Complete larval development of the rare porcellanid crab, Novorostrum decorocrus Osawa, 1998 (Crustacea: Decapoda: Anomura: Porcellanidae), reared under laboratory conditions

The complete larval development of Novorostrum decorocrus Osawa, 1998, is described and illustrated on the basis of laboratory‐reared material. Two zoeal stages and one megalop stage were recorded. Zoeas of N. decorocrus closely resemble those of N. indicum in the appendage characters, including the endopod of the maxillule with only a single stout seta on the distal margin. This character is unique to Novorostrum zoeas. However, N. decorocrus is distinguished from N. indicum by the setation on the endopod of the maxilla and the basis of the second maxilliped in both zoeal stages. The megalops of N. decorocrus are characterized by having a strongly elongate carapace, and differs considerably from the adults in the structure of the carapace, rostrum and third thoracic sternite, and in the armature of the pereiopods. The larval duration of N. decorocrus suggests that this rare porcellanid is more widely distributed than currently known.


Introduction
The porcellanid genus Novorostrum Osawa, 1998 was established for a species formerly attributed to the genus Petrolisthes Stimpson, 1858, N. indicum (de Man, 1893) (type species) and two new species, N. decorocrus Osawa, 1998 andN. phuketensis Osawa, 1998, based on the morphology of the carapace, rostrum and ocular peduncle. Osawa (2000) subsequently described the zoeal stages of N. indicum and reported that they possessed two characters that supported the establishment of the genus Novorostrum. These characters were the exopod of the antenna possessing only small spines and the endopod of the maxillule with only a single stout seta on the distal margin. Recently, Fujita et al. (2002) pointed out that the former character is also observed in zoeas of Petrolisthes unilobatus Henderson, 1888, but the latter character is still useful for discriminating zoeal larvae of N. indicum from those of P. unilobatus.
Novorostrum decorocrus appears to be very rare, and has only been observed on a few rocky shores on Iriomote Island in the Ryukyu Archipelago, southern Japan (Osawa 1998;unpublished data). Fortunately, we were able recently to collect ovigerous females and to rear the larvae in the laboratory from hatching to the megalopal phase. We herein describe and illustrate the complete larval development of N. decorocrus, and compare the morphological characters with larvae of the congener, N. indicum, and with larvae of species of the allied genus Petrolisthes.

Materials and methods
On 20 July 2001, two ovigerous females of N. decorocrus were captured by hand from the intertidal rocky zone at Sonai beach of Iromote Island. Each crab was transported to the laboratory at the University of the Ryukyus and isolated in a 1.4-litre plastic aquarium until the larvae hatched. Hatched zoeal larvae were mass-cultured in a circular plastic tank (30 cm diameter) filled with filtered sea water (8 litres). Newly hatched Artemia nauplii were provided as food for zoeal and megalopal larvae. Water temperature and salinity ranged from 28.0 to 28.5uC and 34.5 to 35.0%, respectively. Approximately one-third of the water in the tank was changed daily, and then moults of larvae were checked to determine the duration of each stage.
Larvae were fixed and preserved in 50% ethylene glycol for morphological observation. Observations and drawings were made using a Nikon Optiphot-2 binocular microscope equipped with a drawing tube. Five specimens of each larval stage were examined for variations in appendage setae. Body somites are described from anterior to posterior, appendages from endopod to exopod, and segments and setae from proximal to distal. Setal formulae for the segments and distinctions between setae and spines are shown in Osawa (1997a). The long, plumose natatory setae on the exopods of the first and second maxillipeds in the zoeal stages are not fully illustrated but are drawn truncated. The terminology of setae generally follows that of Ingle (1991). Usage of the terms 'zoeas (plural)', 'megalop (singular)', 'megalops (plural)' and 'basial endite' of appendages follows the recommendation by Clark et al. (1998). Methods for measuring carapace length (CL), rostral spine length (RSL) and posterior spine length (PSL) in zoeas followed those of Osawa (1995), whereas measurement of CL of megalops followed that of Fujita et al. (2002).
The dissected or undissected larvae and the spent females are deposited in the National Science Museum, Tokyo, under the registration numbers of NSMT-Cr 15245 (first zoeas), 15246 (second zoeas), 15247 (megalops) and 15248 (spent females), respectively.

