Effects of Temperature and Salinity on the Metamorphosis of Nauplius of a Planktonic Shrimp Acetes Intermedius Omori, 1975

FISHERIES SCIENCE 2002; 68: 117–122

Original Article

Effects of temperature and salinity on the metamorphosis of nauplius of a planktonic shrimp Acetes intermedius Omori, 1975

Yung-Hui CHEN AND I-Ming CHEN*

Institute of Marine Resources, National Sun Yat-sen University, Kaohsiung, Taiwan 804, Republic of China

ABSTRACT: The effects of temperature and salinity on the metamorphosis of nauplii into protozoea I of a planktonic shrimp Acetes intermedius were studied. Eggs or nauplii I, hatched under optimal conditions (30¡C, 30 ppt), were incubated under 15 different combinations of three temperatures (20°C, 25°C, and 30°C) and ﬁve salinities (15 ppt, 20 ppt, 25 ppt, 30 ppt, and 35 ppt) until reaching protozoea I stage. At salinities of 20Ð35 ppt, eggs required 28 h, 45 h and nearly 4Ð5 days to develop into protozoea I at 30¡C, 25¡C and 20¡C, respectively. Nauplii metamorphosed into protozoea I at salinities of 25 ppt and 30 ppt earlier than at 20 ppt and 35 ppt at temperatures of 25¡C and 30¡C, whereas no regularity in time length existed among salinities at 20¡C. Irrespective of hatching conditions, the rate of metamorphosis reached approximately 90Ð100% at temperatures of 25¡C and 30¡C at salinities of 20Ð30 ppt, but was signiﬁcantly reduced at 20°C for all salinities examined. The metamorphosis rate of nauplii was generally higher at salinities of 20Ð30 ppt at all temperatures. Mul- tiple linear regression analysis revealed that the temperature experienced by nauplii accounted for most of the variation in the metamorphosis rate.

KEY WORDS: Acetes, metamorphosis, nauplius, salinity, temperature.

INTRODUCTION The larvae of genus Acetes occur seasonally and abundantly in estuaries,7–10 suggesting that larval The planktonic shrimp Acetes intermedius Omori development may be affected by variations in both 1975 has long been a commercially and ecologi- temperature and salinity, as has been documented cally important species in the coastal waters off in other decapods. Survival rate of nauplii of the south-western Taiwan.1 Gravid shrimps occur shrimp Metapenaeus bennettae are significantly abundantly within estuaries throughout the year.2,3 higher at temperatures greater than 24°C and salin- As a result, the larvae that occur during the differ- ities above 30 ppt.11 The larvae of prawn Pandalus ent seasons experience seasonal changes of water borealis, maintained under a regimen of increasing temperatures from 20°C in winter to 28–30°C in temperature from 4.2°C to 6.0°C in 60 days, grows summer.4 Similarly to other Acetes spp., the larvae more rapidly than those maintained at a constant of A. intermedius undergo four developmental temperature.12 Survival rate of the semiterrestrial stages: (i) nauplius; (ii) protozoea; (iii) mysis; and grapsid crab, Armases miersii, is frequently higher (iv) postlarvae before reaching the juvenile stage.5,6 at salinities of 15–25 ppt than at 35 ppt.13 Penaeus Larvae at the naupliar stage depend on an internal chinensis juveniles grow best in salinities ranging energy source to survive before they metamor- between 20 ppt and 30 ppt and at temperatures of phose into feeding larvae;3 that is, the protozoea I 30°C.14 In general, either temperature or salinity stage. Abiotic factors are particularly important alone can influence the growth and survival of the in the development of larvae at the non-feeding larvae, but the best larval growth usually occurs naupliar stage. when both salinity and temperature are optimal. Previous studies on the spatial distribution of *Corresponding author: Tel: 886-07-525-2000 ext. 5055. larvae of Acetes spp. have focused mainly on larvae 5,15 Fax: 886-07-525-5020. Email: [email protected] at the zoeal and/or mysid stages, and not on Received 9 February 2001. Accepted 8 August 2001. naupliar larvae. This is because of the difficulties in 118 FISHERIES SCIENCE Y-H Chen and I-M Chen

