Nudibranchs lay their eggs in flat ribbons attached to rocks or other objects (dorids) or in tangled masses attached either to the sea bottom or to algae and other objects (aeolids). The eggs, often multiply enclosed in capsules (as opposed to singly enclosed), are embedded in a mucous matrix that both supports them and protects them.

Spawn of an aeolid nudibranch Dendronotus sp. 0.3X

Mating and egg-laying aggregation of Okenia rosacea 1X.
Photo courtesy Kevin Lee, Fullerton, California diverkevin

Dorid nudibranch Triopha maculata laying eggs 1X.
Photo courtesy Kevin Lee, Fullerton, California diverkevin

Details of spawning times, egg counts, and egg-mass morphology are available for 22 species of nudibranchs around Monterey Bay, California.  One feature of interest is that most, but not all, egg masses deposited by dorid nudibranchs form a counterclockwise spiral, as viewed from above and starting from the centre and working out (see drawing on Left).  If you look closely at the photograph on the Right you can see the end of the counterclockwise spiral, in the same position as shown in the drawing. The spirals are of the Archimedes type, that is, with equally spaced whorls.  Why a spiral?  The author offers 2 suggestions, both of which are considered in the quiz below : 1) ease of laying, 2) being less spread out gives a greater chance of survival.  Costello 1938 J Morph 63: 319.

NOTE  the single exception in the study is Okenia (Hopkinsia) rosacea, which produces a clockwise spiral (see photos in Research Study 1 above and Research Study 4 below)

NOTE  a Greek mathematician and physicist who lived in the 3rd Century BC.  Among his many inventions and discoveries are the Archimedean screw for raising water, and elucidation of the principles of lever and buoyancy.  The spiral he described can be best visualised as a tightly coiled rope lying flat, with each successive whorl evenly spaced from the one preceding it. Although there are many different kinds of spirals, another one familiar to marine biologists is the logarithmic spiral, where the successive whorls increase in breadth in steady ratio.  This is how a Nautilus shell spirals

Why are dorid egg ribbons generally laid in a counterclockwise Archimedes-type spiral? Here are some options, 2 of which are proposed by the author of Research Study 3 above relating to the spiraling part. Consider which options are most convincing, then CLICK HERE for explanations.

Ease of laying.

Being less spread out gives a greater chance of survival.

The position of the gonopore favours a counterclockwise spiral.

An Archimedes-type spiral gives equal space between the parts of the ribbon for good gas exchange.

In the comprehensive study of egg masses of Californian nudibranchs just noted (Research Study 3 above), the author comments that all dorid species examined lay their masses in a counterclockwise direction.  This actually also includes Okenia rosacea, which as noted in the foregoing Research Study, lays its egg mass in a clockwise-spiralling mass (see the egg ribbon on farthest Right in the photo). However, it does this uniquely from the outside in, moving in a counterclockwise direction and thus ending up with a clockwise-spiralling mass (see the egg ribbon on farthest Right in the photo). If a laying dorid is disturbed and/or displaced, it will often return to its original position and resume its laying from the point of disruption.  Examples of rates of deposition are Anisodoris sp. at 50mm ^.  h^-1 and Hermissenda crassicornis at 20mm ^.  h^-1.  Number of eggs in a mass is species-specific and highly variable.  At the distal or oldest end of the mass there may be 8 or more eggs per capsule, while at the proximal or newest end there may be only 1 egg per capsule.  Why lay in a concentrated mass?  The author comments that a concentrated mass of eggs may provide better chance of survival than if they are strewn about over a larger area.