Results
Novorostrum decorocrus passed through two zoeal stages and reached the megalop phase, but no larvae metamorphosed to first-stage crab. The total duration of zoeal stages, from hatching to the end of the second stage, ranged from 12 to 14 days. The minimum durations of the first and second zoeal stages were 4 and 8 days, respectively. Morphological characters of the larvae are described below and summarized in Table I  Carapace ( Figure 1A-C). Typical porcellanid; rostral spine extremely long, 3.99 times CL, with numerous spinules along entire length; posterior spines with spinules along entire length, ventral spinules becoming large on proximal part; posteroventral margins of carapace with 11-16 spinules; eyes sessile.
Second maxilliped ( Figure 2H). Biramous; coxa without setae; basis with two setae on ventrodistal margin; endopod four-segmented, first and second segments each with two simple/sparsely plumose setae on ventral margin, third segment with 1+2 simple/sparsely plumose setae each situated at median and distal parts of ventral margin, fourth segment with five plumodenticulate setae distally and one plumose seta at dorsoproximal angle, fine setules present on dorsodistal margin of second segment and dorsomedian margin of third segment; exopod as in first maxilliped.
Abdomen ( Figure 1B, D). Five somites; fifth somite longest; second to fifth somites with pair of short simple setae near posterolateral angles and distinct serration on posterodorsal margin; third to fifth segments with pair of posterolateral spines; pleopods absent.
Colour in life. Carapace, abdomen, telson and appendages essentially transparent; median gastric region red-brown; dorsal and ventral margins of rostrum and posterior spines pale orange; orange and/or red-brown chromatophores present on mandibles, proximal part of maxilla, first to fifth abdominal somites and proximal part of telson.
Telson ( Figure 3B, D-I). Similar to first zoea except for posteromedian margin with a pair of short plumodenticulate setae and dorsal surface with four pairs of short plumose setae as illustrated.
Colour in life. Similar to first zoea. Carapace with scattered small red or orange chromatophores; antennal endopod and pereiopods with orange chromatophores.
Antennule ( Figure 6A). Peduncle three-segmented; first (proximal) segment broad, with small granules on anterolateral margin and row of simple/plumose setae on lateral margin and ventral surface; second and third segments with few short simple/plumose setae; flagellum biramous; endopod two-segmented, proximal segment slightly longer than distal segment, with seven or eight simple setae, distal segment with 10 simple setae; exopod sixsegmented, with row of aesthetascs on second to fifth segments, numbering proximal to distal, 0-3+7-8, 7-9+4-5, 3+3, 3, respectively, fourth and fifth segments with two or three short simple setae, distal segment with one long and short simple setae.
Third maxilliped ( Figure 6H). Biramous; coxa with 11-14 plumose setae and two serrate setae; basis narrow, with three or four plumose setae; endopod five-segmented; ischium with seven or eight long plumose and 14-18 short simple setae, distolateral projection weakly produced; merus with broad, rounded lobe on mesial margin, and 14-16 long plumose and 10-17 short simple setae; carpus with 11-15 long plumose, eight or nine serrate and 10-14 short simple setae; propodus with 17-20 long plumose, six to nine serrate and five to eight short simple setae; dactyl with 14-16 long plumose setae, four to six serrate setae and one to three short simple setae; exopod elongate, overreaching distal margin of ischium, incompletely two-segmented, with one or two proximal and zero to two terminal short simple/plumose setae. Figure 5A, E-G). All legs fully developed, with numerous short simple/plumose setae and covered with minute denticles. Cheliped (first pereiopod) approximately as long as carapace, flattened dorsoventrally, with scattered short plumose setae on dorsal surface; carpus with one large proximal tooth and four to six small teeth on dorsoflexor margin, all teeth acutely pointed, extensor margin slightly serrated; propodus with rows of small acute teeth on extensor and flexor margins, dorsal surface with several small denticles; dactylus with row of small acute teeth on flexor margin, dorsal surface with small denticles. Ambulatory legs (second to fourth pereiopods) flattened, with scattered short plumose and simple setae on lateral and dorsal surfaces, lateral surfaces of merus, carpus and propodus with small denticles; merus with row of small teeth on extensor and flexor margins; carpus slightly crenulated on extensor margin; propodus with three to five small movable spines on flexor margin, lateral spine of distal pair largest, extensor margin nearly smooth; dactylus with three small movable spines on flexor margin. Fifth pereiopod short, slender, chelate; propodus with six to nine long, distally curved, serrate setae and scattered short simple/plumose setae as illustrated. No male or female gonopores.
Colour in life. Body and pereiopods generally transparent, carapace and abdomen with scattered orange or red chromatophores; antennule, antenna and third maxilliped with orange chromatophores; chelipeds and ambulatory legs with orange-red coloured spines, denticles and fringe of setae.

