Larval development to the first eighth zoeal stages in the deep-sea caridean shrimp Plesionika grandis Doflein, 1902 (Crustacea, Decapoda, Pandalidae)

Abstract The larvae of the deep-sea pandalid shrimp Plesionika grandis Doflein, 1902 were successfully reared in the laboratory for the first time. The larvae reached the eighth zoeal stage in 36 days, both of which are longest records for the genus. Early larval stages of P. grandis bear the general characters of pandalid shrimps and differ from the other two species of Plesionika with larval morphology known in the number of spines on the anteroventral margin of carapace, number of tubercles on antennule, endopod segmentation in antenna, and third maxilliped setation. Although members in Plesionika are often separated into species groups, members of the same species group do not necessarily have similar early larval morphology. Since the zoea VIII of P. grandis still lacks pleopods and fifth pereiopod, this shrimp likely has at least 12 zoeal stages and a larval development of 120 days.

Plesionika grandis Doflein, 1902 is a widely distributed species in the Indo-West Pacific from Japan to NE Australia and Madagascar at depths of 110-375 m (Chan and Crosnier 1991) and is rather common amongst the deep-sea catches in Taiwan. The present work succeeded in obtaining a live ovigerous female of P. grandis and maintained it in the laboratory until its eggs were hatched. Larvae developed into the eighth zoeal stage in 36 days, enabling the larval morphology of P. grandis is described and illustrated for the first time.

Materials and methods
The ovigerous female of P. grandis was collected by a commercial trawler at depths of 220 m off northeastern Taiwan (24°52.352'N; 121°58.010'E). The berried female was reared in a 100 L aquarium and raised in sea water (salinity of 35) at 14 ± 1°C. Once the eggs hatched, approximately 400 actively swimming larvae were transferred to two beakers (5L). Each beaker contained similar number of larvae, with aerated seawater maintained at a temperature of 23 ± 1°C and a 12:12 hour photoperiod. Specimens of each zoeal stage were collected after the larvae moulted and preserved in a 70% ethylene glycol solution. At least two larvae from each stage were dissected and examined on glass slides under a stereo microscope (OLYMPUS SZX12) using fine entomological needles. Appendages were drawn using a camera lucida installed on a compound microscope (Olympus BX50). The descriptions and figures are arranged according to the standards proposed by Clark et al. (1998). Morphological terminology follows Yang and Ko (2004) and Landeira et al. (2010). Abbreviations of larval measurements are as follows: carapace length (CL), from the postorbital margin to the posteromedian end of the carapace; body length (BL), from the postorbital margin of the carapace to the posterior end of the telson; and total length (TL), from the tip of the rostrum to the tip of the telson. These are all given as mean values followed by the range (in parentheses). The female and larvae are deposited as vouchers in the National Taiwan Ocean University (NTOU M02079). Telson (Fig. 1A) subtriangular, posterior margin minutely spinulated except on distolateral parts, with 7 + 7 plumose setae, outermost two pairs only plumose on inner margin.
Carapace (Fig. 3A, B, D) rostrum shorter than in previous stages but still curved, 0.85 times as long as carapace length; ventrolateral margin with four spines posterior to pterygostomial spine; other unchanged.
Antennule (Fig. 3E) peduncle 2-segmented: basal segment with two long and three short plumose setae; distal segment with two groups of setae, one consisting of six plumose and other with two simple setae; endopod with one long plumose seta; exopod unsegmented with two aesthetasc, one plumose and one simple setae.  Carapace (Fig. 4A, B, D) rostrum not curved and shorter than in previous stages but longer than frontal lobe, 0.35 times as long as carapace length; other unchanged.
Antennule (Fig. 4E) peduncle 3-segmented: basal segment with four (one proximal, one terminal long and two terminal short) plumose setae plus one subterminal strong spiniform seta; medial segment with five (one long, four short) plumose setae; distal segment with two groups of setae, one consisting of six plumose (two subterminal + four terminal) setae and other with two simple setae; endopod unchanged; exopod unsegmented with two aesthetascs and two plumose setae.
Mandible (Fig. 4G) incisor having four teeth; molar process with numerous small teeth; other unchanged.
Maxillule  Pleopods absent. Uropod (Fig. 4A, C) protopod without setae; endopod well developed with ten plumose setae; exopod with 12 plumose setae and one simple seta at outermost apex. Telson (Fig. 4A) less triangular than in zoea III; one lateral simple seta, posterior margin with five pairs of plumoserrulate setae, and two outer simple setae on each side.
Mandible (Fig. 8G) incisor with four or more terminal teeth; molar process with numerous small teeth; other unchanged.

