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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 Navanax & OTHER CEPHALASPIDS is considered here, while AEOLIDIA , ALDERIA, HERMISSENDA, and APLYSIA & OTHER ANASPIDS are considered in their own sections.
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Research study 1

photograph and schematic explaining egg-trading and sperm-trading models for mate selection in the cephalaspid Navanax inermisThe cephalaspid Navanax inermis is a simultaneous hermaphrodite that mates in pairs, with one individual acting as a sperm donor and the other as a sperm recipient. An individual will follow another’s mucous trail, initiate courtship, then copulate as a male. Studies at the Scripps Institution Of Oceanography, La Jolla, California show that individuals appear always willing to receive sperm, but willingness to be male varies over time. Size plays no role in determining which individual initiates a copulatory bout. The authors suggest that the mating system in Navanax is based on sperm- trading, that is, individuals donate sperm in order to receive sperm, and in the case of Navanax they are more willing to receive than donate. During a copulatory bout the individual acting as female controls the duration of the bout. The individual acting as male cooperates by maintaining penis contact until its partner initiates courtship behaviour signifying that it now wants to trade roles. A pair of animals may swap roles several times during a single copulatory session, with no evident tendency to specialise in one sexual role or the other. The authors speculate that the amount of sperm transferred in a single copulation is enough to fill only a portion of the receiving individual’s seminal receptacle, thus explaining the need for multiple copulations with the same partner. Leonard & Lukowiak 1985 Can J Zool 63: 2719 & Leonard & Lukowiak 1991 Anim Behav 41: 255; see also Leonard & Lukowiak 1984 Am Nat 124: 282 and Leonard & Lukowiak 1984 Zeitschrift fur Tierpsychologie 65: 327 for more on reproductive and other behaviours in Navanax inermis, and Leonard 1991 Amer Malacological Bull 9: 45 for review of mating systems in hermaphroditic gastropods.

NOTE just as in sea hares Aplysia, an individual Navanax in front in a copulating pair is acting as a female (receiving sperm), while the individual in back is acting as a male. Chains of copulating individuals are fairly common in laboratory situations under crowded conditions, but are rarely seen in the field

NOTE the opposing model, egg-trading, predicts that simultaneous hermaphrodites will mate, not to get their own eggs fertilised, but to fertilise a partner’s eggs; in other words, they will prefer the male sexual role. In a later article the authors test 3 hypotheses germane to the notion that sperm-trading rather than egg-trading is, in fact, occurring. The tests and results in this second article are complex and hard to understand: Leonard & Lukowiak 1991 Anim Behav 41: 255

Copulatory chains of three Navanax inermis, where 2
individuals receive sperm and 2 give sperm, are used in
the authors’ second cited here ( Leonard & Lukowiak
1991 Anim Behav 41: 255)
to test their hypothesis of
sperm trading in Navanax. Use of 3 individuals provides
opportunity for an individual to choose between
behaving as a male or as a female. If only 2
individuals are used, one of them has no initial choice

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Research study 1

photographs of cephalaspid Bulla gouldiana showing copulatory and egg-laying behavioursA somewhat different mating strategy is exhibited by the related cephalaspid Bullia gouldiana, where size does play a strong role in determining frequency and duration of mating, and in gender choice. Researchers at the Universities of California and Colorado State observe no size-assortive mating in 7mo of field tests using groups of Bullia in shallow tidal pools at the Scripps Coastal Reserve in southern California, but laboratory results tell a different story. Here, 127 mating experiments with a group of 254 individuals reveal that pairs are more likely to copulate if one of the participants is larger, and that this larger one tends to copulate as a male. In contrast, a pair of equally large animals tends to switch once during a copulatory bout so that both mate in both sexual roles. When the partners are disparately sized, mating duration increases with the size of the smaller sperm recipient. The authors compare their results with those from similar studies on other hermaphroditic invertebrates (earthworms, flatworms, opisthobranchs, pulmonates) and discuss generally the selective value of size-assortive mating. Chaine & Angeloni 2005 Behav Ecol Sociobiol 59: 58. Photographs courtesy Kevin Lee, Fullerton, California diverkevin.






An aggregation of Bulla gouldiana in southern
California displays pre-nuptial, copulatory, and
egg-laying behaviours. Individuals are 6cm in length

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