35 ppt). The temperature range spans that of the seasonal temperature changes which occur in the coastal waters off south-western Taiwan.4 Chen and Chen have indicated previously that the lowest temperature and salinity for egg hatching of A. intermedius is 20°C and 15 ppt, respectively.18 After hatching, the development of naupliar larvae was examined under an anatomical microscope at certain time intervals until they metamorphosed into protozoea I. The metamorphosis rate of nauplii was calculated as the percentage (%) of the number of protozoea I to the sum of naupliar larvae and protozoea I. The rate was determined when it did not obviously fluctuate with time. The required time length was determined accordingly. Fig. 1 Map of the south-western coast of Taiwan indi- On average, 100 eggs were used in each beaker. cating the collecting site of gravid female shrimps Acetes At least three beakers were used to estimate the intermedius used in the experiments. metamorphosis rate for each condition. Each experiment was carried out at least twice. separating Acetes nauplii from field samples mixed Nauplius to protozoea I with other decapod naupliar species that having a similar external morphology.5 Until now, studies The proportion of internal energy consumed by related to naupliar stages of Acetes spp. have eggs during development depends on hatching concentrated on describing the morphological conditions. It can be considered that napulii which changes that occur during larval development hatch under different environmental conditions under constant conditions.16,17 The purpose of the contain different amounts of energy for their present study is to understand the effects of tem- subsequent development. In order to understand perature and salinity on the metamorphosis of A. the effects of temperature and salinity on the intermedius nauplii into protozoea I. metamorphosis of nauplii exclusively from the influences of environmental conditions on egg hatching, the nauplii I, which were newly hatched MATERIALS AND METHODS at a temperature of 30°C and a salinity of 30 ppt, were randomly introduced into the 15 different Collections conditions described earlier. Chen and Chen have demonstrated previously that eggs of A. inter- Gravid shrimps of A. intermedius Omori 1975 medius reach maximum hatching success in the were collected along the bank of Kaohsiung Port, shortest time at a temperature of 30°C and in Taiwan, Republic of China (Fig. 1), using a bag net. salinities ranging between 25 ppt and 30 ppt.18 They were introduced individually into a 0.5 L The experimental procedures and estimation of the beaker to spawn at room temperature (25–27°C) metamorphosis rate are mentioned as described at a salinity of 25 ppt (filtered seawater, 0.22 m). m earlier. Because gravid females spawn throughout the night, only those eggs that were spawned within a similar time period of 15–25 min were used. After Statistic analysis spawning, eggs were siphoned off, pooled together and then distributed randomly into 0.5 L beakers The significance of temperature and salinity containing 400–450 mL of filtered seawater at dif- interactions on metamorphosis rate of nauplii ferent salinities. The beakers were incubated in a was examined using two-way ANOVA. If significant, temperature-controlled water bath. one-way ANOVA was employed to examine the significance of the differences in metamorphosis rate among different the levels of each factor (tem- Egg to protozoea I perature/salinity) under the same levels of other factors (salinity/temperature). Duncan multiple Eggs were subjected to 15 combinations of three comparisons test was used in order to sort out the different temperatures (20°C, 25°C, and 30°C) and metamorphosis rate into different groups where five salinities (15 ppt, 20 ppt, 25 ppt, 30 ppt, and necessary. Multiple linear regression analysis was Metamorphosis of Acetes FISHERIES SCIENCE 119

used to analyze the relative importance of each ity. The metamorphosis rate of nauplii ﬂuctuated variable on the basis of variation of metamorpho- without regularity. The protozoea I usually showed sis rate. weak mobility, and their spines of uropod were either absent or bent. The abnormality was especially obvious at salinities 15 ppt and 35 ppt. RESULTS

