As with all elasmobranchs, skate and rays are internal fertilizers. Internal fertilization is beneficial because it increases the likelihood and efficiency of fertilization by reducing sperm wastage. In addition, it ensures that the energy-rich eggs produced by the female are not consumed by predators, and that all the energy allocated to reproduction is passed to the embryos and not lost to the environment. This is especially the case for species that retain their embryos until the embryos have completely developed, a reproductive mode termed viviparity. Elasmobranches that practice viviparity are called viviparous (or live-bearing). There are many types of viviparity, which can be divided into two broad categories: aplacental and placental viviparity. Placental viviparity is the most advanced mode of reproduction, during which the embryos are initially dependent on stored yolk but are later nourished directly by the mother through a placental connection. This type of reproduction is not exhibited by any type of batoid. Ovoviviparity (or aplacental viviparity), on the other hand, is the only mode of reproduction employed by rays. In rays, the embryos rely on the substantial yolk within the ovulated egg only during the initial stages of development. After the nutrients stored in the egg have been consumed, the embryo ingests or absorbs an organically rich histotroph (or "uterine milk") produced by the mother and secreted into uterus. The most highly developed of these strategies occurs in some rays in which the lining of the uterus forms tiny, finger-like projections (termed trophonemata) that increase the surface area for histotroph secretion. This form of nutrient supply (or maternal investment) results in very large offspring, which is characteristic of most species of ray.
Other elasmobranchs retain their eggs for shorter periods of time and then deposit them on the ocean floor in tough, keratinized egg cases, a reproductive mode termed oviparity. These elasmobranchs are called oviparous (or egg-laying).
The young of both oviparous and viviparous forms hatch, or are born, as fully developed, miniature copies of their parents that are capable of actively avoiding predators, unlike the larval stages of many teleost fishes. The relatively large size reached by all elasmobranchs prior to hatching, or birth, also ensures that they are capable of consuming a wider variety of potential prey.
All skates exhibit the most primitive form of oviparity, termed extended oviparity. Extended oviparity is a mode of egg-laying where embryonic development occurs primarily outside of the mother, and can last for very long periods of time. More specifically, the energy-rich ova are fertilized within the oviducts; quickly encapsulated in keratinoid egg cases secreted by the shell (or nidamental) glands; and then deposited on the substrate sequentially in pairs of two. Once the egg cases are deposited on the bottom (termed oviposition), the embryos receive no further parental care, and are nourished solely from yolk stored in the yolk sac. The tough egg case provides an osmotically independent environment until the embryo is capable of urea retention and osmoregulation. It also forms the only protective barrier against predators and the outside environment that the embryo has. Development within the egg case is a slow process that can last for a period of up to 15 months, after which the embryo (or miniature skate) pushes its way out of the capsule. Approximately 42% of living chondrichthyans (or cartilaginous fishes) have extended oviparity.
Oviparity is thought to be the more primitive or ancestral condition in elasmobranchs. A gradual evolutionary shift from oviparity to viviparity in some species may have served to better shield young against potential predators and other hazards, as well as to offer a more uniform developmental environment. The young of oviparous elasmobranchs also tend to be smaller than those of viviparous elasmobranchs because the amount of nutrients available to the developing embryo is limited to what is stored in the yolk sac. Larger young, in turn, are better able to avoid predators, perhaps explaining an overall evolutionary shift away from oviparity, especially in more pelagic species.
Although oviparous species provide no parental care in the form of nutrients after the egg is deposited, little is known about "parental care" in oviparous elasmobranchs. None are known to actively guard their eggs or build nests, but it is possibel that egg-laying elasmobranchs select appropriate substrate for their eggs so as to maximize the chances of survival of the developing embryos. The females of at least one species of bullhead shark (family Heterodontidae) are known to pick up their eggs in their mouths and then wedge them into rocks and marine vegetation.
Mating behaviour has been observed in the wild in very few species of elasmobranch. Only five of the more than 450 species of batoids have been observed copulating in the wild, leaving most aspects of their natural sexual behaviour unknown. Female receptivity to copulation likely depends on hormonal status. Behavioural and/or chemical cues may then signal this receptivity to the males. For example, the process of parturition might produce some form of sexual pheromone or olfactory attractant for male rays. Precopulatory mating behaviour (i.e. grasping and biting by the males) occurs in many species of elasmobranch - including many species of batoid - and is evident in scars and wounds on the females. For example, male eagle rays (Aetobatis narinari) dive upon and gouge the back of females during courtship, and the copulatory bites of the male roughtail stingray (Dasyatis centroura), round stingray (Urolophus halleri) and the bullseye stingray (Urolophus concentricus) - among others - can inflict lasting scars on the female. Mating and insemination is often a long, drawn-out process, especially in batoids, where copulation can last for several hours. In some species of elasmobranch, sperm are then stored in the nidamental gland of the female, in some cases for months or longer, before they are released to fertilize ovulated ova.
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