Reproductive biology

Most greenlings, sculpins, and related species lay adhesive masses of eggs that either adhere to rocky substrate, cluster around plant or animal stalks and tubes, or are wedged into crevices. The eggs always adhere to each other, but not always to the spawning substrate. Among the sculpin relatives, only the comephorid sculpins of Lake Baikal give live birth to hatched larvae. Perhaps the most remarkable reproductive specialization among cottoid fishes is the phenomenon of internal gametic association, which has been demonstrated for sculpins (Munehara et al., 1997) and poachers (Munehara, 1997). With internal gametic association, the fishes copulate, but the sperm does not fertilize the egg until introduced to the calcium ions in seawater. This enables a female to repeatedly deposit small egg masses in specific ways over a long period of time, up to dozens or hundreds of depositions over weeks of time, based on one mating. Many highly specialized spawning substrates have evolved in various cottoid species, perhaps on the basis of internal gametic association in more cases than have yet been demonstrated. Proof of internal ga-metic association merely requires dissection of eggs from ovaries and placement in seawater, then incubation and observation for embryonic development.

Another typical characteristic of sculpin reproduction is the guarding of a cluster of different egg masses by a single territorial male. Male greenlings and lingcod also guard one or more egg masses. In some sculpins, however, the male exhibits haremic behavior in which both the nest site and a group of females are guarded together. In haremic species there is no evidence yet of internal gametic association. With the scalyhead sculpin, Artedius harringtoni, this author, while diving, observed several rotund little fish, presumed females, dart inside the empty shell of a giant barnacle, immediately followed by a much larger individual, presumed male, which curled its body and spread its fins to close off entry to the barnacle shell. This species lays its eggs inside giant barnacle shells, and there are typically several different colors of egg mass. Females of various sculpin species lay only one of various characteristic colors of egg. In haremic species of the genus Artedius, a cluster of egg masses may number over a dozen masses, but only with a few different colors of egg mass, corresponding to the number of females in the harem. In other genera (e.g. Scorpaenichthys, Enophrys, Hemilepidotus) where the male guards clusters of egg masses of differing colors, it is not known whether haremic behavior is involved, because groups of females have not been observed remaining near the guarding male.

Whereas haremic species tend to lay egg masses that adhere to a rock surface or to previously laid egg masses in a cluster at the guarded nest site, species with internal gametic association tend to lay smaller egg masses of less characteristic size. That is, the female can extrude whatever number of eggs is required to fill an interstitial space or to form a ring around a stalk, then move on to search for another deposition site, the sites being dispersed according to availability. In agonid species there appears to be a tendency to spawn inside sponges, whereas liparidid species tend to spawn in seaweeds or inside shells.

As much as the spawning characteristics vary among sculpins and relatives, their larvae also demonstrate diverse adaptations. Lacking a swimbladder at any stage, these species have evolved diverse mechanisms to enable early growth while inhabiting the water column. Enlarged pectoral fins exist in larvae of many species, whereas larvae of other species have a flaccid, globular body with relatively large volumes of low-density, buoyant body fluid.

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