THE ZOEA OF ACANTHODROMIA ERINACEA A. MILNE-EDWARDS; THE FIRST DESCRIPTION OF A DYNOMENID LARVA (DECAPODA, DROMIOIDEA)

Abstract

The systematic position of the dromiacean crabs and, in particular, whether they should be included within the Brachyura remains the subject of considerable controversy among carcinologists (see, for instance, Guinot, 1978, and Saint Laurent, 1980). Students of decapod larvae have generally felt that the zoeal characters of the dromioids sensu stricto, that is, excluding the Homoloidea, ally them to the anomuran groups rather than to the brachyurans (see Rice, 1980). However, of the three extant families into which the dromioids are usually divided, that is, the Dromiidae, the Dynomenidae and the Homolodromiidae, larval descriptions are available only for the first. Although these larvae are clearly anomuran, it has been suggested (Williamson, 1976) that the Dynomenidae and Homolodromiidae might have more homolid larvae and might therefore to some extent bridge the gap between the Dromiidae and the undoubted brachyuran groups. A knowledge of dynomenid or homolodromiid larvae would therefore be very desirable. Consequently, during a recent visit to the Paris Museum, I gratefully accepted the offer of some advanced embryos of Acanthodromia erinacea A. Milne-Edwards which Mme. M. de Saint Laurent had been permitted to remove from a female in the collections of the National Museum of Natural History, Smithsonian Institution, Washington (catalog no. USNM 68165), collected by the Johnson-Smithsonian Expedition off Mona Island, West Indies, in 1933. The embryos were cleared in lactic acid and carefully dissected from the egg membranes, some of which had already been shed. The eggs must have been very close to hatching at the time of collection, but the contained larvae are in a pre-zoeal condition, with much of the segmentation not clear, the integument crumpled and misshapen, and the spines and setae partly or completely invaginated. Although the material is therefore rather unpromising, it is possible to make out sufficient detail (Fig. 1) to demonstrate that the zoeae of Acanthodromia are very similar to those known from the family Dromiidae (see Lebour, 1934; Pike and Williamson, 1960; Rice and Provenzano, 1966; Sankolli and Shenoy, 1967; Kircher, 1970; Rice et al., 1970; Wear, 1970; Lang and Young, 1980). Thus, the carapace (Fig. 1A) is elongated, without a prominent rostrum and without either dorsal or lateral spines (Fig. 1B). The abdominal somites lack dorsolateral protuberances and the sixth somite, which is fused with the telson, is much longer than broad. The telson (Fig. IC) itself is triangular, with a median notch flanked by seven pairs of posterior processes of which the second is greatly reduced and difficult to see. The antennules (Fig. ID) are elongated and simple, with an indeterminate number of terminal aesthetascs. The antennal exopod is a well-developed scale with 10 marginal setae; the endopod carries three long terminal setae and a very short subterminal one; the protopod carries a short spine at the base of the endopod (Fig. 1E). The maxillules (Fig. IF) in all the specimens examined are particularly tightly folded and details of their armature cannot be seen clearly. However, the endopod appears to be two-segmented and there is no sign of a lateral seta on the basis. The maxillae (Fig. 1G) are also tightly folded, but the scaphognathite will apparently carry about 12 marginal setae in the first stage, while the endopod and each of the endites will also be armed with