The sexes are separate in all cephalopods. The unpaired gonads, either ovary or testis, are always located in the posterior region of the mantle cavity. Females may have one or two oviducts depending on their major taxonomic group. Each oviduct has an oviducal gland that secretes a primary coating around the egg. Many female cephalopods have an additional nidimental gland that adds extra layers of protective coating to the egg. This gland is found beyond the end of the oviduct.
In males, the sperm passes through the spermatophoric gland complex, which packages the sperm into a packet known as a spermatophore. The hectocotylus is a modified arm on the males of many types of cephalopods that is used to transfer spermatophores to the female. The modifications of the arm take many forms, some quite bizarre. The males of some species also exhibit modifications of other arms in addition to the hec-tocotylus. Females of some species also develop such modified structures as arm-tip photophores when mature. Other forms of sexual dimorphism vary among species within families. These forms include such characters as greatly elongated arm pairs or tails; enlarged suckers; and special photophores. Although researchers have speculated about the functions of these features, little is known about them as of 2003.
Spermatophores are implanted in specific locations on the females by the male hectocotylus in species possessing one, or by an elongated penis (the terminal organ of the sperm duct) in nonhectocotylized species. The time required for the process ranges from seconds in some squids to hours in some octopods. The spermatophores may be implanted in the female's oviducal gland; inside the mantle cavity; around the mantle opening on the neck; in a pocket under the eye; or around the mouth. The mode of reproduction and egg-laying for many cephalopods is unknown, especially oceanic and deep-sea species.
All cephalopod eggs have substantial amounts of yolk. Embryonic development is unlike that of any other mollusk. Nau-tilid eggs are as much as 1.14 in (2.9 cm) in length. Neocoleoid eggs vary in size from about 1.6 in (4.2 cm) in some Granele-done and Megaledone species (among the largest of any invertebrate) to 0.03 in (0.8 mm) long in Argonauta. The eggs have one or more layers of protective coatings and generally are laid as egg masses. Egg masses may be benthic (laid on the ocean bottom) or pelagic, varying among major taxonomic groups. The time span of embryonic development also varies widely from a few days to many months, depending on the species and temperature conditions. Hatching may occur synchronously from a single clutch or extend over a period of 2-3 weeks.
Except for nautilids, which reproduce repeatedly over a period of years, extant cephalopods apparently mate only once. Their reproductive period, however, may comprise the brief terminus of their life span, may extend for a considerable period, or may be intermediate between these extremes. Different species, even within a single family, are found at different points along this continuum but generally cluster at the two extremes. Similarly, the number of eggs produced by a female range from a few dozen to hundreds of thousands. Hatchlings from benthic eggs may either be benthic or temporarily planktonic, eventually settling back to the adult benthic habitat. Pelagic hatchlings are planktonic.
The development of cephalopod embryos is direct, which means that the embryos do not have true metamorphic stages. The hatchlings of species with large eggs look like miniatures of the adult, whereas hatchlings of species with small eggs undergo changes in body proportion during development. The young of some species differ conspicuously in the proportion of their body parts and development of specialized structures (e.g., photophores; modification of suckers into hooks) from the adults. Thus, researchers have coined the term "paralarva" for early stages of cephalopods that differ morphologically and ecologically from later stages.
Most incirrate octopods and, as has recently been discovered, some squids care for their egg masses until the eggs hatch. Pelagic species that exhibit such behavior swim while holding the egg mass in their arms. Benthic octopods use such structures as the interiors of mollusk shells, rock crevices, or discarded bottles as dens. They manipulate the eggs to keep them free of detritus and blow water across them, presumably to aerate them.
Reproduction, at least among coastal species, is seasonal, although there may be either one or two peak periods of seasonal reproduction. Because of their short life spans, neo-coleoid cephalopods tend to exhibit strong seasonality in the occurrence of their different life-history stages.
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