Crustacea, Decapoda: Dromiacea

THE METAMORPHOSIS OF A SPECIES OF HOMOLA (CRUSTACEA, DECAPODA: DROMIACEA) 1 ANTHONY L. RICE Institute of Marine Science, University of Miami ABSTRACT Large decapod zoeas and megalopas taken in the Straits of Florida and held in the laboratory through one moult are described and identified as the species of Homola designated "barbata" by Rathbun (1937). However several differences are noted between these zoe as and homolid zoeas described from the Mediterranean, where H. barbata is the only species of the genus known to occur. It is suggested that H. barbata, as it is currently considered, may consist of more than one subspecies, or even species, with distinct larvae. INTRODUCTION The dromiacean section of the Brachyura, containing the superfamilies Dromiidea and Homolidea, consists of a number of rather primitive crabs which are generally held to be close to the stock from which the rest of the Brachyura arose (Glaessner, 1960). Although the group is therefore of considerable phylogenetic importance the larvae are relatively poorly known. Even in those species of which larvae have been described. only one or two stages have usually been dealt with, and in only one case (Dromia vulgaris Milne-Edwards) has anything approaching the com- plete larval development been described (see Pike & Williamson, 1960). Larvae attributed to the genus Homola have been described by Boas (1880), Cano (1893), Thiele (1905), Aikawa (1937) and Pike & Wil- liamson (1960), but none of these larvae have been identified with certain- ty, since in no case have they been linked with a definitely identifiable stage. Living homolid zoeas and megalopas taken in the Straits of Florida and held through one moult in the laboratory have been identified tenta- tively as Homola barbata (Fabricius) and are described here. MATERIAL AND METHODS Six terminal zoeas: Straits of Florida off Miami, 25° 45' N, 79° 52' W to 25° 35' N, 79° 52' W; estimated depth 133 m; Isaacs-Kidd mid-water trawl fished from 1930 hrs. to 2130 hrs. RjV GERDA cruise 6338, station G-195, September 9, 1963. Three megalopas: Straits of Florida off Miami, 25° 35' N, 79° 52' W to 25° 33.5' N, 79° 52' W; estimated depth 333 m; Isaacs-Kidd mid- IContribution No. 539 from The Marine Laboratory, Institute of Marine Science, Uni- versity of Miami. This investi~ation was supported in part by Public Health Service Research Grant GM ] 1244-01 from the National Institute of Health. Research Grants G-16298 and G-20355 from the National Science Foundation, and a Harkness Fellowship of the Commonwealth Fund. 222 Bulletin of Marine Science of the Gulf and Caribbean [14(2) water trawl fished from 2208 hrs. to 2305 hrs. R/V GERDAcruise 6338, station G-196, September 9, 1963. All of these larvae were held at 20° C and about 36 ppt salinity, and fed on Artemia nauplii and small pieces of shrimp. The water was changed every two days. Two of the zoeas died without moulting, one died in the process of moulting to the megalopa and three moulted successfully to the megalopa after 4-1'0 days in the laboratory but died before reaching the first crab stage. One of the megalopas died without starting to moult, one died after moulting incompletely and one moulted successfully to the first crab on the 27th day after capture and died five days later. Exuviae and dead animals were preserved in 70 per cent alcohol with 10 per cent glycerine added. They were stained with chlorazol black, dis- sected in glycerine and examined under a binocular microscope. Measurements were made with the aid of an ocular micrometer. The total length (TL) was measured from the tip of the rostrum to the posterior median margin of the telson, excluding the telson processes, and the carapace length (CL) was measured from the tip of the rostrum to the posterior median margin of the carapace. Drawings were made with the aid of a camera lucida. In many of the illustrations the secondary setules are omitted for clarity, and even where they are indicated their representation is diagrammatic since no attempt at quantitative accuracy was made in the case of these structures. DESCRIPTION Terminal zoea (Fig. 1) .