of A. tsuensis and water temperature in this study, the constant α, which represents the biological zero for the species (Uye, 1980b), was calculated as Values of the constant α obtained for other Acartia species is 8.2 for A. clausi (McLaren, 1969), 5.8 for A. clausi (=A. omorii) (Uye, 1980b) and 3.2 for A. steueri (Uye, 1980b), which were all lower than that of A. tsuensis

12 136 T. Takahashi and A. Ohno obtained in this study. Therefore, A. tsuensis is adopted to warmer environment than another Acartia species described above. Although the instar duration of N-1 is shorter than and that of N-2 is longer than the other developmental stages, A. tsuensis is considered to develop on the basis of isochronality from N- 3 to C-6 (Fig. 7) as suggested previously in our outdoor experiments (Ohno and Okamura, 1988). Nauplius stage 1 is shorter than other stages, which were observed in genus Acartia and other copepod (Miller et al., 1977; Uye, 1980b; Landry, 1983; Peterson and Painting, 1990). A similar pattern of development, namely isochronal development with relatively long instar duration of the nauplius stage 2, was observed commonly in genus Acartia species (Heinle, 1969; Johnson and Miller, 1974; Landry, 1975a, b; Miller et al., 1977; Uye, 1980a, b; Landry, 1983; Hart, 1990). The isochronal development of copepods was explained as an adaptation leading to a shorter life cycle in food abundant coastal environment and higher rate of production (Miller et al., 1977). In the present study, the instar duration of N-5 at 17.5 C and C-3 at 30.0 C were exceptionally longer than those of other stages (Fig. 7). From the viewpoint of water temperature at which A. tsuensis employed the normal development mode, the water temperature between 20.0 and 27.5 C is considered to be optimal for this species. Mortality rate of A. tsuensis was high during N-1 to N-3 and then moderate rate thereafter in all water temperatures tested (Fig. 9). The high mortality during the early stages of the nauplii seem to be due to the difficulty in the smooth shifting of energy sources from yolk-related endogenous food sources to an exogenous one as suggested for A. californiensis (Trujillo and Arroyo, 1991), based on the life table analysis. This study indicated that the mortality of A. tsuensis particularly during N-3 to C-6, was highly affected by the water temperatures applied (Figs. 10 and 11). Although no detailed examination has been made for temperature-dependent mortality in copepods, Landry (1978) observed in A. clausi that the natural mortality could be decreased by regulating food and the water temperature suitable for the species. In terms of minimum temperature-dependent mortality, mortality coefficient Mt2 in this study, water temperature of 25.0 C is said to be optimum for A. tsuensis. Fecundity of A. tsuensis obtained in this study was the highest at 25.0 C at 13.5 eggs/day/ female (Table 1). It is probably underestimated because of the cannibalism of adult and older copepodid; however, this could not be demonstrated in this study. It seems plausible that the fecundity of A. tsuensis was also affected by water temperature and that the species lay maximum eggs at water temperature of 25.0 C (Table 1). Overall, the water temperature at which A. tsuensis showed high hatching success, normal isochronal development, minimum mortality and high fecundity was obtained consistently at around 25.0 C. This water temperature corresponds to that of coastal ponds at the Inland Sea of Japan area from May to July when the species is predominant among all the Acartia species (Ohno, 1992). In conclusion, A. tsuensis seems to adapt to warm-water environment with an optimal water temperature of 25.0 C. Acknowledgements We wish to thank Mr. Keigo Maruyama and Seiichi Tsumura of the Momoshima Experimental Station of the Japan Sea Farming Association for their cooperation. We wish to thank Dr. Hiroshi Ueda, University of Ehime, for his instruction pertaining to species identification. We also thank Dr. Masanori Doi and Dr. Riza S. Ordonio-Aguilar for the critical reading of this manuscript. This study was part of a joint project with the Japan Sea Farming Association.

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