Japanese nephropid lobster Metanephrops japonicus lacks zoeal stage

Authors


*Tel: 81-54-620-8911.
Fax: 81-54-629-1255. Email: kazutoshi1_okamoto@pref.shizuoka.lg.jp

Abstract

ABSTRACT:  Larvae of the Japanese nephropid lobster Metanephrops japonicus hatched in the laboratory were reared at 15°C, and the development and feeding were observed. All larvae hatched at the ‘prezoea stage’ with no natatory setae on the exopodite of the pereiopods. Without feeding, 50% of prezoea molted into the megalopa stage, having small buds as the exopodite, within 1 h and all molted within 22 h. The megalopa fed with Artemia nauplii, shrimp meat and pelleted food molted into the first juvenile stage with no exopodite after approximately 17 days. The average carapace lengths of prezoea, megalopa and the first juvenile stage were 3.2, 3.6 and 4.4 mm, respectively. The survival rate from hatching to the first juvenile stage was high (90–100%). This lobster may be the only known nephropid species with no zoeal stage.

INTRODUCTION

Nephropid lobsters, known as ‘scampi’, belong to the genera Nephrops and Metanephrops of the family Nephropidae, and all species are important marine products. Among the 18 species identified from the two genera,1,2 three from the genus Metanephrops, M. japonicus, M. thomsoni and M. sagamiensis, are known to occur in the Japanese waters. Metanephrops japonicus, which is also known as ‘Japanese scampi’, is a high-value edible lobster with an elongated body of approximately 20 cm in length; it inhabits the sandy-mud sediments located at depths of 200–400 m off the Pacific coast of Japan from the middle region of Japan to Kyushu.3 It is an economically important local fishery resource in Suruga Bay, Shizuoka Prefecture, Japan. Overfishing results in a decrease in lobster resources.4 Restocking with artificially cultured juveniles is considered to be a promising method for the recovery of depleted marine lobster resources.5 A project has been initiated with the aim of developing land-based aquaculture to produce a large number of M. japonicus juveniles and to release them into Suruga Bay.6,7 Currently there is no commercial aquaculture of the nephropid lobster in the world.8–10 A major hurdle in establishing such an enterprise is the difficulty in sustaining the survival of lobster larvae.9,11–15 In the genera Metanephrops and Nephrops, development of larvae hatched in the laboratory was reported in six species, i.e. M. andamanicus,16M. challengeri,13M. thomsoni,14M. sagamiensis,15M. japonicus17,18 and N. norvegicus.11 Among these species, rearing the larvae to the juvenile stage has been reported for only two species: M. thomsoni14 and N. norvegicus.11 Although larval rearing of M. japonicus has been attempted,17,18 the metamorphosis of larvae to the juvenile form has not been achieved to date. The present study reports the successful rearing of larvae from this lobster from hatching to the juvenile stage with morphological observation.

MATERIALS AND METHODS

Capture and maintenance of ovigerous females

During January 2002 to February 2003, 11 ovigerous female lobsters were captured in Suruga Bay by using lobster pots submerged at depths of 200–400 m. The females were transferred to the laboratory and kept individually in 5-L tanks filled with running sea water. The water temperature was maintained at approximately 15°C. Each day, the lobsters were fed on frozen sergestid shrimp Sergia lucens. Residual food and excrement were removed daily, with occasional observation to check for hatching.

Monitoring of hatched larvae

Batch hatching was observed on 27 May, 24 June and 30 June 2002, where 11, 60 and 20 newly hatched larvae were obtained, respectively. The larvae were transferred to a 1.5-L glass finger bowl filled with filtered sea water and placed in a temperature-controlled bath maintained at 15.2°C ± 0.3°C (mean ± standard deviation, SD). The temperature adopted in the present study was similar to that observed at the depth of main habitats of M. japonicus in Suruga Bay. The number of molted individuals was counted at 10- to 30-min intervals until 50% of individuals were molted; subsequently, occasional counting was carried out until all the individuals were molted.

Cultivation of hatched larvae

Newly hatched larvae obtained from a single female on 7 May 2003 were used for investigating survival and growth. Forty larvae were kept singly in 0.5-L beakers filled with filtered sea water and placed in a temperature-controlled bath maintained at 15.2°C ± 0.2°C (mean ± SD). Four feeding conditions were arranged: (i) Artemia spp. nauplii at a density of 5 inds/mL (hereafter referred to as Artemia); (ii) finely chopped muscle of Sergia lucens (chopped shrimp); (iii) kuruma prawn pellets (prawn pellet, Higashimaru Co., Kagoshima, Japan); and (iv) no food (non-food). Ten individuals were used for each feeding condition. The larvae were transferred every morning to newly prepared beakers containing filtered sea water by using a large-mouth pipette.

