Atlas of Crustacean Larvae

44. Amphionidacea

general: The sole reported species of the order Amphionidacea and family Amphionidae is Amphionides reynaudii, a small (to 30 mm long) pelagic shrimp-like crustacean found in all oceans between latitudes 36° N and 36° S. The species, although morphologically quite variable, is characterized by having a very thin, almost membranous carapace that is somewhat inflated and fringed with setae; reduced or vestigial mandibles and maxillules in adult females; biramous thoracopods , the first of which (and no others) is modified into a maxilliped; and large first pleopods in females, which extend forward to form a brood pouch with the carapace (Fransen 2010). Males are rare; nearly all records are of females or larvae . Larvae are also uncommon, although D. Williamson (1973) examined 251 larvae from the International Indian Ocean Expedition. According to Fransen (2010), “no author has seen an intact adult male or female and the figure provided by [D.] Williamson (1973) [reproduced here as fig. 44.1H] is a composite drawing based on several specimens.” Relatively little is known about adults or larvae. The life cycle includes eggs (assumed to be fertilized and then carried loose in the female’s brood pouch) that probably hatch as an amphion (zoeal) larval stage (number unknown, but probably 20), followed by post-larvae (metamorphosed larvae), which are essentially small adults (Heegaard 1969; Kutschera et al. 2012). larval types Amphion Larva: It is not known if a free naupliar stage exists , but this is considered unlikely, due to the retention of eggs by the female and the advanced nature of the zoeal stages (Fransen 2010). Amphionides reynaudii passes through several (number unknown, but possibly as many as 20) zoeal stages (Kutschera et al. 2012), somewhat similar to what is seen in the Caridea, with which they were once grouped (Heegaard 1969). Although called a zoea by D. Williamson (1973) (and a mysis by Heegaard 1969), this larva differs from the true zoea of the Decapoda in that only the first pair of thoracopods is developed as a maxilliped (fig. 44.2A–C, F, G). For this reason it has been called an amphion larva by some workers (e.g., D. Williamson 1973, F. Schram 1986). Nearly all we know of larval development of A. reynaudii comes from the work of Heegaard (1969), who unfortunately employed the terms promysis , mysis, and post-larva. The recent work of Kutschera et al. (2012), displaying the first photographs of Amphionides larvae, applies a more neutral terminology and refers to stages. Morphologically, amphion larvae progress more or less gradually toward the adult form. morphology: Larval morphology was described by Heegaard (1969). Gurney (1942) reported a very early larval form that lacked a rostrum; Heegaard (1969) could not confirm this, but referred to the possibility of this promysis stage. The first zoeal stage (fig. 44.1A) has antennules, antennae, three mouthparts (all well developed), and three thoracopods. Only the first thoracopod develops into a true maxilliped; thoracopods 2 and 3 are natatory (fig. 44.2A–C, E–H). The fourth thoracopod appears as a bud in the second zoeal stage (fig. 44.1B), and uropod buds also begin to develop (fig. 44.1C). Other appendages appear more or less gradually in subsequent stages (figs. 44.1D–F; 44.2A–C), until the adult form is reached. Thoracopods bear large spines (fig. 44.1G) that form a feeding basket in most stages (fig. 44.2E). Also, the carapace bears a post-rostral spine and a vestigial dorsal organ in most stages (Heegaard 1969). Sexual differentiation begins in zoeal stage 9. Later stages can be differentiated into two distinct forms, based on the thickness of the antennular peduncle and the lengths of the antennular flagella (see D. Williamson 1973; Fransen 2010). According to Fransen (2010), “specimens in the last two zoeal stages with a robust antennular peduncle always have a uniramous eighth thoracopod, a small pleurobranch on the eighth somite, and the first pleopod short and biramous, and in the last zoeal stage a paired testis and paired genital openings in the coxae of the eighth thoracopods are present [in males].” In contrast, “those specimens in which 44 Joel W. Martin Verena Kutschera Joel W. Martin and Verena Kutschera Amphionidacea Amphionidacea 227 the last two zoeal stages have a slender antennular peduncle possess neither an appendage on the eighth somite, nor a pleurobranch, and have a uniramous first pleopod. Ovaries develop and the genital pores on the coxae of the sixth pair of thoracopods become visible in the last zoeal stage...