The spawn and embryonic development of colour variants of Dendrodoris nigra Stimpson (Mollusca: Nudibranchia)

Abstract

Colour variants of Dendrodoris nigra from New South Wales produce morphologically different planktotrophic larvae. All colour variants deposit transparent gelatinous egg ribbons which are arranged spirally onto hard substrata. Embedded into the matrix of the ribbons are yellow zygotes. Colour variants which are brownish-black, gray or black with white speckles (form 1) can produce up to 251,000 zygotes (diameter 74–75 μm) per spawn. Form 1 embryos hatch 9.6 days after oviposition at 22–23°C. Newly liberated veligers (mean length 114 μm) are shaped like most planktotrophic opisthobranch larvae; each veliger has a transparent sinistrally coiled shell, operculum, hyperstrophically arranged visceral mass and a cephalopedal region which can be retracted into the shell. The subvelum of form 1 veligers is not well defined like that of form 2. Colour variants which are jet-black or black with red-rimmed mantles (form 2) produce up to 88,600 zygotes per spawn. Zygotes of jet-black variants (mean diameter 129 μm) are significantly larger (P<0.05, Student’s t-test) than those of variants with red-rimmed mantles (mean diameter 121 μm). Differences in diameter of zygotes have no effect on the shape of the veligers. Form 2 embryos hatch 9.2 days after oviposition at 22–23°C. Mean shell length of newly liberated veligers produced by jet-black variants is 153 μm, 9 μm greater than that of veligers produced by variants with red-rimmed mantles. Compared to most planktotrophicopisthobranch larvae, the shape of form 2 veligers is atypical in that each lacks an operculum and possesses an oversized cephalepedal region which can not be retracted into the shell. The shell of form 2 is darkly pigmented when embryos are reared in the laboratory. Colour variants do not appear to vary their developmental mode and in th laboratory copulation occurs only between individuals with the same colour pattern. Differences in larval morphology suggests that certain colour variants of D. nigra may be separate (sibling) species.