Morphological observation

Larvae obtained from the same batch hatched on 7 May 2003 were subjected to morphological observation. For morphological observation and the measurement of body size, 40 hatched larvae were kept singly in 0.5-L beakers filled with filtered sea water and placed in a temperature-controlled bath maintained at 15.2°C ± 0.2°C (mean ± SD). Artemia were fed to these larvae in the same manner mentioned above. Each morning, these individuals were transferred to newly prepared beakers containing filtered sea water by using a large-mouth pipette. The larvae of each stage were fixed with 5% neutralized formalin for 24 h and preserved in a 70% ethanol solution. The carapace and body lengths of 5–13 individuals were measured under a microscope at each stage of development. Carapace length was measured from the posterior margin of the orbit to the posterior margin of the carapace; body length was measured from the posterior margin of the orbit to the posterior end of the telson, excluding telsonal processes.

Although the terminology for larval development in the Decapoda Crustacea remains a topic of debate, some of the terms may be defined as follows from the viewpoint of locomotion and swimming appendages.19,20 In the prezoeal phases, locomotory ability is limited and requires abdominal flexure because the natatory setae on the pereiopods are not well developed. Locomotion in the zoeal phase occurs almost invariably by using cephalothoracic pereiopods; in this phase, the natatory setae on the exopods are well developed and numerous. In the megalopa or postlarvae, locomotion is achieved using the abdominal pleopods. In the first juvenile stage and subsequent stages, locomotion is again by thoracic pereiopods that are now well developed. The terms ‘prezoea’, ‘zoea’, ‘megalopa’ and ‘juvenile’ have been used in previous studies on nephropid lobster.10,13–15,17,18 Hence, the same terms are used in this study. These stages of nephropid lobster are described as follows. In the prezoeal stage, the natatory setae on the pereiopods are not developed yet.13–16 In the zoeal stage, the pereiopods bear a long exopodite with natatory setae.13–15 The exopodites of the pereiopods regress and change to small buds in megalopa stage13–15 and are lost in the juvenile stage.14

RESULTS

Monitoring of hatched larvae

Hatching from five of the ovigerous females was observed during April to July 2002; all newly hatched larvae were observed to be at the ‘prezoea’ stage (Fig. 1). Hatching was observed at or after sunset between 17:00 and 20:00 hours when the female fanned out the pleopods. Without feeding, the prezoea molted to a megalopa (Fig. 2). The percentages of the megalopa stages during the course of development following hatching are shown in Figure 3. Approximately 50% of prezoeae in the glass finger bowls had molted to megalopae within 1 hour post hatch, and all prezoea molted into megalopae within 22 h.

Figure 1.

Prezoea of Japanese nephropid lobster Metanephrops japonicus. (a) lateral view, (b) rostrum, (c) pereiopod with exopodite (arrowhead), and (d) telson and rudiment of uropod (arrowhead). Scale bar, 1 mm.

Figure 2.

Megalopa of Japanese nephropid lobster Metanephrops japonicus. (a) dorsal view, (b) pereiopod and exopodite (arrowhead), (c) rostrum, carapace and teeth (arrowheads), and (d) telson and uropod. Scale bar, 1 mm.

Figure 3.

Accumulated number of megalopa individuals (%) of Japanese nephropid lobster Metanephrops japonicus during development post hatch on (●) 27 May (n = 11), (▴) 24 June (n = 60) and (○) 30 June (n = 20) 2002.

Cultivation of hatched larvae

Development from prezoea to the first juvenile stage (Fig. 4) was observed in all feeding conditions (Table 1). Of ten individuals, only one survived to the juvenile stage in the non-feed group, whereas 9–10 individuals reached the juvenile in the other feeding groups. The larvae in the fed groups reached the juvenile stage in an average of 17 days, while a single survivor in the non-feed group took 23 days from hatching to reach the juvenile stage.

Figure 4.

Juveniles of Japanese nephropid lobster Metanephrops japonicus. (a) dorsal view, (b) rostrum, carapace and tooth (arrowhead), (c) ventral view of posterior cephalothorax region, and (d) telson and uropod. Scale bar, 1 mm.

Table 1.  Survival and periods between hatching and developing into the juvenile stage under different feeding conditions
Feeding conditionsNo. of prezoeaNo. of successful molts to megalopaNo. of successful molts to juvenilePeriod (average day)
Artemia10101016–18 (16.7)
Chopped shrimp1010916–19 (17.0)
Prawn pellet10101016–18 (16.7)
Non-food1010123.0

Morphological observations

The carapace and body lengths of five prezoea specimens (Fig. 1a) ranged 3.1–3.3 mm (average 3.2 mm) and 8.9–9.3 mm (average 9.1 mm), respectively. In these specimens, the carapace, rostrum and abdomen were smooth and lacked spines (Fig. 1a,b); all the exopodites of the pereiopods (Fig. 1c) and pleopods lacked setae. The telson had a spine on each side and one in place of the median notch. The rudiments of uropods were visible under the cuticle (Fig. 1d).

