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photograph of 2 nudibranchs Hermissenda crassicornisNudibranchs are hermaphroditic, and copulation leads to reciprocal transfer of sperm between 2 individuals.  Each individual’s gonopore is located on the right-hand side, so body alignment is part of the pre-copulatory ritual.  Eggs are encapsulated (with up to a dozen or more per capsule depending upon species), embedded in a protective gelatinous mass, and usually attached to the sea bottom.  Hatching occurs in a week or so depending upon temperature, and veliger larvae swim free to feed on phytoplankton for several weeks before settling.  Little has been written on how the veligers excape the gelatinous mass, but by the time the veligers break out of their capsules the mass is generally infested with bacteria, protists, and other organisms, and is in a state of at least partial degradation. Of particular interest in nudibranchs has been the identification of a natural metamorphosis inducer, and some research on this topic has been done on west-coast species. 

A pair of Hermissenda crassicornis 1X

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Mate selection & copulation

  Mate selection & copulation are dealt with in this section, while topics of EGG-LAYING, EMBRYONIC DEVELOPMENT, HATCHING & LARVAL LIFE, SETTLEMENT & METAMORPHOSIS, SETTLEMENT CUES, and ONTOGENETIC DEVELOPMENT OF BEHAVIOUR are considered elsewhere.
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  This section on mate-selection & copulation is arranged alphabetically by genus. The genus Aeolidia is considered here, while HERMISSENDA , ALDERIA, NAVANAX & OTHER CEPHALASPIDS, and APLYSIA & OTHER ANASPIDS are considered in their own sections.
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Research study 1

drawings of copulatory behaviour in the aeolid nudibranch Aeolidia papillosaStudies at Friday Harbor Laboratories, Washington show that mating of Aeolidia papillosa involves the following behaviour.  After first contact on any part of the body, 2 individuals ready to mate turn head-to-head.  Direct contact of mouths usually then precedes a left turning by each individual and the 2 advance until their gonopores are aligned.  These behaviours take about 1min.  Penises may be everted simultaneously or one individual’s eversion may stimulate the other’s to evert.  Small stiffish projections located on the posterior parts of everted reproductive apparatuses help align the 2 penises opposite the respective oviducal openings.  In these studies, the researchers observe no actual penis penetration during copulation, and sperm is transferred from the penis tip across a short space to the oviducal opening in each individual. The sperm is contained within discrete packets known as spermatophores. 

Total copulatory time from penis eversion to retraction is correlated with individual size, and is of relatively short duration.  Note in the graph below that copulatory times are generally between 5-10min.  The slope, b, of the regression line is 0.66, suggesting that copulation time scales isometrically with metabolic rate.

graph showing relationship of copulatory time and size in the aeolid nudibranch Aeolidia papillosa

photograph of aeolid nudibranch Aeolidia papillosaCould the relatively short-duration copulatory bouts in Aeolidia papillosa result from sperm depletion?  Counts of number of sperm packets transferred during an average copulation in Aeolidia papillosa (154) multiplied by the average number of sperm in each packet (180,000) gives a total of less than 30 million sperm per copulation.  Since an average-sized Aeolidia has over 400 million sperm stored in its seminal vesicle (termed ampulla by the authors), it seems likely that depletion of sperm is not a causal factor in termination of copulation. Longley & Longley 1984 Can J Zool 62: 8.

Aeolidia papillosa crawling on a kelp frond. The small white
patches are bryozoan colonies, probably Membranipora sp. 1X

NOTE  although copulation with penile penetration is usual in other nudibranch species, the authors reject the notion that absence of this in Aeolidia may be an artifact of laboratory conditions.  Back-lighting allows the authors to view (and count) the sperm packets (0.5mm in length) as they exit the penis of one individual and enter the oviducal opening of the other individual.  The penis enlarges 100-fold in volume during eversion (increase of 3mm to 3cm in length).

NOTE  individual live mass in the graph is that measured for the smaller of the copulating pair.  This is because it is the smaller individual that generally breaks off from copulation first. The regression line does not include a small-sized outlier, shown in yellow

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photograph of Aeolidia papillosa in copulatory mode, courtesy Longley & Longley, Santa Cruz, California CLICK HERE to see a video of Aeolidia papillosa copulating. Video courtesy Roger & Alison Longley, Friday Harbor Laboratories, Washington.
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