Developmental time Egg to protozoea I

Nauplii metamorphosed into protozoea I at night At the temperature 30°C, the metamorphosis rate of in all conditions. At temperature 25°C and 30°C, nauplii was, on average, more than 90% when salin- nauplii started to metamorphose into protozoea I ity was greater than 15 ppt, whereas it was higher around 45 h and 29 h after spawning, respectively. (88–94%) at salinities of 20 ppt and 25 ppt at the The metamorphosis rate reached approximately temperature 25°C. At 20°C, it was greatly reduced for 90% in 2 h and became steady thereafter. Nauplii all salinities, whereby the higher metamorphosis metamorphosed approximately 2 h and 6 h earlier rates (54–69%) occurred at salinities ranging be- at 25 ppt and 30 ppt compared with at 20 ppt and tween 20 ppt and 30 ppt (Fig. 3). The metamor- 35 ppt, respectively (Fig. 2). At 20°C, eggs generally phosis rate of nauplii was signiﬁcantly different took approximately 4–5 days after spawning to among temperatures (P < 0.01) as well as among develop into protozoea I irrespective of the salin- salinities (P < 0.01). At the same temperature, the

Fig. 2 Metamorphosis rate of nauplii in different temperature (25°C and 30°C) and salinity (20 ppt, 25 ppt, 30 ppt, and 35 ppt) conditions, which changed over time. The initial times were: (a) 44 h after spawning for the 25°C treatment group; and (b) 24 h for the 30°C treatment group. 120 FISHERIES SCIENCE Y-H Chen and I-M Chen

Table 1 Duncan’s multiple comparisons of the metamorphosis rate (%) of nauplii in which eggs were incubated in different conditions until reaching protozoea I stage. Metamorphosis rates among salinities/temperatures were compared while maintaining the other variables (temperature/salinity) constant Salinity (ppt) (a) Temperature (°C) 15 20 25 30 35 20 1.43 69.94 60.14 54.40 32.00 CAAAB Fig. 3 Metamorphosis rate of nauplii (%, mean ± SD) 25 63.02 94.30 88.86 73.78 53.28 incubated at the same temperature and salinity CAABD conditions as that for eggs. (See Table 1 for detailed 30 63.62 88.11 94.31 95.37 97.15 comparisons). BAAAA

Temperature (°C) (b) Salinity (ppt) 20 25 30 15 1.43 63.02 63.62 BAA 20 69.94 94.30 88.11 BAA 25 60.14 88.86 94.31 BAA 30 54.40 73.78 95.37 CBA 35 32.00 53.28 97.15 CBA

Fig. 4 Metamorphosis rate of nauplii (%, mean ± SD), For a given temperature/salinity (row), values indicated by hatched at 30°C and 30 ppt, in different temperature identical letters do not differ significantly at the level P = 0.05. and salinity conditions. (See Table 2 for detailed comparisons). was significantly different among temperatures lowest metamorphosis rate always occurred at the (P < 0.01), as well as among salinities (P < 0.01). The salinity 15 ppt. The metamorphosis rate was gener- significance of the interactions between tempera- ally similar among salinities ranging from 20 ppt to ture and salinity (P < 0.01) suggested that the 30 ppt at 20°C and 25°C, except from 20 ppt to 35 ppt variations in the metamorphosis rate were not at a temperature of 30°C (Table 1a). At the same influenced by temperature or salinity alone. At the salinity, the metamorphosis rate was always lowest same temperature level, the metamorphosis rate at temperature 20°C among the various salinities. It was similar among salinities at temperatures of was similar between 25°C and 30°C at salinities 25°C and 30°C, but was significantly lower at salin- ranging between 15 ppt and 25 ppt, but was signifi- ities of 15 ppt and 35 ppt compared with salinities cantly different between salinities 30 ppt and 35 ppt at a temperature of 20°C (Table 2a). At the same (Table 1b). salinity, the metamorphosis rate was not significantly different between the temperatures 25°C and 30°C, but it was lowest at the temperature Nauplius to protozoea I 20°C, except at 20 ppt (Table 2b). All data were pooled together for linear multiple At the temperature 30°C, nearly 100% of nauplii regression analysis using a stepwise procedure. For metamorphosed successfully into protozoea I for salinities ranging between 15 ppt and 35 ppt and all salinities. The metamorphosis rate ranged from temperatures ranging between 20°C and 30°C, 79.84% to 95.63%, in which the higher rates oc- we obtained a multiple linear regression model as curred at salinities ranging from 20 ppt to 30 ppt follows: at 25°C. It significantly decreased at 20°C for all salinities examined, especially at 15 ppt (9.80%) MR =-280 + 24.. 19 ¥MT - 0 44 ¥ MT2 + 0 . 05 ¥ MT and 35 ppt (29.55%) (Fig. 4). Two-way ANOVA ¥=HT()rFnP2 045., = 5226 ., =< 199 , 001 . revealed that the metamorphosis rate of nauplii (1) Metamorphosis of Acetes FISHERIES SCIENCE 121

