Larval development of Pandalus gracilis Stimpson (Crustacea: Decapoda: Pandalidae) reared in the laboratory

The complete larval development of Pandalus gracilis Stimpson is described based on larvae reared in the laboratory. The species has four larval stages (stages 1–4) and one postlarval stage (stage 5). The survival rate of the postlarva stage is very high. Of the 40 larvae reared individually, 39 larvae moulted to the postlarva stage within approximately 21 days at a rearing temperature of 15°C. Larval morphological characteristics of the species are described and figured. Larval development of P. gracilis is compared with other Pandalus and Pandalopsis species. Komai (1999) divided species of Pandalus into four species groups based on adult morphology. Pandalus gracilis belongs to the P. hypsinotus group. In this study, using larval characters, the P. hypsinotus group can be divided into two groups by the appearance of the mandibular palp and the shape of the telson: the first group contains P. hypsinotus and P. danae, and the second group contains P. gracilis, P. prensor, and P. nipponensis.


Introduction
The genus Pandalus contains 19 species, which are found at all depths on the continental shelf and slope in the northern hemisphere. Members of this genus are of commercial importance and of scientific interest because of their reproductive patterns. Most species within the genus are protandric hermaphrodites (Holthuis 1980;Komai 1999;Bergström 2000).
The present study provides a complete description of the larval development of P. gracilis based on laboratory-reared material. Since Berkeley (1930) classified the genera Pandalus and Pandalopsis based on the adult phase, Komai (1999) indicated that there was a close phylogenetic relationship between the P. platyceros group and Pandalopsis based on the adult phase. On the other hand, Christofferson (1989) suggested that Pandalopsis is a subordinate clade within the genus Pandalus by comparing the characters of larval morphology. In this study, we also compare the larvae of P. gracilis with those of other Pandalus and Pandalopsis species.

Materials and methods
An ovigerous female of P. gracilis was collected on 10 February 2001 using a shrimp trawl at Yongcho Island (34u309N, 128u409E), Tongyeong, Korea. The female was transported to the laboratory and kept in a 4-litre jar with aerated seawater. The diameter of eggs was 1.9-2.961.3-1.9 mm. Sixty-eight larvae hatched on 27 April 2001. Of these, 40 larvae were reared individually in 100 ml glass bottles placed in an incubator at 15uC and 33.7 psu. For drawing and descriptions, 28 larvae were reared in a 4-litre jar.
Larvae were fed with newly hatched Artemia nauplii each day. Moulting and mortality were recorded daily, and each day all larvae were transferred to fresh containers. At each stage, dead larvae and exuviae were fixed and preserved with 3% buffered formalin.
Four live specimens of each stage were dissected for observation. Drawings were made with a microscope equipped with a drawing tube. Measurements were made to the nearest 0.01 mm as follows: carapace length (CL), from the anterior tip of the rostrum to the postero-median carapace margin; total length (TL), from the anterior tip of the rostrum to the postero-median margin of the telson, excluding telson processes. At least 10 exuviae were measured using a calibrated ocular micrometer. The ovigerous female, the undissected larvae, and dissected appendages are deposited in NFRDI (National Fisheries Research and Development Institute, Busan, Korea).

Duration and survival of the larvae
In the complete larval development of Pandalus gracilis, there are four larval stages and one postlarva stage. The duration of each larval stage and survival from an initial 40 larvae is shown in Figure 1. Of the 40 larvae reared individually, 39 larvae moulted to the postlarva stage within approximately 21 days.
Abdomen (Figure 2A, B, Q): dorsal surface of each somite smooth, without armature. Sixth somite with pair of blunt posterolateral spines.
Carapace ( Figure 3A, B): rostrum well developed, straight, overreaching anterior margin of eye, about 0.8 times as long as carapace. Dorsal edge of rostrum and anterior part of carapace armed with 10-12 minute teeth. Antennal and pterygostomian spines on anterolateral margin.
Mandible ( Figure 3E): mandibular palp present, but unsegmented. Incisor and molar processes well developed and distinctly separated by deep cleft.
Maxilliped 3 ( Figure 3J): endopod with numerous short setae; exopod shorter than that of stage 1, not reaching middle of first segment of endopod, without terminal plumose setae.
Pereiopod 2 ( Figure 3L): carpus of left pereiopod with five articulations, that of right with three articulations. Ischium, propodus, and dactylus with several short setae. Exopod falling short of distal margin of ischium.   Carapace ( Figure 4A, B): rostrum almost as long as carapace, and slightly curved upwards. Dorsal margin of rostrum and anterior part of carapace armed with 15-16 teeth; anterior-most tooth well separated from other spines. Ventral margin of rostrum with four teeth. Antennal and pterygostomian spines well developed.
Pereiopod 2 ( Figure 4L): left carpus with seven articulations and right carpus with four articulations. Carapace ( Figure 5A, B): rostrum upcurved distally, about 1.1 times as long as carapace. Dorsal margin of rostrum and anterior part of carapace armed with 15-17 teeth; ventral margin of rostrum with five teeth. One short seta present between tooth and tooth, or not.
Telson and uropods ( Figure 5A, B, U): posterior margin of telson rounded, narrower than anterior end, armed with two pairs of dorsolateral spines and five pairs of terminal setae; setae on dorsal side stronger than those of stage 3. Exopod of uropod with acute posterolateral spine. Carapace ( Figure 6A, B): rostrum upcurved distally, 1.25 times as long as carapace. Dorsal margin of rostrum and anterior part of carapace armed with 16-18 teeth; ventral margin of rostrum with six teeth. One short seta present between tooth and tooth.
Telson and uropods ( Figure 6A, B, U): telson with three pairs of dosolateral spines and three pairs of terminal setae. Exopod of uropod with transverse articulation (diaeresis) from base of posterolateral spine.

