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Posted by Merry on April 20, 2015 at 10:35:08:
This year, we’ve had extraordinary luck finding nudibranchs that inhabit the blades of giant kelp or other types of low-growing algae. The adults are very small, well-camouflaged, and easily overlooked, but their eggs give them away.
The egg masses of these diminutive slugs appear in sharp contrast to their habitat and our eyes are easily drawn to the luminous white shapes. Delicate strings, flat whorls, wide ribbons, and glistening orbs contain developing embryos that herald another generation of nudibranchs. During these chance encounters, I imagine that I’ve caught a glimpse into their hidden, ephemeral lives. During courtship and mating, the black dorid, Polycera atra, periodically gave its mate a nudge or a nip. Or perhaps it was a nuzzle.
Nudibranchs are hermaphrodites, that is, they possesses both male and female reproductive organs. During copulation, the slugs orient themselves so that their genital apertures meet. After reciprocal insemination, each slug may lay eggs.
Polycera atra mating and their characteristic egg ribbon.
These vivid Trapania velox achieved quite a reach with their male reproductive apparatuses. That’s all I will say.
Some species of nudibranchs have a patent on their style of egg mass. One example of this is seen in Corambe pacifica and Corambe steinbergae. The two are closely related, similar in appearance, but produce different egg masses.
Both species competitively feed on the bryozoan, Membranipora membranacea, and they reproduce at the same time.
Corambe pacifica and its distinctive coiled egg mass.
A pair of Corambe pacifica with egg masses.
The slightly smaller Corambe steinbergae.
Both species serve a useful purpose in that by preying on Membranipora they help keep the encrusting bryozoan in check, which is an advantage to giant kelp.
Corambe steinbergae egg masses on Membranipora, showing the empty zooid compartments where the adults had been feeding. Corambe covers each individual zooid with its mouth, uses its radula to slice open the animal’s protective membrane, and then sucks out the soft parts. Yum.
A Corambe pacifica adult next to Corambe steinbergae eggs.
Although Corambe steinbergae larvae settles on Membranipora later than C. pacifica settles, their efficient reproduction makes up for lost time. (P. Yoshioka, 1986).
What a mob of C. steinbergae eggs on decimated bryozoan!
After an egg mass is laid, the embryos of most opisthobranch species develop fairly rapidly, roughly in 1 – 2 weeks. In 91% of 126 west coast benthic species, what hatches from the egg is a larvae that does not resemble the adult. Rather, the embryo develops into a shelled veliger larvae, no larger than 1/10th – 1/20th of a millimeter.
The nearly transparent veliger larvae have protective shells and a ciliated velum, which is used for swimming, food particle collection and concentration. Veligers propel themselves upwards in order to prey on single-cell algae near the surface.
Picture this microscopic traveler as enters the cosmopolitan world of plankton, to drift and to swim upwards for hours, days, or weeks, depending on the species. Although we can’t see them while diving, they’re probably zipping all around us. And they’re not alone. A proportion of zooplankton contains other molluscan veligers as well as opisthobranch veligers.
A planktonic larval veliger is the transition state before becoming a recognizable slug. After developing sufficiently, and when the veliger reaches a state of competence, it responds to a chemical signal from its preferred food source and settles to the bottom. In a day or so, it metamorphoses into a juvenile slug.
During the past two weeks, legions of juvenile Falbellina trilineata have appeared on many different types of algae. They’re almost microscopic and about as easy to spot as an eyelash.
Knowing the journey that a nudibranch undergoes before finally becoming a photo subject, makes it all the more exciting to find its eggs. The lush eggs of Cuthona lagunae take 8 days* to develop and hatch into a planktonic veliger. *J. Goddard and B. Green, 2013.
These well-developed C. lagunae embryos show the features of veliger larvae.
You’ve probably seen the distinctive flower-shaped egg mass of Melibe leonina, the lion nudi, which is laid on kelp blades. Nearly transparent juveniles feed on minute crustacean larvae and other zooplankton.
Melibe leonina veligers develop and hatch in only 4 days*. The well-developed embryos in this egg mass resemble popcorn kernels. *J. Goddard and B. Green, 2013.
If I had to guess, I'd say that this tangled egg ribbon probably belongs to Dendronotus venustus (formerly D. frondosus).
This assortment of egg masses comprises at least 3 different nudi species, including Eubranchus rustyus (6 cryptic juveniles in photo) and Doto form A.
A single egg mass of Hermissenda crassicornis yields 7,000 – 1,000,000 veliger larvae, which are planktonic for 30-40 days before metamorphosing. (Avila et al.,1997, J Exp Mar Biol Ecol 218:243). Photo by Phil Garner.
My video of Hermissenda crassicornis veligers; there are three per egg capsule. | 