Table 2 Duncan’s multiple comparisons of metamor- estuaries. The use of brackish habitats for the early phosis rate (%) of nauplii hatched at 30°C and 30 ppt, but development of larvae has also been found for incubated under different combinations of temperature the congeneric species Acetes erythraeus.9 and salinity conditions. Metamorphosis rates among Eggs took nearly 4–5 days to develop into salinities/temperatures were compared while maintain- protozoea I at 20°C but only 28–30 h and 44–50 h ing the other variable (temperature/salinity) constant at 30°C and 25°C, respectively. Regardless of the Salinity (ppt) time needed for eggs to hatch under different con- (a) Temperature 18 (°C) 15 20 25 30 35 ditions, nauplii required nearly 3–4 days to metamorphose into protozoea I at 20°C, whereas they 0 9.80 89.20 69.34 71.20 29.55 took only approximately 15–16 h at temperatures ABBBA of 30°C and 25°C. The metamorphosis rate was sig- 25 79.84 93.90 95.63 95.64 82.00 nificantly lower at the temperature 20°C compared AAAAA 30 97.69 100.00 99.35 100.00 98.93 with at 25°C and 30°C for all salinities examined, ABABBAB regardless of the experimental conditions in which the nauplii were hatched. These observations indi- Temperature (°C) cate that 25°C is close to the minimum limit of the optimal temperature range for the metamorphosis (b) Salinity (ppt) 20 25 30 of nauplii of A. intermedius. The surface tempera- 15 9.81 79.83 97.69 ture in the coastal waters off south-western Taiwan ABBcan reach 28–29°C in summer, but might be as low 20 89.20 93.90 100.00 as 20°C in winter.4 The results of the present study AAAshowed that the metamorphosis of A. intermedius 25 69.34 95.63 99.35 nauplii is adversely affected by incubation tem- ABB 30 77.20 95.64 100.00 peratures below 25°C. It is likely that the recruit- ABBment of A. intermedius is reduced in winter as a 35 29.55 82.00 98.93 result of the adverse effects on the metamorphosis ABBof nauplii. This hypothesis is supported by Wen’s studies, which indicate that the recruitment of For a given temperature/salinity (row), values indicated by A. intermedius in the south-western coastal waters identical letters do not differ significantly at the level P = 0.05. off Taiwan is weaker during winter than during summer.2 Linear multiple regression analysis revealed whereby MR is the metamorphosis rate of nauplii, that approximately 43% of variations in the meta- HT is the hatching temperature (°C), and MT is morphosis rate of A. intermedius nauplii could the temperature experienced by the nauplii. The be explained by the temperatures (MT) at which metamorphosis temperature(MT),overall,explains nauplii were incubated after hatching. Salinity was nearly 43% of variations in the metamorphosis rate statistically excluded from the regression equation of nauplii. Lack-of-fitness was significant (F = 7.90, because it did not contribute significantly in ex- P < 0.01), suggesting that other factors exist which plaining the variation of the metamorphosis rate affect the metamorphosis rate of nauplii. when compared with other variables. This indi- cates that temperature, especially at the naupliar stages, affects the metamorphosis of nauplii more DISCUSSION significantly than does salinity. This does not, however, totally exclude the influence of salinity on Irrespective of hatching conditions, nauplii usually the metamorphosis of nauplii. As shown earlier, have a higher metamorphosis rate at salinities nauplii took less time in reaching a higher meta- ranging from 20 ppt to 30 ppt than those at 15 ppt morphosis rate at the salinities 25–30 ppt com- and 35 ppt for all temperatures. In addition, ab- pared with at other salinity levels. normal individuals of protozoea I were commonly The results of the present study showed that the found at the salinities 15 ppt and 35 ppt at 20°C. nauplii of A. intermedius that hatched at optimal Nauplii incubated at 25 ppt and 30 ppt metamor- conditions (30°C, 30 ppt) usually had a higher phosed into protozoea I earlier than those at other metamorphosis rate than those hatched at other salinities at temperatures of 25°C and 30°C. These suboptimal conditions (Figs 3,4). This is because observations indicate that the optimal salinity the nauplii of A. intermedius share a common range for metamorphosis of nauplii is approxi- energy source with eggs, and the proportion of mately 25–30 ppt. It is suggested that nauplii energy reserved for nauplii for subsequent devel- might be found abundantly in the lower parts of opment depends on the utilization conditions of 122 FISHERIES SCIENCE Y-H Chen and I-M Chen