Now, laying from the inside-out (counterclockwise) from a central start-point has another advantage in that it is easier to control the symmetry of the mass (if that is important...which it seems to be), because each successive whorl lies neatly against the previous. If a laying is outside-in, as with Okenia, there are 2 potential sources of "error". The first is how to regulate the circumference of the initial whorl, such that the egg-layer arrives exactly at the centre of the spiral with egg-supply exhausted. Second, how does the nudibranch navigate along, laying a perfect spiral, without the reference guide of a previously laid whorl?. Both of these "problems" are illustrated in the photograph above. Note the raggedy appearance of the uppermost coil, where the egg-layer meanders up and out of the photo on its first pass, finally to reestablish (more-or-less) the correct direction, but ending up well shy of a perfect spiral and with the job only half completed. The spiralling of the ribbon at the extreme Right of the photo is comparatively good, but the egg-layer gets sloppy towards the end, and the spiral is also incomplete, with the end of the ribbon tailing off to the right. Costello 1938 J Morph 63: 319. Photo courtesy Kevin Lee, Fullerton, California diverkevin.

NOTE if Coriolis forces were to influence egg-laying, then we might expect clockwise spirals in the northern hemisphere and counterclockwise spirals in the southern hemisphere. However, there appear to be no clockwise-spiralling species (recall that Okenia actually lays in a counterclockwise direction) on the west coast of North America. As to the question, "how many counterclockwise-spiralling species are there in the southern hemisphere", the answer is: we don't know yet, and are waiting for some enterprising student to go through photographs of dorid egg masses in the Sea Slug Forum, Australian Museum, Sydney, sponsored by Bill Rudman

NOTE  an egg mass from Anisodoris sp., 360 x 12mm in size, is reported to contain 1,350,000 eggs; another, from Diaulula sandiegensis (740 x 15mm), 16,000,000 eggs 

Upright or flat egg ribbons? Most west-coast nudibranch species lay their ribbons so that they stand upright on the substratum. This, combined with the Archimedean even-spacing of the whorls, is thought to favour good gas-exchange for the embryos as well as easier egress from the mass at the time of hatching. However, at least 5 west-coast species, including Okenia rosacea (see Research Study 4 above) and Doriopsilla gemela (photo on Left), lay flat egg masses. Nothing seems to be known about the implications of this strategy with respect to egg-capsule densities or hatching behaviour, and there would seem to be scope for research in these regards. Photos courtesy Jeff Goddard, UC Santa Barbara and seaslugforum.

NOTE in addition to the 2 species mentioned, the nudibranchs Conualevia alba, Crimora coneja, and Limacia (=Laila) cockerelli lay flat egg masses

Newly hatched veligers of Doriopsilla
gemela ofshell diameter 180um

One additional "flat-depositing" opisthobranch is the anaspidean Phyllaplysia taylori. Its usual habitat is on eelgrass blades and it also lays its egg masses there. Perhaps because its habitat surface is rich in oxygen released as a byproduct of photosynthesis from its "host", Phyllaplysia may not have the same potential gas-exchange constraints as described in the previous Research Study 5. Photo of egg mass courtesy Jeff Goddard, UC Santa Barbara, California.

Flat egg mass of Phyllaplysia. The ones shown here
are actually from P. padinae in the Gulf of California

Egg ribbons of Doris (Archidoris) montereyensis are helically spiralled, 1.5-2.5cm in width, and are made up of up-and-down loopings of a continuously extruded gelatinous cylinder containing the egg capsules.  Each capsule has 1-2 eggs. Hurst 1967 Veliger 9: 255.

Attachment of a ribbon to the substratum is remarkably strong.
It is not known whether a special glue is used for this purpose, or if
it is just the inherent stickiness of the gelatinous egg cylinder 0.8X

Aeolidia papillosa lays a tangled mass of eggs consisting of a tubular jelly mass within which are crowded many capsules.  The capsule-containing tubule folds in on itself in a tangled mass. The capsules are embedded in and protected by the jelly matrix. Each capsule contains about 6 eggs.  Hurst 1967 Veliger 9: 255.