The carapace length of seven megalopa specimens (Fig. 2a) ranged 3.5–3.7 mm (average 3.6 mm), and the body length was between 9.5 and 9.7 mm (average 9.6 mm). The exopodites of the pereiopods were regressed and rudimentary (Fig. 2b). The dorsal margin revealed four pairs of teeth, including an anterior pair on the rostrum and three equidistant pairs located posteriorly on the carapace; the third pair was situated immediately above the orbital margin (Fig. 2c). Unlike the previous prezoea specimens, the uropods in the megalopa stage were well differentiated (Fig. 2d). Locomotion was accomplished using natatory pleopods located in the abdominal segment. The chelae of the first pereiopods were symmetrical and significantly increased in length (Fig. 2a).

Thirteen juvenile specimens (Fig. 4a) exhibited a carapace length of 4.1–4.6 mm (average 4.4 mm) and a total body length of 10.1–10.9 mm (average 10.4 mm). The ventral margin on the rostrum has a miniature tooth (Fig. 4b). The exopodites of the pereiopods were lost in this stage (Fig. 4c). No remarkable changes from the previous stages were observed in the morphology of the telson and uropods (Fig. 4d). The juvenile form was similar to the adult form.

DISCUSSION

In this study, M. japonicus hatched at the so-called ‘prezoea’ stage, a phenomenon that has been reported previously in this species17,18 as well as in other Metanephrops spp.13–16 Thus, the prezoeal stage of Metanephrops can be regarded as a normal developmental stage. The prezoeal phase of Metanephrops spp. was short-lived, as demonstrated previously.14,15 Fujii et al.18 reported that they could obtain the juvenile form of M. japonicus from larvae hatched in the laboratory. However, the form depicted in their illustration has now been confirmed as megalopa because of the existence of exopodites on the pereiopods. Moreover, their report lacked a detailed observation of the larval growth of the lobster. Therefore, the present report is the first paper describing the successful rearing of M. japonicus from the time of hatching to the juvenile stage. An in-depth study of larval development will be conducted in future.

In this study, the survival of individuals in the non-feed group was low; however, the attainment of the juvenile stage by one individual showed that there was the potential for development in the absence of food. This occurrence might depend upon the energy reserve in the yolk of the large eggs produced by this species.6 It was found that sufficient yolk could be deposited in each egg to help in sustaining the complete development of juveniles from the prezoeal stage in the absence of food. The survival rates from hatching to the juvenile stage were very high under the common feeding conditions such as feeding of Artemia spp. nauplii, finely chopped shrimp muscle and prawn pellets. These results showed that the cultivation of this lobster was not difficult.

Among the 18 known species from the genera Metanephrops and Nephrops, larval development from hatching to the megalopa stage has been reported in four species (Table 2). The number of zoeal stages in these species varies from zero to three: it was found that M. japonicus was the only species that lacked the zoeal stage. Gore20 defined two types of abbreviated development, namely, direct development and advanced development. In direct development, no free-swimming larval stages or zoea exist. The young hatch more or less in the adult form and possess distinct larval morphology. In advanced development, the young hatch as zoea but in a state considerably developed, so that larval development is often foreshortened in comparison, both durationaly and with respect to the ecdysiasts. According to Gore's20 classification, larval development of M. japonicus belongs to the direct type. Thus, this study indicates direct development in M. japonicus, which appears to lack a free-swimming larval stage. Direct development occurs commonly in some freshwater brachyuran crabs and astacidean crayfish.20,21 The cases of direct development were frequently observed in habitats where food resources might be limited such as the deep sea and fresh water. The mode of direct development might vary among the species depending on their life-history strategy.

Table 2.  Comparison of larval stage characters of nephropid lobsters
SpeciesNumber of larval stagesSurvival rate from hatching to molting to megalopa (%)Period from hatching to molting to megalopa
ZoeaMegalopa
Nephrops norvegicus11,12310–7.3∼1 month
Metanephrops challengeri131103–4 days
Metanephrops thomsoni1421104 days
Metanephrops sagamiensis152116–8 days
Metanephrops japonicus01100a few hours

CONCLUSION

From the viewpoint of aquaculture, the duration and complexity of the larval phases as well as the fecundity of cultivable crustaceans have a major effect on hatchery design, operating costs and technical skills required to maintain a predictable output of postlarvae.21 Commercial aquaculture of the nephropid lobster has not been accomplished to date because of the difficulty in broodstock maintenance6 and in cultivating individuals through the larval stages.9,11–15 However, it has been reported that pots were found to be more suitable than trawl nets for catching live lobsters for use as broodstock.6Metanephrops japonicus is expected to be considered as an aquaculture species because of the improved survival and hatching of broodstock captured by pots, the absence of zoeal stages, and their high rate of survival from the point of hatching into the juvenile form, as outlined in this paper. Thus, aquaculture of the nephropid lobster is potentially viable depending on improvement of cultivation techniques.

ACKNOWLEDGMENTS

I am grateful to members of the Shizuoka Prefectural Research Institute of Fishery for support in this study. This paper is a contribution (No. 1122) from the Shizuoka Prefectural Research Institute of Fishery, Japan.

Ancillary