INTRODUCTION

Box crabs of the genus Calappa of the family Calappidae are remarkable because of their unique cheliped shape and their predatory behavior against gastropods and hermit crabs [18, 20, 22]. It is of interest as to at which point in their life history the calappid crabs attain their specialized form and behavior. Knowledge of larvae of the box crab larvae is, however, fragmentary [15]. The first larval description in the family was made by Smith [19] for Calappa flammea (Herbst), under the name of C. marmorata, but no figures were given. Gurney [7] provided a complete morphological description for C. lophos (Herbst) larva, while Lebour [12] described the planktonic megalopa stages of C. flammea and Cycloes bairdii Stimpson from Bermuda. Several authors have studied calappid zoeas: C. lophos [1, 14, 22], C. philargius (L.) [21], Hepatus chilensis (H. Milne Edwards) [3], Matuta lunaris (Forskål) [8, 14, 21], and Orithya sinica (L.) [9]. Recently, Seridji [17] gave a description of planktonic zoeas which were assigned to this family from the western Mediterranean. The complete larval development within the Calappidae is known only for Hepatus epheliticus (L.) of the subfamily Matutinae [2].

The larval morphology has been documented in 3 species found in Japanese water [1, 21]. Nevertheless, information on calappid larvae is still poor since this family includes 22 species [13], and it is difficult to identify calappid larvae in plankton samples.

In this paper we describe the first stage zoeas of C. japonica Ortmann and C. gallus (Herbst), compare larval characters within the Calappidae, and provide a tentative key to zoeas of the calappid subfamilies.

MATERIALS AND METHODS

Ovigerous females of C japonica and C. gallus were obtained from Shima, Mie Prefecture in April and October 1993, respectively. Collected females were kept in aquaria at 17–21°C. A female of C. japonica laid eggs on September 1 and released the first stage zoeas on September 25. In C. gallus, the crab also carried eggs on October 25 and shed the first stage zoeas on November 23. Newly hatched zoeas were fixed in 5% formalin and preserved in 70% ethanol for morphological observation. The larvae were fed with Brachionus plicatilis Müller, but all of them died. Total carapace length (TL) was measured between the tips of rostral and dorsal spines. Total width of zoea (TW) was indicated as the distance between the tips of lateral spines. Most of the terminology for setae follows that of Ingle [10]. Setation counts are from proximal to distal. Techniques for dissection, observation and drawings were almost the same as previous work [11]. Differences of size and spinule number of the zoeas were analyzed statistically using Duncan's multiple range test. The specimens used in this study have been deposited at the Zoological Institute, Faculty of Science, Hokkaido University under accession numbers ZIHU 1000 and 1001.

DESCRIPTION OF FIRST ZOEAS

Calappa japonica Ortmann, 1892

Dimensions: TL and TW range 1.48–1.78 mm (mean 1.68 mm) and 1.02–1.18 mm (mean 1.11 mm), respectively.

Carapace (Fig. 1A): A ventrally-curved rostral spine, dorsal spine, and a pair of short lateral spines. Rostral spine with 4–6 pointed spinules along its anterior half.

Fig. 1

Whole animals of two Calappa zoeas in lateral view. A: Calappa japonica Ortmann, 1892, B: Calappa gallus (Herbst, 1803). Scale bar = 0.2 mm.

Antennule (Fig. 2A): Conical with 3 aesthetascs and a short simple seta.

Fig. 2

Appendages and telson of zoea 1 of Calappa japonica Ortmann, 1892. A: antennule, B: antenna, C: mandible, D: maxillule, E: maxilla, F: maxilliped 1, G: maxilliped 2, H: telson. Whole aesthetascs or exopod setae are not shown in A, F, and G. Scale bars = 0.1 mm.

Antenna (Fig. 2B): Biramous, spinous process. Exopod pointed.

Mandible (Fig. 2C): Stout incisor process, molar processes developed.

Maxillule (Fig. 2D): Coxal endite with 6 serrated setae and a simple seta. Basial endite with 2 cuspidate setae, 2 plumodenticulate setae, and a short serrated seta. Two-segmented endopod, with 2 + 4 setae on the distal segment.

Maxilla (Fig. 2E): Coxal and basial endites bilobed, with 5 + 3 and 4 + 4 plumodenticulate setae, respectively. Unsegmented endopod with 2 + 5 setae. Scaphognathite with 4 marginal setulate plumose setae and a long plumose projection.

Maxilliped 1 (Fig. 2F): Coxa with a short seta. Basis with 2 + 2 + 2 + 2 simple setae. Endopod five-segmented, with 2,1,0,2,4 + I setae (Roman numeral indicates a dorsal seta). Exopod unsegmented, with 4 long plumose natatory setae distally.