eggs during the hatching process. The high-energy Sergestidae). Oceanogr. Mar. Biol. Annu. Rev. 1993; 31: contents reserved for nauplii after hatching is 259–444. likely to enhance the metamorphosis success of 6. Oshiro LMY, Omori M. Larval development of Acetes nauplii. americanus (Decpaoda: Sergestidae) at Paranagua and The significance of lack-of-fitness implies that Laranjeiras Bays, Brazil. J. Crust. Biol. 1996; 16: 709–729. 7. Pillai VK. On the larval development of Acetes indicus there might be other factors affecting the meta- H. Milne-Edwards (Crustacea: Decpaoda: Sergestidae) from morphosis rate of nauplii that were not examined Bombay waters. J. Mar. Biol. Assoc. India 1973; 15: 587–595. in the present study. Recent studies have shown 8. Grabe SA, Lees DC. Macrozooplankton studies in Kuwait that egg quality, as a function of egg size, not only Bay (Arabian Gulf). II: Distribution and composition of affects the hatching success of eggs but also the planktonic penaeidea. J. Plank. Res. 1992; 14: 1673–1686. survival of newly hatched fish larvae before they 9. Le Reste L. Biologie de Acetes erythraeus (Sergestidae), dans start to feed.19 Fish larvae hatched from big eggs une baie du N. W. de Madagascar (Baie d’Ambaro). Cah. O. usually have a better survival rate than those R. S. T. O. M. Sér. Oceánogr. 1970; 8: 33–56 (in French with hatched from small ones.20 This could also be the English abstract). case for A. intermedius because the size of eggs 10. Jo SG, Omori M. Seasonal occurrence and vertical distribu- 3 tion of larvae and post-larvae of the pelagic shrimp Acetes varied from 168 mm to 210 mm. It is likely that the japonicus Kishinouye (Sergestidae) in the central part of the metamorphosis rate of A. intermedius nauplii is Seto Inland Sea, Japan. Bull. Plank. Soc. Jpn. 1996; 43: 75–87. related not only to the hatching conditions of eggs 11. Preston N. The combined effects of temperature and but also egg quality. 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