Discussion
In the genus Pandalus, the larval development of 11 species have been reported completely and that of one species incompletely. P. jordani by Rothlisberg (1980), P. eous by Kurata (1964), P. borealis by Haynes (1979), P. goniurus by Haynes (1978), P. stenolepis by Needler (1938), P. hypsinotus by Haynes (1976), P. danae by Berkeley (1930), P. prensor by Mikulich and Ivanov (1983), P. nipponensis by Taishaku et al. (2001), P. platyceros by Price and Chew (1972), and P. latirostris by Kurata (1955) have complete descriptions; P. montagui has an incomplete description. There is a wide variation in the number of larval stages within the genus, from 2 to 13 (Table I). Pandalus gracilis has four larval stages. Although Bergström (2000) mentioned that most species of Pandalus have planktonic larvae which spend their early stages in or near the euphotic zone and then gradually assume a more demersal life with age, P. latirostris and P. prensor are exceptions from this general pattern. These two shallow water species have large eggs and relatively advanced first larvae with abbreviated development (Kurata 1955;Mikulich and Ivanov 1983). Taishaku et al. (2001) mentioned that P. nipponensis also shows ''bottom-dwelling behaviour'' after hatching with welldeveloped pereiopods. In P. gracilis, only 64 larvae were hatched from one ovigerous female. They did not swim in the rearing water; they sank to the bottom of the rearing bottles and remained there, with well-developed pereiopods: pereiopods 3-5 are segmented as in the adult form. However, they have no pleopods. Mandibular palps are present in the second stage. These traits suggest that the larval development of P. gracilis follows the abbreviated pattern and that larvae in the field would stay close to the bottom. Komai (1999) revised the genus Pandalus and divided the species of this genus into four species groups based on adult morphology as follows: We compared characters of Pandalus larvae which are known so far (Table I), and there are common characters which can be used to distinguish subgroups within each group (except the P. stenolepis group). In the P. montagui group, the larvae share poorly developed pereiopods. In the P. hypsinotus group, pereiopods are well developed, but larval pleopods are absent or undeveloped. In the P. platyceros group, the larvae have more developed pleopods than those of other groups. The larvae of the P. stenolepis group have the same characters as those of larvae of the P. montagui group. Komai (1999) mentioned that the adults of the P. stenolepis group are placed in an intermediate position between the P. montagui and the P. hypsinotus groups. Therefore, the characters of the larvae of each group generally support the suitability of Komai's (1999) groups in adult classification.
In addition, using larval characters, Komai's (1999) P. hypsinotus group can be divided into two groups: the first group contains P. hypsinotus and P. danae, and the second group contains P. gracilis, P. prensor, and P. nipponensis. The first group has diagnostic features which differentiate them from the second group as follows: (1) they have five to six larval stages, (2) the shape of the telson is triangular; (3) the number of spines on the telson is 7+7; (4) the mandibular palp appears in the fourth stage, and (5) somite 6 is separated in the second stage. The second group, including the present species, has common characters as follows: (1) they have from three to four larval stages; (2) the shape of the telson is circular; (3) the number of spines on the telson is more than 8+8; (4) the mandibular palp appears from the first to second stages, and (5) somite 6 is separated in the first stage. In the P. hypsinotus group, the larvae of P. gracilis are very similar to those of P. prensor and P. nipponensis. Some morphological differences are found in the following characters: (1) in the first stage, P. gracilis has eight pairs of setae on the distal margin of the telson, whereas P. prensor has nine pairs and P. nipponensis has 23-25; (2) in P. gracilis, the pleopods appear in the third stage, whereas these appear in the first stage in P. prensor and P. nipponensis, and (3) in P. gracilis and P. prensor, the mandibular palp appears in the second stage, whereas it appears in the first stage in P. nipponensis. Berkeley (1930) classified the genera Pandalopsis and Pandalus based on their adult phase. According to her classification, Pandalopsis has long antennules that are twice as long as the carapace and possess a laminate expansion on the merus of the third maxilliped and the ischium of the first pereiopod. Komai (1999) also mentioned that Pandalus is immediately distinguishable from Pandalopsis in the ventral laminar expansion of the ischium of the first pereiopod, which bears spinules ventrally, the slender, conical propodus of the first pereiopod, and the greatly unequal second pereiopods. However, Christofferson (1989) suggested that Pandalopsis is a subordinate clade within the genus Pandalus. In this study, we compared larval development in 13 Pandalus species with three Pandalopsis species (mainly first larval stage). As a result, no diagnostic characters for Pandalopsis were observed in the larval phase. Pandalopsis dispar has no laminate expansion of the third maxilliped and first pereiopod, which Park et al. (2004) noted as a diagnostic feature of Pandalopsis dispar. The characters of Pandalopsis larvae are similar to those of Pandalus as follows: (1) developed pereiopods are close to the P. hypsinotus and P. platyceros groups; (2) the absence of the laminate expansion on the third maxilliped is close to all groups in Pandalus; (3) developed pleopods, and (4) the number of setae of the telson is more than eight pairs, which is close to P. platyceros. Therefore, Pandalopsis and Pandalus are not distinguished by larval morphology. This means there is close relation between Pandalopsis and Pandalus, although Rice (1980) pointed to the potential danger in the uncritical use of larval information for estimating phylogeny in decapods. Further study on the larval development of Pandalopsis species and characters of the adult morphology within two genera are required.