Maxilliped 2 (Fig. 2G): No coxal seta. Basis with 1+ 1 + 1 + 1 setae. Three-segmented endopod with 1,1,3 setae. Exopod as in maxilliped 1.

Maxilliped 3 and pereiopods: Rudimentary bud.

Abdomen: Five somites plus a forked telson. Abdominal somites 2–5 each with a pair of dorsoposterior setules, and somites 2 and 3 with lateral knobs. No pleopods on abdominal somites.

Telson (Fig. 2H): Forked, each furca with 2 outer spines and a dorsal spine, and 3 long posterior setae: posterior outer spine thin and short.

Calappa gallus (Herbst, 1803)

The morphology of this congener species is almost identical with that of C. japonica except for its overall size and the morphology of the rostral spine (cf. Fig. 1). TL and TW range 1.28–1.46 mm (mean 1.35 mm) and 0.84–0.98 mm (mean 0.88 mm), respectively. The number of spinules on the anterior half of the rostral spine is 5–10, and these are longer than those of C. japonica. In one specimen, the basial endite of the maxillule had a short distal spinule instead of a seta (Fig. 3D′). The whole animal and the appendages are illustrated in Figure 1B and Figure 3, respectively.

Fig. 3

Appendages and telson of zoea 1 of Calappa gallus (Herbst, 1803). A: antennule, B: antenna, C: mandible, D: maxillule, D′: maxillular basial endite with short distal seta, E: maxilla, F: maxilliped 1, G: maxilliped 2, H: telson. Whole exopod setae are not shown in G. Scale bars = 0.1 mm.

DISCUSSION

Table 1 summarizes the morphology of the known zoeas of the Calappidae. All Indo-Pacific Calappa species are almost identical except for the overall size of C. japonica and the rostral spines. The TL and the number of spinules on the rostral spine are significantly different between the two species described here (p < 0.01), and the larvae of both species are larger than those of C. lophos and C. philargius. Calappa japonica and C. gallus also appear to differ from C. lophos and C. philargius in possessing a third outer spine on the telsonal furca, though it is possible that earlier authors overlooked this minute spine. The first zoea of C. granulata (L.), which was described from planktonic material from the western Mediterranean, is considerably different from the other known Calappa species both in its remarkable body size and the setation of the maxillule and maxillipeds.

Table 1

Comparison of main larval characters of the first stage zoeas of Calappidae

On the other hand, there are remarkable differences between the zoeas of different subfamilies within the Calappidae. We herewith give a tentative key to the early zoeas of the calappid subfamilies based on laboratory-reared materials.

The matutinid genera, Hepatus and Matuta, differ from each other in the main larval characters. This support Guinot's conclusion [4, 5] which united the genera Hepatus, Aethra, Osachila, Hepatella and Actaeomorpha in Hepatinae, distinguishing it from the Matuninae.

The zoeas of the subfamily Calappinae closely resemble those of the families Portunidae and Xanthidae in general morphology, having telsonal furca with outer spines, the antenna with an exopod, and lateral knobs on abdominal somites 2 and 3. The Calappa zoeas are distinguished from the xanthid zoeas by setal number of proximal endopod segment in maxilliped 1:2 in the former and 3 in the latter. The portunid zoeas also differ from Calappa zoeas in having a single seta on the proximal segment of the maxillular endopod and only a seta on the second endopod segment of maxilliped 1. The Calappidae, Leucosiidae, Dorippidae, and Raninidae have been grouped into the higher taxon Oxystomata in the classification of adult crabs, whereas their larvae show great morphological diversity. In recent trends in adult brachyuran taxonomy, however, this taxon is no longer valid [6]. On the basis of larval morphology, a similar conclusion was proposed by Rice [16]. Seridji [17] also adopted it from the view point of larval morphology, suggesting that the Oxystomata seemed more likely to be an assemblage of diverse taxa adapted to a similar mode of life.

All zoeas of the present two species died without feeding on the marine rotifers provided. Terada [21, 22] had the same experience with C. lophos and C. philargius. It is not clear at present whether calappid zoeas feed on zooplankton or not, although Terada [op. cit.] suggested that they might feed on phytoplankton. Yatsuzuka and Iwasaki [23] succeeded in rearing small pinnotherid larvae with specialized morphology using a combination of Chlorella and bivalve trochophore larvae as diets. Further calappid larval nutritional studies are required, since no complete larval development has been described in the subfamily Calappinae.