Reference: Biol Bull. 180: 355-371. (June. 1991) Embryonic Development of the American Lobster (Homams americanus): Quantitative Staging and Characterization of an Embryonic Molt Cycle S. M. HELLUY AND B. S. BELTZ Department of Biological Sciences. Wellesley College, Wellesley, Massachusetts 02181 Abstract. The growth of a single brood of lobsters (Homarus americanus Milne-Edwards 1837) maintained at constant temperature is studied from the naupliar stage to hatching, and the sequence of appearance of morpho-logical, anatomical, and behavioral characteristics ob-served. A percent-staging system based upon Perkins' eye index (1972) is presented, and ten equally spaced embry-onic stages are illustrated and characterized at different levels of resolution: whole eggs, dissected embryos, an-tennulae and telsons. The tegumentary and setal changes in the telson show that a complete molt cycle takes place in the egg starting at about 12% embryonic development (El 2%) with the molt of the nauplius into the metanau-plius and ending just after hatching when the metanau-plius molts into a first stage larva (LI, first zoea). At E30%, the cuticle begins to separate from the setae in the telson; this signals the start of Orach's (1939) stage D of the metanaupliar embryonic molt cycle. At that time, the first sign of organogenesis of the LI, the formation of the en-dopod of the antennulae, becomes visible; presumed sen-sory neurons and their axons are observed at the tip of the exopod of the antennulae where a giant sensillum is differentiating. During D the setae of the first larval stage are forming proximally and medially in the bilobed telson under the metanaupliar cuticle. At E90%, these setae are retracting, and the embryo has entered stage D,. After hatching (100%), the telson of the free metanauplius (prelarva) shows the characteristics of stage D : _ 3 and ec-dysis soon follows. The arrested development observed at constant temperature in the experimental brood occurred at stage D of the metanaupliar molt cycle, whereas de-velopment was resumed as the embryos entered stage D, . These changes in developmental pace from D to D! in the embryonic molt cycle are parallel to those occurring Received 3 December 1990; accepted 8 March 1991. in older lobsters (Aiken, 1973). The quantitative staging of lobster development from extrusion to hatching, and the description of the embryonic molt cycle will facilitate future investigations on particular aspects of the embryo-genesis of Homarus such as neural differentiation. Introduction Studies on lobsters and other crustaceans have made a significant contribution to our understanding of neural organization and the control of behavior (see Wiese el ai. 1990). There is increasing interest in examining the on-togenesis of particular behaviors and the cellular archi-tecture that is the basis for those behaviors (Kravitz, 1988; Govind. 1989; Sandeman and Sandeman, 1990). Re-search on neural development at the embryonic level in Homarus is flourishing (Cole and Lang. 1980; Beltz and Kravitz, 1987; Beltz el a/.. 1990; Helluy and Beltz, 1990; Meier and Reichert, 1990), but progress has been limited by the lack of adequate documentation on the general development of this organism in the egg, as well as by the absence of a staging system for the total embryonic period. These two problems are addressed in this paper. Recent developmental studies in Homarus have dealt primarily with the perihatching period (Davis, 1964; En-nis, 1975; Charmantier and Aiken, 1987), and larval and postlarval life (Phillips and Sastry, 1980; Charmantier, 1987), whereas most of the literature concerned with the prehatching period dates back to the nineteenth century (Bumpus, 1891; Herrick, 1895). The latter studies are a remarkable achievement of patient and detailed obser-vation and are illustrated by elegant drawings (Herrick, 1895), but the modern microscopic and photographic methods used in this study are necessary to provide added resolution. The nineteenth century studies also tend to focus on early embryogenesis while providing little or no information about middle and late development in the 355
BioStor