Discussion
The first eight zoeal stages of Plesionika grandis were obtained in 36 days after hatching, representing the longest larval rearing record for the genus. Previous longest larval culture for Plesionika shrimps was P. edwardsii by Landeira et al. 2009a, lasting 20 days, and reaching the Z7 stage. It is suspected that feeding and/or rearing temperature may be the main causes for the mortality of the larvae as discussed in our previous work on the larval rearing of another deep-sea pandalid shrimp Heterocarpus abulbus Yang, Chan andChu, 2010 (Jiang et al. 2016).
Features of each larval stage as well as changes in appendage setation and setal types in P. grandis are summarized in Table 1. The major characters of each zoeal stage are: (ZI) sessile eyes, three pairs of maxillipeds, pleon with five somites and telson subtriangular; (ZII) eyes stalked; (ZIII) uropod with exopod well developed, first pereiopod appeared, and pleon with six somites; (ZIV) antennular peduncle segmented, uropod with endopod; (ZV) endopod of antenna 3-segmented, second pereiopod appeared, and anal spine present on sixth abdominal somite; (ZVI) third pereiopod appeared, telson becoming rectangular; (ZVII) endopod of antenna with more than three seg-ments, and fourth pereiopod appeared as a bud; (ZVIII) endopod of antenna with more than ten segments, and fourth pereiopod biramous.
The early zoeal morphology of P. grandis has the common characters of pandalid larvae, such as eye peduncle narrowed at base, carapace with two dorsal protuberances and anteroventral margin bearing spines, antennule with peduncle strongly concave and exopod bearing spatulate seta, antenna with segmented exopod, rostrum elongated in earlier stages (see Thatje and Bacardit 2000;Landeira et al. 2010;Jiang et al. 2014). Only two species of Plesionika have their larvae reported. They are P. edwardsii by Landeira et al. 2009a [ZI-ZVII] and P. narval by Landeira et al. 2009b. The zoeae of these three species mainly differ in the following characters: (1) Number of spines on anteroventral margin of carapace: P. edwardsii with two spines in ZI, but disappeared in ZII and later stages; P. narval with three spines in ZI to ZV; P. grandis with three spines in ZI, increased to four spines in ZIII, and then, seven spines in ZVI.
(2) Number of tubercles on antennule in ZI: P. edwardsii and P. narval with two tubercles, P. grandis with one. (3) Endopod segmentation of antenna: P. grandis 3-segmented in ZV, 5-segmented in ZVI, 8-segmented in ZVII, 18-segmented in ZVIII; P. edwardsii segmented only in ZVII and 3-segmented; P. narval segmented since ZV and 2-segmented. (4) Third maxilliped setation in ZI: Basis with three setae in P. edwardsii and P. grandis, whilst P. narval with four setae. Moreoever, P. edwardsii has a somewhat different endopod setation at the third maxilliped (1, 1, 2, 4 vs. 2, 1, 2, 4 in the other two species). Furthermore, the ZVII of P. grandis appeared to be more developed than that of P. edwardsii by having the first three pairs of pereiopods well developed (vs. only first two pereiopods well developed in the latter). This indicates that the larval development of P. edwardsii may have even longer duration. Although the numerous species in Plesionika are often separated into species groups (see Chan andCrosnier 1991, 1997;Chan 2004) with P. narval and P. grandis belonging to the same species group, their early larval stages are not more similar to each other than to P. edwardsii.
Species of Plesionika likely have very long larval development (see Landeira et al. 2014). Compared to the long larval development in other caridean shrimps such as Rhynchocinetes conspiciocellus Okuno & Takeda, 1992 (eleven zoeal stages to decapodid in 112 days, Matoba and Shokita 1998) and Macrobrachium lar (Fabricius, 1798) (eight zoeal stages to decapodid in 110 days, Lal et al. 2014), their pleopods only firstly appeared at three stages before the final zoeal stage (i.e. R. conspiciocellus in ZVIII; M. lar in ZV). Since the ZVIII of P. grandis still lacking pleopods, it implies that there are likely at least 12 zoeal stages with a duration of more than 120 days for the larval development in this species.