Embryonic Development of the American Lobster (Homarus americanus): Quantitative Staging and Characterization of an Embryonic Molt Cycle

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 morphological, anatomical, and behavioral characteristics observed. A percent-staging system based upon Perkins' eye index (1972) is presented, and ten equally spaced embryonic stages are illustrated and characterized at different levels of resolution: whole eggs, dissected embryos, antennulae 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 (E12%) with the molt of the nauplius into the metanauplius and ending just after hatching when the metanauplius molts into a first stage larva (L1, first zoea). At E30%, the cuticle begins to separate from the setae in the telson; this signals the start of Drach's (1939) stage D0 of the metanaupliar embryonic molt cycle. At that time, the first sign of organogenesis of the L1, the formation of the endopod of the antennulae, becomes visible; presumed sensory neurons and their axons are observed at the tip of the exopod of the antennulae where a giant sensillum is differentiating. During D0 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 D1. After hatching (E100%), the telson of the free metanauplius (prelarva) shows the characteristics of stage D2-3 and ecdysis soon follows. The arrested development observed at constant temperature in the experimental brood occurred at stage D0 of the metanaupliar molt cycle, whereas development was resumed as the embryos entered stage D1. These changes in developmental pace from D0 to D1 in the embryonic molt cycle are parallel to those occurring 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 embryogenesis of Homarus such as neural differentiation.

Estagiamento de embriões de Macrobrachium olfersi (Wiegman) (Crustacea, Palaemonidae) através de critérios morfológicos nos dias embrionários

“…Em outros decápodes com ovos centrolécitos, a seqüên-cia de desenvolvimento da clivagem ao o início da formação do olho corresponde a até 50% do tempo total de incubação, como observado em Cherax destructor (SANDEMAN & SANDEMAN 1991), Homarus americanus (HELLUY & BELTZ 1991) e Palaemonetes argentinus (NAZARI et al 2000).…”

Functional differentiation of adult neural circuits from a single embryonic network

“…This folding of the ectoderm to form the stomodeum occurs very early during development and can be observed as early as E5 in H gammarus. Working on H. americanus, Bumpus (1891) noted that the sto-175 cycles modeum is present from stage "J," the equivalent to E4-5 on the staging scale of Helluy and Beltz.(1991). This early development of the foregut is consistent with general crustacean ontogeny, since in many species hatching occurs at the nauplius stage (corresponding to E5 in Homarus), and consequently the stomodeum must be well developed to permit feeding in these free swimming larvae.…”

“…possible that the nonfunctional embryonic stomodeum expresses a fictive swallowing-like activity as a phylogenic consequence rather than for any immediate functional reasons. Second, the lobstet embryo passes through as many as seven molts in ovo (Goudeau et al, 1990;Helluy and Beltz, 1991). Since the stomodeum is ectodermal in origin, and therefore sheds its cuticle at each embryonic molt, the rhythmic movement of the stomodeum could likely play a main role in this process.…”

“…The eye index is first measurable at 70 pm and increases to 580 pm at hatching. According to Helluy and Beltz (1991) each value of EI can be converted into a percentage of the embryonic development by dividing the measured EI at a given time by EI at hatching. Thus, hatching corresponds to 100% of the embryonic development (ElOO), E50 to halfway through embryogenesis and EO to fertilization.…”

“…The developmental stages of Homarus had been extensively described since the pioneering work of Herrick (1895) and Bumpus (1891), and more recently by Helluy and Beltz (1991) and Beltz et al (1992). The principal features of the lobster Homarus gammarus development are summarized in Figure 2.…”