An Aeolidia papillosa viewed from below
through a glass aquarium extrudes a
tangled egg mass from its gonopore.
The animal is curled around on itself
and is sitting on the egg mass 2X

The proximal stimulus for egg-laying in sea hares Aplysia californica is release of a peptide hormone from the neurosecretory bag-cell organs.  A few minutes after injection of a crude extract in buffer solution of these tissues, test animals slow their rate of locomotion, and begin a pattern of mouth-puckering and head-waving indicative of the onset of egg-laying.  Egg-laying begins about 60min after injection in experimental animals, while control animals that receive an equivalent amount of buffer solution exhibit no behavioral changes.  The authors suggest that target sites of the hormone may be muscle tissue surrounding the ovotestis and elements of the nervous system.  Arch & Smock 1977 Behav Biol 19: 45. Photo courtesy Kevin Lee, Fullerton, California diverkevin.

NOTE  these are neuroendocrine cells associated with the abdominal ganglion in Aplysia.  The authors provide a comprehensive reference list of past research done on the neurosecretory control of egg-laying in Aplysia

NOTE  in sea-hare parlance this is actually termed “weaving” because during actual egg-laying the head moves side-to-side interspersed with right-side tucking movements.  These movements ensure that the egg strand is deposited as a compact knot firmly glued to the substratum

Head of sea hare Aplysia californica showing oral tentacles, rhinophores,
and the approximate location of the opening of the genital groove (arrow),
the latter also serving to conduct sperm and eggs from the ovotestis

Sea hares Aplysia lay a long egg string or cordon with egg capsules containing variable numbers of eggs packed loosely along its length (see drawing). A laboratory study on egg-packaging in Aplysia californica finds that the number of eggs per capsule directly relates to an individual’s body size.  Thus, an individual during its reproductive lifetime encloses as few as 7 or as many as 80 eggs within a capsule (see graph on Right). There doesn’t appear to be any functional disadvantage to this in terms of hatching success, but the question arises as to whether overall egg production by an individual correlates with individual size.  The answer is ‘yes’, but with quite high variability in the data (data not shown here).  Kandel & Capo 1979 Veliger 22: 194. Original drawing courtesy Kriegstein et al. 1974 Proc Nat Acad Sci, USA 71: 3654.

NOTE  an early report on fecundity in sea hares A. californica gives a value of 478 million eggs deposited over a 17-wk period from Nov-Mar by a 2.6kg live-mass individual.  Average number of eggs per capsule is 188, more than reported in the laboratory study above. The specimen featured in the 1934 study releases its egg cordon at an average rate of 6cm (= 41,000 eggs) per minute.  MacGinitie 1934 Biol Bull 67: 300

The west-coast barnacle-eating Onchidoris bilamellata is circumpolar in distribution, so studies on its reproduction at similar latitudes might be of interest.  On the coast of Yorkshire, England it has an annual life cycle, just as on the west coast of North America.  A single spawning (2 in some areas) in late winter/early spring leads to settlement of larvae in early summer.  Growth is rapid during the summer and there appears to be considerable body shrinkage during spawning (see graph on Right). 

The author suggests that individuals find one another for mating through chemosensory means, either distance perception using the rhinophores, or perhaps mucus-trail following.  However, a test of the second possibility, using 10 pairs of sexually mature but unmated individuals, show that such tracking is unlikely.  A typical negative result is shown in the figure below Left. Note that the tracker (the second snail) has 3 opportunities to follow the trail laid down by the marker snail, but fails to follow  Of the 10 trials, 7 are negative, 2 are inconclusive, and only one is positive.  Other research on nudibranchs shows that mate-finding is mainly by distance chemoreception, mediated by the oral tentacles or the rhinophores.  Todd 1979 J Exp Mar Biol Ecol 41: 213.

NOTE  each test involves placing a first individual, the “marker”, in the centre of a clean glass aquarium and allowing it to crawl along for 15-35min.  After this, the snail is removed, the seawater decanted, and the trail traced onto paper from below.   After adding fresh seawater, a ssecond, “tracker” snail is then placed in the centre of the aquarium and its trail similarly recorded.  Each run is terminated when the nudibranch being tested butts against the side of the aquarium tank and stops moving