Atlas of Crustacean Larvae

50. Gebiidea and Axiidea (= Thalassinidea)

general: Gebiids and Axiids are widely referred to as ghost shrimps (Callianassidae), mud shrimps (Upogebiidae), mud lobsters (Thalassinidae), or lobster shrimps (Axiidae) (McLaughlin et al. 2005). The two groups presently include 13 families, containing 615 extant species that represent about 4.2 percent of all decapods (De Grave et al. 2009). Recent molecular studies have recognized thalassinideans as a paraphyletic group, now separated into the infraorders Axiidea and Gebiidea (Bracken et al. 2009b; Robles et al. 2009). But because of common usage, in this chapter we still refer to them collectively as thalassinideans. Thalassinideans appear shrimp-like, with elongate and flexible subcylindrical bodies and variable sclerotization of the exoskeleton. The group has a near-global distribution—from 71° N to 55° S latitudes (Robles et al. 2009)—within estuarine and marine habitats. Here they inhabit soft sediments, mainly within intertidal and shallow subtidal waters, although they are also found to depths exceeding 2,500 m (Heard et al. 2007). Many are specialized to construct and live in elaborate burrows in sand or mud, where they constitute an important bioturbation component that affects sediment properties and processes (Griffis and Suchanek 1991). The life cycle consists of a pre-zoea, a zoea, a decapodid (megalopa), a juvenile, and an adult, all of which are free living. larval types: The larvae of thalassinideans can be separated into two distinct groups, those resembling anomuran larvae, now treated as the Gebiidea, and those resembling the larvae of nephropid lobsters, which form the “homarine group,” now treated as the Axiidea (Gurney 1938, 1942). Collectively , larval information is very limited, available only for approximately 13 percent of the known species and 25 percent of the genera. Complete larval descriptions are available for only about one third of the larval accounts, and the larvae of at least 4 families (the Callianideidae, Ctenochelidae, Michelidae , and Thomasiniidae) presently are unknown (Pohle et al. 2011). Also, larval descriptions of some species are based on plankton-collected material, and thus are of uncertain parentage . Consequently, assignment of these larvae to genera/ species should be viewed with caution. Pre-Zoea: As in other decapods, a pre-zoea can be the hatching stage in some thalassinidean species (fig. 50.2A–C). This stage is non-feeding, of very short duration, and thought to have very poor mobility (Vaugelas et al. 1986; Strasser and Felder 2000). Zoea: The subsequent zoeal phase (fig. 50.1A, K) consists of two to seven stages. While usually characteristic for a species, the number of zoeal stages can vary intraspecifically, which appears to be more common in callianassids (Strasser and Felder 1999, 2000). Megalopa: The zoea is followed by a decapodid (megalopa ), which resembles the adults more than the previous larval stages (fig. 50.1Q, R). The decapodid has also been referred to as the first post-larva. Subsequent stages change more gradually in morphology than, for example, in the brachyurans. Consequently, they are at times referred to as second and third post-larvae and the like (e.g., Shenoy 1967), but they effectively represent juvenile stages (Felder et al. 1985). An apparent exception to this developmental sequence is Upogebia savignyi, a sponge commensal that lacks a free larva and has hatchlings that are basically in adult form (Gurney 1937). morphology Pre-Zoea: Few larval accounts present information on this short-lived stage (e.g., Pohle et al. 2011). The cephalothorax is rounded and generally without spines, other than an incipient folded rostrum (fig. 50.2A–C). The carapace and appendages are covered by a thin cuticle, with the setae and spines of the first zoea still invaginated. Zoea: The zoeae are elongated shrimp-like stages that are characterized by having thoracic appendages, with natatory setae, that are used for locomotion (fig. 50.1A, K). Chromatophores with distinct patterns, colors, and locations are usually present (fig. 50.5) but often go unreported, because they 50 Gerhard Pohle William Santana Gerhard Pohle and William Santana Gebiidea and Axiidea (= Thalassinidea) 264 Gerhard Pohle and William Santana fade in preserved specimens. The first zoeal stage has its eyes fused to the carapace and bears five abdominal somites plus a telson (fig. 50.2E). The spinal and/or setal armament of the telson changes during zoeal development, as may its shape in some taxa (e.g., compare the telson of fig. 50.2E with 50.2G). Subsequent stages have movable eyes, and a sixth abdominal somite is added no later than the last zoeal stage (fig. 50.2G). Typically, and in common with anomurans...