Reproduction
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  Egg-laying
 

Egg-laying is dealt with in this section, while topics of MATE SELECTION & COPULATION, EMBRYONIC DEVELOPMENT, HATCHING & LARVAL LIFE, SETTLEMENT & METAMORPHOSIS, SETTLEMENT CUES, and ONTOGENETIC DEVELOPMENT OF BEHAVIOUR are considered elsewhere.

After some brief introductory material, the entries are arranged alphabetically by genus.

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Research study 1
 

photograph showing spawn string of aeolid nudibranch Dendronotus sp.
photograph showing nudibranchs Okenia rosacea mating and egg-laying courtesy Kevin Lee, Fullerton, Californiaphotograph of nudibranch Triopha maculata laying its egg mass courtesy Kevin Lee, Fullerton, CaliforniaNudibranchs 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

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Research study 2
 

photograph of an egg mass of a nudibranchDetails 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 drawing of an Archimedes spiral, typical of nudibranch egg-mass depositionspaced 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; for an up-to-date review of developmental modes in opisthobranchs from southern California see Goddard & Green 2013 Bull South Cal Acad Sci 112 (2): 49.

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

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

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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.

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Aeolidia

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Research study 1
 

drawing of an aeolid nudibranch Aeolidia papillosa viewed from below as it extrudes an egg massphotograph of aeolid nudibranch Aeolidia papillosa with a drawing of a single egg capsuleAeolidia 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 partially sitting on the egg mass 2X

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Aplysia

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Research study 1
 

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 experimental 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 photograph of head of Aplysia californica courtesy Kevin Lee, Fullerton, Californiaanimals 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. Later publications indicate that these neurosecretory peptides are similar in structure and function in the related west-coast anaspids Aplysia californica, A. vaccaria, and Phyllaplysia tayloriLi et al. 1999 J Exp Biol 202: 2961

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

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Research study 2
 

graph showing relationship between body size and number of eggs per capsule in the sea hare Aplysia californicadrawing of Aplysia californica with details of egg string and egg capsules courtesy Kriegstein et al. 1974 Proc Nat Acad Sci, USA 71: 3654.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

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Research study 3
 

histogram comparing egg production in different cultured cohorts of sea hares Aplysia californicaA study on sea hares Aplysia californica at the University of Miami Aplysia Facility determines, among other things, the extent of variation in egg production in 3 cohorts cultured under identical conditions.  The entire study is much larger, but this part is most germane to this section of the Odyssey.  Results show that while number of individuals in the 3 cohorts are similar (45-48), spawn production varies by a factor of over 30 (see graph).  For whatever reason of reduced fitness (genetics, disease, or whatever), members of the least reproductive cohort (coloured orange in the histogram) also exhibit significantly less growth than the other 2 cohorts.  The study is important in reminding us of what we know already, but never really pay attention to, that individuals in wild populations may differ in genetic makeup and fitness much more than we think.   Gerdes & Fieber 2006 J Am Assoc Lab Anim Sci 45 (1): 40.

NOTE   the cohorts represent individuals hatched from 3 separate egg strands each obtained from a wild-caught adult.  Because the history of sperm donor or donors for the 3 adults is unknown, the hatchlings in each cohort will be at least half siblings to one another.  “Conditions” include ad libitum feeding on red algae Gracilaria ferox, temperature of 13-15oC, and standardised light/dark, oxygen concentration, pH, and so on.  A fourth cohort is fed on a test ration in a separate experiment and will not be considered here

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Doriopsilla

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Research study 1
 

photograph of coiled egg-ribbon of nudibranch Doriopsella gemela courtesy Jeff Goddard, UC Santa Barbara. CaliforniaUpright 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. photograph of newly hatched veliger larvae of nudibranch Doriopsella gemelaHowever, at least 5 west-coast species, including Okenia rosacea (see entry in the Okenia section below) 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

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Doris

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Research study 1
 

photograph of copulating nudibranchs Doris montereyensis with egg ribbon
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

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Janolus

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Research study 1
 

Researchers at the Oregon Institute of Marine Biology, Charleston investigate the relationship between the nudibranch Janolus fuscus and its common1 copepod parasite Ismaila belciki.  Most often a single female parasite accompanied by 0-2 smaller males will reside in the snail’s hemocoelic cavity loosely clinging to the ovotestis organ.  The relationship is obligate for the copepod, but whether hosts other than J. fuscus is not made clear by the authors.  Questions addressed in the research include whether parasitic castration2 occurs and, if it doesn’t, what effects does the parasite have on the host’s reproduction, as well as on its growth.  Results show that parasitised Janolus do mate, do transfer and receive sperm, and do spawn; however, overall reproductive output is reduced by up to 54%, and egg strings of parasitised sea slugs have fewer capsules (by 38%) and fewer viable embryos (by 34%) than uninfected ones.  Overall, the researchers calculate that up to 34% of the host’s reproductive output is redirected into production of eggs by the parasite.  Contrary to expectation, growth of infected snails does not differ significantly from growth of uninfected ones.  The authors comment that while Ismaila belciki is not a parasitic castrator, its presence causes significant reductions in all ways related to reproductive output of its host.  This work on Ismaila belciki and other research on related copepod parasites suggest that the cause of the reduction does not owe to consumption of the host’s ovotestis (reduced mouthparts) or competing with it for space, but rather to consumption3 (or absorption) of the host’s hemolymph.  Further research will need to be done on this topic.  Wolf & Young 2014 Int J Parasit 44: 391.

