Brachyura larvae
Introduction
A proper knowledge of zooplankton, including its larval members, is of fundamental importance since it forms a vital link between primary producers and different consumer levels in the food chain (Wickstead, 1976). Crustacea are among the most prominent animals of the marine zooplankton (Hardy, 1958), to which they often contribute 50% or more of the biomass (Friedrich, 1969a). During their life cycles most decapod crustaceans spend some time as part of this community. With about 10,000 species (Bowman and Abele, 1982), the "ten-footed" Decapoda (Greek deka: ten, and pous: foot) represent the largest and most varied order of crustaceans, encompassing about one-third of known crustacean species. This order includes the typical larger and well known crustaceans, many of which live on or close to the bottom of the sea as juveniles and adults, but spend their larval life as part of the plankton.
Decapods have two basic adult body plans. Shrimps and lobsters possess well developed tail sections. In contrast, the Brachyura, or short-tailed decapods, have a flat abdomen flexed under the body. This group comprises the true crabs. With about 5,000 species worldwide (Melo, 1996), the true crabs represent half of the Decapoda. Crabs reach their greatest diversity in tropical regions, although a significant number are also found in temperate waters. Thus, within the South Atlantic alone, 328 crab species are presently recognized that belong to 170 genera among 24 families (Table 1 South Atlantic brachyuran species). The recent accounts of Boschi et al. (1992), Zolessi and Philippi (1995), Martins and D’Incao (1996), and Melo (1996) include most of these species.
Perhaps surprisingly, most of the larvae of these crabs remain unknown. Larval information presented here is available for 102 species, representing less than one third of all known crab species within the South Atlantic. While far from complete, this denotes significant progress over the last 16 years. In comparison, the last coverage of decapod larvae from the area (Boschi, 1981) included only about one quarter of the brachyuran species covered here. Nevertheless, it is clear that we presently still have limited ability in identifying the decapod larvae from the South Atlantic. For example, larvae of several families, including the Cymonomidae, Raninidae, Cyclodorippidae, Geryonidae, Goneplacidae and Palicidae, are unknown within the South Atlantic (Table 14 Species with known larval development).
Among crustaceans, decapods are considered to be amongst the most advanced groups. Within the lineage of crawling decapods, or Reptantia (Latin reptare: to creep), the brachyurans represent the most evolutionary advanced forms, together with their sister group, the Anomala, comprising the hermit crabs and their relatives (Scholtz and Richter, 1995). Our understanding of the evolution of different brachyuran and other decapod groups, however, is still quite poor. Knowledge of ancestor-descendant relationships is largely based on adults. They display a vast array of highly specialized adaptations that may mask their evolutionary relationships. Evidence from larvae may help resolve these problems and early developmental stages are now increasingly used in phylogenetic reconstruction (Rice, 1980; 1983; Clark and Webber, 1991; Marques and Pohle, 1995; Pohle and Marques, 1998).
—General life history and larval stages
Even though larval types of most decapods are described below, the brachyuran larval stages found within the plankton are the focus of this chapter. During this phase of their life-cycle, larvae bear very little resemblance to the juvenile and adult form, and the inexperienced observer would be hard pressed to recognize the developmental stages of crabs. In fact, naturalists of an earlier day believed that such larvae represented different animals (Schmitt, 1971).
Before hatching, eggs of true crabs are extruded and brooded by the female in the space between the thorax and cupped abdomen. The number of eggs produced per brood varies widely, from as little as 200 (Telford, 1978), to as many as 8,000,000 (Prager et al., 1990; Mantelatto and Fransozo, 1997). In tropical and subtropical areas, most species spawn and hatch batches of eggs throughout the year (Negreiros-Fransozo and Fransozo, 1995; Negreiros-Fransozo et al., in press; Mantelatto and Fransozo, 1998). In these regions incubation periods may be as short as 1-2 weeks (Pohle, 1994), egg size and temperature being determining factors. Larval development is also temperature dependent (Christiansen, 1973), a higher temperature shortening the period, and salinity also affects the duration of the larval phase (Fransozo and Negreiros-Fransozo, 1986; Negreiros-Fransozo and Fransozo, 1990). However, the number of larval stages is another determining factor in the length of the larval period. A warm-water species with 5 larval stages can reach the last larval stage in as little as 9-10 days after hatching (Marques and Pohle, 1996a; Fransozo and Negreiros-Fransozo, 1997).