-CL 5.2-5.8 mm; TL 10.4-11.4 mm. In addi- tion to the acute forwardly directed rostrum the carapace has three blunt dorsal projections in the mid-line and well developed paired supra-orbital and branchiostegial spines anteriorly. On each side there is a pointed anterolateral spine, a shorter pointed posterolateral spine, a blunt dorsolateral projection, and a prominent downwardly directed spine at the posteroventral corner. Other prominent features of the carapace include a small bulge on either side of the anterodorsal projection, a larger median hemispherical bulge behind this projection, and a system of low ridges connecting the major carapace spines. Abdominal somites 2-5 each have a mediodorsal spine and paired dorsolateral spines, all of which decrease in size posteriorly. The first somite has only a blunt dorsal projection and somite 6 has a pair of long dorsolateral spines but no dorsal spine. Ventrolaterally on abdominal somites 1-6 there are posteriorly directed projections which become more prominent posteriorly. The telson is wider than long (L/W ratio 0.64-0.70) with concave posterior and lateral margins and acute posterolateral corners. There 1964] Rice: Metamorphosis of Homola 223 FIGURE 1. Homola barbata: Terminal zoea. a, Dorsal view.-b, Lateral view. --c, Posterior dorsal view of carapace. Bar scale represents 5 mm. 224 Bulletin of Marine Science of the Gulf and Caribbean [14(2) are 37-46 plumose processes articulated to the posterior margin, every 4th to 6th such process being rather larger than the rest. There is a pair of large dorsal spines on the telson, close to the lateral margin. Abdominal somites 2-5 each carry a pair of biramous, non-setose pleopods (Fig. 2f and g) which are considerably better developed in late terminal zoeas than in earlier examples of the same stage. The sixth somite carries well developed uropods, the external rami each with about 50 plumose setae and the inner rami each with 4'0-45 plumose setae. The eyestalks consist of a narrow proximal portion and a swollen terminal portion on the anterior margin of which there is a prominent spine immediately proximal to the corneal region. The antennular peduncle (Fig. 2a) is completely segmented and has a rudimentary statocyst in the proximal portion. The three-segmented inner flagellum is unarmed and the outer flagellum consists of nine or ten segments of which all but the proximal two carry a row of aesthetes (about 50-60 in all). The antennal endopod (Fig. 2b) consists of 14-16 segments and carries a single terminal seta. The sub-ovate scale carries 34-39 long plumose setae. There is a single unarmed spine on the basipodite at the base of the endopod. The mandibles (Fig. 2c) each have an unsegmented palp which may carry a non-plumose terminal seta. The endopod of the maxillule (Fig. 2d) is two-segmented, bearing a total of six long plumose setae. The endites are armed with many fairly stout setae. The scaphognathite of the maxilla (Fig. 2e) has a large, bilobed posterior projection and carries about 110 plumose marginal setae. The endopod is unsegmented and carries 10-12 long plumose setae. The basal and coxal endites are both bilobed and are armed with many marginal and sub-marginal setae. The exopods of the maxillipeds (Fig. 3a, band c) each have a con- striction at about the mid-point and carry 21-26 nata tory setae. The endopods of all three maxillipeds consist of five segments, though the septa between the second, third and fourth segments in the third maxilliped are not very clear. The medial margin of the basipodite bears 20-24 setae in the first maxilliped, four in the second and two in the third. The segmentation of the developing pereiopods (Fig. 3d, e and f) is visible beneath the zoeal cuticle and the first pair are already cheliform. The first, second, and third legs each carry a small exopod, but this struc- ture is apparently missing in the fourth and fifth pairs. The zoeas are generally orange-red in color, with at least part of this coloration being deep in the thorax and in the eyestalks. However, the surface of carapace and abdomen have numerous red chromatophores 1964] Rice: Metamorphosis of Homola 225 f b c f g a d e FIGURE. 2. Homola barbara: Terminal zoea. a, Antennule.-b, Antenna.-c, Mandible.-d, Maxillule.-e, Maxilla.-f-g, Pleopods of second and fifth abdominal somites, respectively. Bar scales represent 1 mm. 226 Bullet;n of Ma,;ne Sdence of the Gulf and Car;bbean [14(2) FIGURE 3. Homola barba/a: Terminal zoea. a, b, c, First, second, and third maxillipeds, respectively.-d, e, f, First, second, and third pereiopods, rcspe<:t- ively. Bar scale represents 1 mm. which also extend into the proximal parts of the appendages. Megalopa (Fig. 4).-CL 7.25-7.50 mm; TL 12.0-12.75 mm. The downwardly directed rostrum projects slightly farther than the paired frontal spines. The carapace has three low blunt projections in the mid-line and several other paired knobs and bulges. There is a well-marked groove separating the gastric and hepatic regions. The whole surface of the carapace carries short setae, those in the gastric region each arising from a small tubercle. These surface tubercles are enlarged in certain localized areas of the anterolateral region of the carapace. The abdominal tergites are well rounded from before backwards and 1964] Rice: Metamorphosis of Homola 227 FIGURE 4.

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