NOTE1  the authors note that on the Oregon coast as many as 80% of J. fuscus may be infected by a single female copepod

NOTE2  this is the complete elimination of the host’s reproduction by a single parasite.  This frees up energy to be used both by the parasite and by the host, in the latter instance potentially prolonging the life span of the host and, by association, the life span of the parasite

NOTE3a propo to this topic, a graduate student at Oregon State University, Corvallis notes that the gut system in both sexes of Ismaila (monstrous) belciki is incomplete (stomach only, no intestine or anus) and, in fact, in the female much of the interior body is devoted to reproductive organs.  The odd-looking appendages on the female are thought to be used for clinging onto an organ, such as the ovotestis, during the female’s residency in the hemocoelic space of its host (see drawings below).  Their apparent softness would minimize damage to the host.  Note that the author Francis Belcik received additional kudos in having the species he worked on named after him.  Belcik 1965 MSc Thesis, Oregon State University, 36pp.

 
photograph of nudibranch Janolus fuscus parasitised with a copepod Ismaila belciki The splayed out cerata of Janolus fuscus often make it difficult to differentiate head from tail. The single large rhinophore in the centre of the photograph suggests that the animal has rolled in on itself. A strip of blood-red or orange pigment is often present, running from the top of the head and along the back between the paired rhinophores. The egg sacs of the copepod Ismaila belciki are visible drawings of male and female copepods Ismaila belciki
Male and female Ismaila belciki drawn to scale
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Melibe

 
Research study 1
  drawing of a hooded nudibranch Melibe leoninadrawing of egg mass, eggs in their capsules, veliger larvae of the nudibranch Melibe leonina Nudibranchs Melibe leonina deposit egg ribbons on subtidal kelp, or more often on eelgrass Zostera blades.  The ribbons may be tightly coiled or hang down in wavy folds.  Each capsule contains about 15-25 eggs.  It requires about 2wk in laboratory culture at spring seawater temperatures in San Juan Islands, Washington for veliger larvae to hatch from the capsule.  An early reference to development in M. leonina states that on leaving the capsule the juvenile begins the life of a “so-called naked mollusk”.  In fact, just like other nudibranchs, Melibe spends several weeks feeding in the plankton before metamorphosing.  Kjerschow-Agersborg 1921 The Amer Nat 55: 222; Hurst 1967 Veliger 9: 255; drawing of Melibe courtesy Bert Elliot and Trevor Kincaid, University of Washington.
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Okenia

 
Research study 1
 

photograph showing nudibranchs Okenia rosacea mating and egg-laying courtesy Kevin Lee, Fullerton, CaliforniaIn the comprehensive study of egg masses of Californian nudibranchs the author comments that all dorid species examined lay their masses in a counterclockwise direction.  This also includes Okenia rosacea, which 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 note that 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 

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Onchidoris

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Research study 1
 

graph showing size of nudibranch Onchidoris bilamellata at spawning time in winter in Yorkshire, Englanddrawing of dorid nudibranch Onchidoris bilamellataThe barnacle-eating Onchidoris bilamellata is circumpolar in distribution.  Studies on the species in Yorkshire, England show that it has an annual life cycle, just as on the west coast of North America.  A single spawning period (two 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 mucous-trail following.  However, a test of the second possibility, using 10 pairs of sexually mature but unmated individuals, showed that this is unlikely.  Each test involves placing one individual, the “marker”, in the centre of a clean, glass aquarium and allowing it to crawl along for 15-35 minutes.  After this, the snail is removed, the seawater drawing showing tracks made by one nudibranch Onchidoris bilamellata following the mucous trail of a conspecificdecanted, and the trail traced onto paper from below.   After adding fresh seawater, a “tracker” snail is then placed in the centre of the aquarium and its trail similarly recorded.  A typical negative result is shown in the figure shown bottom Right. Note that the tracker has at least three opportunities to follow the trail laid down by the marker.  Of the 10 trials, seven are negative, two are inconclusive, and one is positive.  Other research by different investigators shows that mate-finding in nudibranchs is mainly by distance chemoreception, possibly mediated by the oral tentacles or, in some cases, by the rhinophores.  Todd 1979 J Exp Mar Biol Ecol 41: 213.

NOTE  each run is terminated when the nudibranch being tested butts against the side of the aquarium tank and stops moving. 

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  Phyllaplysia
 
Research study 1
 

photograph of anasidean Phyllaplysia tayloriphotograph of eggs masses of an anaspidean opisthobranch Phyllaplysia sp. from the Gulf of California courtesy Jeff Goddard, UC Santa Barbara, CaliforniaOne 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 for certain other flat-depositing species. 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

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