Discussion
Zoeas of Novorostrum decorocrus closely resemble those of N. indicum as described by Osawa (2000) but are distinguished from the latter by the following characters: (1) the posteroventral margin of the carapace has 11-16 and 5-11 (usually seven to nine) spinules in the first and second zoeas, respectively (in N. indicum, there are 8-10 and four or five spinules in the first and second zoeas, respectively); (2) the endopod of the maxilla bears 3+4 setae throughout two zoeal stages (in N. indicum there are 3+2+3 setae); and (3) the basis of the second maxilliped bears two ventrodistal setae throughout two zoeal stages (in N. indicum there are two distal setae and one proximally separate seta). In addition to these distinctions, N. decorocrus sometimes lacks a very short subdistal mesial seta on the endopod of the maxillule throughout two zoeal stages and a dorsal plumose seta on the second segment of the second maxilliped endopod in the second zoea. Osawa (1995) divided 19 Petrolisthes species, for which larval morphology was known at that time, into six groups based on the zoeal characters. Fujita et al. (2002) updated Osawa's (1995 grouping for 28 species including P. unilobatus and added a new group for the latter species. As discussed for zoeas of N. indicum by Osawa (2000), N. decorocrus zoeas also have close affinities with Petrolisthes Group 1 of Osawa (1995), which contains P. japonicus (de Haan, 1849) and P. ornatus Paulson, 1875. All three species share the following characters: in the first zoea, (1) the basial endite of the maxillule has seven marginal spines; (2) the maxilla usually has five (sometimes four or six) plumose setae on the posterior lobe of the scaphognathite; (3) the endopod of the first maxilliped is fivesegmented, the fifth segment possesses a plumose seta on the dorsoproximal angle; (4) the telson is longer than broad, with all posterior plumose setae bearing distal hooklets (spinules); and in the second zoea, (5) the mandible lacks a palp; and (6) the telson has six pairs of plumose setae on the posterior margin. However, the setation on the endopod of the maxillule throughout the zoeal stages immediately distinguishes Novorostrum species from the other porcellanid genera including Petrolisthes. In Novorostrum species, the endopod of the maxillule has only a single stout seta on the distal margin, but it usually possesses three to seven setae in other porcellanid species for which larval morphology is known (see Osawa 2000). In addition to this distinction, Novorostrum differs from Petrolisthes Group 1 in having 8-16 (first zoea) and 4-11 (second zoea) spinules on the posteroventral margin of the carapace. Petrolisthes japonicus has only one or two spinules (see Osawa 1995) and P. ornatus is illustrated as unarmed (see Yaqoob 1977, Figure 1A, B).
The present study contributes the first description of the megalop of Novorostrum species. The megalops of N. decorocrus appear to be distinguished from those of both P. japonicus and P. ornatus, discussed above, by having a more strongly elongate carapace (see Yaqoob 1977; Osawa, personal observation). The shape of the carapace also distinguishes N. decorocrus from P. elongatus (H. Milne Edward, 1837) and P. unilobatus, two related species based on morphological similarities in larvae and adults (see Wear 1964;Greenwood 1965;Osawa 1998;Fujita et al. 2002).
The megalopal morphology of N. decorocrus is considerably different from that of the adults: (1) the carapace is elongate-oval in the dorsal view (subtrapezoidal in the adults); (2) the rostrum is subtriangular (trilobate in the adults); (3) the median lobe of the third thoracic sternites is indistinct (developed as long as the lateral lobes in the adults); and (4) the palms of the chelipeds and carpi and propodi of the ambulatory legs lack distinct projections on the extensor margin, which are observed in the adults. In the adults, the armature on the chelipeds and ambulatory legs immediately distinguishes N. decorocrus from two congeners, N. indicum and N. phuketensis. However, the megalop of N. decorocrus lacks such a unique character.
In the present study, the zoeal phase of N. decorocrus has a putative planktonic period of 12-14 days at a water temperature of 28.0-28.5uC under laboratory conditions. This duration generally agrees with that of other sympatric porcellanids, such as Petrolisthes asiaticus (13-17 days) and P. hastatus (12-13 days) (at a water temperature of 29.0-29.5uC under laboratory conditions; see Osawa 1997b). These two Petrolisthes species are known to be widely distributed in the Indo-Pacific region (see Kropp 1984;Haig 1989). This indicates that N. decorocrus zoeas have a dispersal potential comparable to P. asiaticus and P. hastatus, and the distribution of N. decorocrus is possibly wider than currently known. However, this species has been observed on only a few rocky shores on Iriomote Island of the western Ryukyu Archipelago, southern Japan. On the other hand, Jensen (1989Jensen ( , 1991 reported that megalops of two north-eastern Pacific porcellanids, Petrolisthes cinctipes (Randall, 1839) and P. eriomerus Stimpson, 1871, settled gregariously in response to conspecific adults. Jensen and Armstrong (1991) subsequently determined that the zonation patterns observed for these two sympatric species were apparently more a passive consequence of physiological limitations and substratum preferences than the result of continuing biotic interaction. These may suggest that N. decorocrus has a much narrower substratum preference as juveniles and adults than sympatric porcellanids such as P. asiaticus and P. hastatus, although detailed ecological data on these species are needed.