Brachyurans have two distinct types of larvae, the zoea and megalopa. Zoeae emerge from eggs that usually hatch at night. In some species and under certain conditions, eggs may hatch as a prezoea before molting to a zoea. However, these short-lived prezoeal stages, that are still enclosed within a thin cuticle, are not usually found in the plankton.
—The zoea
Zoeae of various species appear very different from juvenile and adult stages (Zoea lateral view) but are themselves superficially alike. Thus all zoeae usually have large paired eyes and a full complement of carapace spines, consisting of a dorsal, rostral and lateral spines that give specimens a triangular upright appearance. A notable exception are dromiid and homolid larvae that look more shrimp-like (Dromia erythropus ov, Cryptodromiopsis antillensis ov,[l][m]Introduction[/m][r]Gb. 6 General remarks on seasonality, biomass and diversity in the pelagic domain of the South Atlantic[/r] Hypoconcha sabulosa ov ,Homola barbata ov). An abdomen consisting of a number of somites and ending in a flat fork-shaped telson protrudes from the carapace, as do two pairs of swimming appendages. Other carapace appendages are less apparent. The number of zoeal stages may vary from a single (Goodbody, 1960) to more than ten (Brossi-Garcia and Rodrigues, 1993; Cuesta and Rodrigues, 1994), depending on the species. Only within the family Majidae are there always only two zoeal stages. Older zoeal stages have the same general appearance but can be recognized by movable eyes, paired buds of appendages on the abdomen, rudiments of claws and legs under the carapace and by an increasing number of swimming setae on the locomotory appendages. However, for a considerable number of species only the first zoea is known.
—The megalopa
The last zoeal stage undergoes a metamorphosis during the molt to the megalopa. The latter also have been referred to as the megalops (Sastry, 1970), megalop (Clark et al., 1998), decapodid (Felder et al., 1985) or postlarva (Gurney, 1942). This stage has a more flattened, crab-like appearance, with legs and claws protruding from the carapace (Megalopa dorsal view). The spines of the carapace have either been lost or are greatly reduced. Unlike adult crabs, however, the abdomen has appendages used for swimming when it is unfolded, with the attachments folded under the carapace while at rest. In comparison to other stages, larval information on the megalopa is the poorest because it is not described in a number of larval publications. This is likely associated with difficulties in rearing, metamorphosis to the megalopa resulting in high mortality. This terminal larval stage is a transitional stage that settles out of the plankton and molts into the first crab instar, the first fully benthic stage. Additional morphological details of larval stages are given in the section dealing with identification.
—Biology
Brachyuran zoeae are only a few millimeters in size, but are vigorous swimmers. Using both the maxillipeds and abdomen to propel themselves upward and forward in pulses, with the dorsal spine often pointing in the direction of swimming, they swim at speeds of about 1-2 cm sö-1 (Warner, 1977). When not active, zoeae sink, and thus they must constantly swim upwards to remain in the same place. In contrast, the megalopa swims smoothly forward with the dorsal spine in a vertical position, using abdominal appendages for propulsion. During swimming the legs are tucked close to the body to minimize resistance. Like many larval forms, zoeae react positively to light and a megalopa is initially also attracted to light but this is no longer the case during settlement.
Zoeal stages will feed on a large variety of phyto- and zooplanktonic organisms, appropriate size being more of a determining factor than type of food. However, evidence suggests that animal food is essential to complete larval development (McConaugha, 1985). Zoeal stages use the abdomen in the capture and manipulation of food. The megalopa feeds on other decapod larvae, copepods and young fish, using the claws for prey seizure and holding.
For decapod larvae, as for all marine animals with planktonic larvae, there is high mortality during larval life. Survival to a newly settled crab has been estimated at less than one tenth of a percent (Warner, 1967) for a single brood. This is compensated for by producing vast numbers of larvae. The advantage of producing planktonic larvae is enhanced dispersal, allowing for rapid colonization in distant areas, and the general great abundance of food in the plankton.
—Important note
Given the present limited knowledge of South Atlantic larvae, it is important for the reader to realize that larvae being identified may not fit any of those described herein. However, in such cases the keys, tables and figures will 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.
For further reading on decapod larvae and their development, the reviews of Gurney (1942), Rice (1980), Williamson (1982) and Gore (1985) are recommended, as well as other references summarized at the end of this section.