Atlas of Crustacean Larvae

41. Cumacea

general: Cumaceans are small benthic peracarids with a distinctive shape. The carapace is large and inflated, and the thorax and abdomen are relatively slender. Currently, 8 families and approximately 100 genera are recognized. There are about 1,500 described species, most of which live in the first few centimeters of sand or mud substrates in marine or brackish waters, from tidal to abyssal depths (Gerken 2005; Watling 2005). Sexual dimorphism is often pronounced. Cumaceans are typically very small, from 1 to 10 mm long, but some species in colder waters can be 30 mm long. All known species have direct development, with the egg hatching in the marsupium and developing into the first manca stage, which is released from the marsupium looking similar to the adults, but lacking the last pair of pereopods. Thus the life cycle consists of an egg, manca I, manca II, a subadult, and an adult, all of which are free living. larval types: Cumaceans have several distinct life stages, but, as in all peracarids, they have no true larval stages. Manca: The cumacean embryo goes through several stages in the marsupium (fig. 41.1A), hatching from the egg and then developing through 5 or 6 stages (fig. 41.1B–E) (Bishop 1982; Akiyama and Yamamoto 2004a) into the first manca stage (fig. 41.1F), which is defined as being entirely without the fifth (last) pair of pereopods. Sars (1900a) observed the development from an egg to a manca stage for several different genera , but he described the process only for the embryos of Diastylis lucifera, while stating that development was “essentially alike” in the other genera (Leucon, Lamprops, and Pseudocuma). The embryos are released from the brood pouch as the first manca stage. Manca I then molts into the second manca stage, where the final pair of pereopods are indicated by a ventral expansion on pereonite 5 (fig. 41.1G). Sars (1900a) suggested that Diastylis lucifera released its young from the brood pouch at the manca II stage, with the limb buds evident when the young were released. While this may be true for D. lucifera, it is not the rule, as it is generally reported that young are released from the brood pouch at the manca I stage, and in field collections it is common to encounter manca I individuals with food in the gut. morphology Manca: The manca is defined by its lack of a fifth (last) pair of pereopods (fig. 41.1F, G). Compared with adults, mancae can be difficult to identify to species, as they tend to have fewer setae, less developed carapace ornamentation, and different proportions. The telson and uropods are more alike in proportion to each other than they are in the adult, and the appendages generally have articles that are more similar in length. The pereopods may be shorter than in the adult, and the first manca lacks the last pair of pereopods. Relative to the carapace, the abdomen is generally shorter than in the adult. morphological diversity: The ceratocumatid genus Ceratocuma entirely lacks the fifth pair of pereopods in adults, making it easy to confuse adults with manca stages. The lampropid genus Stenotyphlops can also be confusing, as the fifth pair of pereopods is represented only by a tiny twoarticle filament; thus manca I and manca II stages are very difficult to identify, except by their smaller size relative to the adults, which is not helpful if there are no other individuals at a different stage of development present for size comparison . Depending on the species, sexual dimorphism may or may not begin to be evident in the manca stage. Juvenile or subadult males and females are quite similar in appearance, with the only differences being the development of additional pereopod exopods, pleopods, and antennae on the males, and the development of brood plates in the females. In males, the developing exopods, pleopods, and antennae typically are formed over several molts, increasing in size and complexity with each molt until attaining their final form in a terminal molt. In the terminal molt, long setae are added to the pereopod exopods and pleopods, the antennal peduncle becomes setose, and the antennal flagellum is present in its final form, 41 Sarah Gerken Joel W. Martin Sarah Gerken and Joel W. Martin Cumacea Cumacea 217 with distinct annulations and setae. Also, the male typically acquires a carapace in the terminal molt that may vary quite remarkably from that of the female, with reductions in ornamentation and texture, and with an...