Cortical reaction in inseminated eggs of the horseshoe crab, Limulus polyphemus L

Book gill development in embryos and first and second instars of the horseshoe crab Limulus polyphemus L. (Chelicerata, Xiphosura)

The scanning electron microscope (SEM) was used to study the development of the opisthosomal appendages and book gills of the horseshoe crab, Limulus polyphemus. Later embryonic stages were examined as well as the first and second instars. The observations are compared with a much earlier light microscopic description of book gill development in the horseshoe crab and with book lung development in scorpion embryos and first and second instars in a recent study with SEM. After the third embryonic molt in the horseshoe crab, the opisthosomal appendages are of sufficient size so they could be fractured or dissected open so internal cells and other structures could be examined. The opisthosomal appendages and book gill lamellae of first and second instars were also opened. The observations support the earlier histological report that the gill lamellae are a hypodermal outgrowth from the posterior surface of the preceding branchial appendages. The genital operculum, branchial appendages and gill lamellae are very thin and consist of external cuticle, hypodermis and space holders. The latter help hold the cuticle walls in place so hemolymph can flow through the narrow channels. The space holders are formed from cell processes that extend into the lumen from the hypodermis just inside the external cuticle. In the recent SEM study in scorpion embryos and in some histological investigations in spider embryos, the book lung lamellae are formed by alignment of cells from an invaginated sac or mass of cells. This clearly differs from the mode of formation of gill lamellae as observed in this and earlier investigations. These reports of differences in embryology refine but do not preclude hypotheses about book gill/book lung homology since addition, deletion or modification of ancestral features often occur for the benefit of the embryos and larvae.

Defending the zygote: search for the ancestral animal block to polyspermy

Fertilization is the union of a single sperm and an egg, an event that results in a diploid embryo. Animals use many mechanisms to achieve this ratio; the most prevalent involves physically blocking the fusion of subsequent sperm. Selective pressures to maintain monospermy have resulted in an elaboration of diverse egg and sperm structures. The processes employed for monospermy are as diverse as the animals that result from this process. Yet, the fundamental molecular requirements for successful monospermic fertilization are similar, implying that animals may have a common ancestral block to polyspermy. Here, we explore this hypothesis, reviewing biochemical, molecular, and genetic discoveries that lend support to a common ancestral mechanism. We also consider the evolution of alternative or radical techniques, including physiological polyspermy, with respect to our ability to describe a parsimonious guide to fertilization.

Paternity in horseshoe crabs when spawning in multiple-male groups

Unpaired or satellite male horseshoe crabs, Limulus polyphemus, are attracted to and often form a group around a pair (a female with an attached male) that is nesting in the high intertidal zone. These males are engaged in sperm competition. We observed nesting pairs and their associated satellites in the wild, collected and reared their eggs and used genetic markers to examine paternity. We found that the unpaired, satellite males are highly successful at fertilizing eggs; two satellites can leave the attached male with few fertilizations. Two satellites together are each as successful as one spawning with a pair. A satellite's location around the female greatly affects his success, and males compete for access to a position over the dorsal canal between the prosoma and opisthosoma of the female and under the front margin of the paired male where they are most likely to fertilize eggs. Although eggs and sperm retain their viability for some time after spawning, nearly all eggs are fertilized by the satellites that are around the nesting pair at the time of egg laying and by the attached male. A number of factors including beach current, female size and male behaviour affect the outcome of sperm competition in this externally fertilizing species. Copyright 2000 The Association for the Study of Animal Behaviour.

Sources of calcium in egg activation: a review and hypothesis

A careful reanalysis of the literature indicates that the initial mechanism of activation in sea urchin eggs is remarkably similar to the mechanism established in medaka eggs: i.e., sea urchin eggs are activated by a qualitatively and quantitatively similar calcium explosion; one which is propagated in a wave sustained by the calcium-stimulated release of calcium from internal sources. These sources are probably in the endoplasmic reticulum. An exhaustive survey of the literature reveals that a wide variety of other activating eggs in the vertebrate line also exhibit secretory waves which are propagated at about 10 microns/sec, and can thus be assumed to reflect the same basic mechanism. Activating protostome eggs on the other hand do not exhibit such waves. This and other systematic differences from deuterostomes suggest that unlike deuterostome eggs, protostome eggs are primarily activated by calcium ions which enter the cytosol from the medium, and do so in response to depolarization of the egg's plasma membrane.

Fertilization in a crab. II. Cytological aspects of the cortical reaction and fertilization envelope elaboration

At fertilization, the egg of Carcinus maenas undergoes cortical vesicle exocytosis, in response to the first contacts between the spermatozoon and the egg plasma membrane. This process was observed in vitro and may be connected with a cortical reaction. Carcinus maenas eggs display two populations of cortical vesicles which, during the reaction, successively release two different exudates: a fine granular material and a mass of ring-shaped granules. During the first steps of exocytosis, the two superimposed vitelline envelopes are detached from the egg surface, and the inner one gradually changes. Thus a new coating, derived from the coalescence of the secreted ring-shaped granules, is progressively elaborated under the vitelline envelopes. These events occur over a 7-8 hr period. The morphological uniqueness of the cortical vesicle exudates and the complexity of the related events are discussed in terms of the cortical reaction and of the formation of the fertilization envelope in Carcinus maenas.

Cortical vesicle breakdown in fertilized eggs of Fundulus heteroclitus

A scanning and transmission electron microscope study has been made of the cortical alveoli of the egg of Fundulus heteroclitus. The study includes both unactivated eggs and fertilized eggs fixed at intervals of 1 second to 10 minutes after insemination. The alveoli appear to vary considerably in size, in contents, and in morphological aspects of their breakdown. As it undergoes dehiscence, each vesicle may form one or several openings in the egg surface; dense granules and particulate, fibrous, or membranous material, apparently in any combination, are liberated to the nascent perivitelline space. It appears that much of the excess membrane externalized during the reaction is strung out in threads and probably lost to the perivitelline space. The evidence does not suggest that the excess membrane either "dissolves" or is retrieved by the egg cytoplasm. That part of the cortical vesicle membrane which remains continuous with the oolemma gradually becomes microvillous and loses it morphological identity. Granules and particulate matter, presumably liberated from the cortical alveoli, are seen adhering to the inner surface of chorions removed from activated eggs. The micropyle appears to be sealed with similar material. Supernumerary sperm are observed inside the chorion in some instances. The cortical reaction appears to play secondary role in the prevention of polyspermy and to be somehow related to the subsequent formation of a normal embryonic blastodisc.