Brachyura larvae
Identification of megalopa of Brachyura

In Table 4 Known larval development the present knowledge of larval development for families of brachyuran crabs from the Southwest Atlantic Ocean is summarized. As mentioned in this chapter before (Page Bl. 1 Introduction, last paragraph: important note), it is important for the reader to realize that because of many unknown larvae, those to be identified may not fit any larvae described in this volume. However, in such cases the keys may help in narrowing the search to higher groups, such as families. It also must be recognized that most of the larval accounts are based on laboratory rearings, and it is still unclear how much variability there is between specimens obtained from the wild and those obtained from culture (Ingle, 1992). Thus definitive identifications should be obtained by consultations with experts in the field.

The megalopa is the last larval stage and is the least well known among crab larvae, unrecognized in some species with known zoeal stages and being poorly described in other cases. While megalopae show many distinct features, these characters are often not suitable for identification because they are not available for enough species to be useful for comparative purposes. There are also few obvious features that readily characterize members of specific families. This complicates identification to a point that does not allow for the construction of keys that are easy to follow. Thus, for identification purposes the reader should instead note the size and gross morphological features of collected specimens (see below), and then compare these with the illustrations provided. Subsequently, Tables 15-19 (see the links at the bottom of this page) can be used for identification verification, by checking patterns of appendage setation.

Gross-morphological features that should be used in the identification process include the following:

1. Relative size of specimens in terms of carapace length or width, some being much larger (e.g. Ocypode quadrata) than others (e.g. Clypeasterophilus stebbingi).
2. Relative shape of the carapace, by comparing length to width, and the shape and width of the anterior margin between eyes (e.g. contrast Aratus pisonii, Armases angustipes, and Acanthonyx scutiformis).
3. Ornamentation of the carapace; this can range from a smooth surface (e.g., Dissodactylus crinitichelis), hairy appearance (Cryptodromiopsis antillensis), to cuticular projections in specific locations that range from rounded knobs (Mithrax hispidus) to “antlers” (Latreillia elegans) and spines of various lengths (e.g., compare Libinia spinosa and Tumidotheres maculatus).
4. Characteristics of pereopods: note the relative length of walking legs to the carapace (e.g., very long for Stenorhynchus, short in Libinia and Taliepus), the absence or presence of long swimming setae on the dactyls of the last pair of legs (e.g., compare Dissodactylus crinitichelis and Tumidotheres maculatus), and if the same dactyl is turned upward (e.g., in dromiids) or not (the common feature).
5. Projections on the sternum and abdomen, such as distinct spines that may be present (e.g. portunids) or absent (e.g. pinnotherids).

Tables with species characters for known megalopa:

Table 15 Mgp Dromiidae Latreilliidae Calappidae Leucosiidae
Table 16 Mgp Hymenosomatidae Atelecyclidae Beliidae Portunidae
Table 17 Mgp Majidae Parthenopidae
Table 18 Mgp Xanthidae
Table 19 Mgp Pinnotheridae Grapsidae Gecarcinidae Ocypodidae