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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, and ONTOGENETIC DEVELOPMENT OF BEHAVIOUR are considered elsewhere.
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Aplysia & other anaspids

  This section on mate-selection & copulation is arranged alphabetically by genus. The genus Aplysia OTHER ANASPIDS is considered here, while AEOLIDIA , ALDERIA, HERMISSENDA, and NAVANAX & OTHER CEPHALASPIDS are considered in their own sections.
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Research study 1

photograph of anaspid opisthobranch Phyllaplysia taylori courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, WashingtonAnaspids, including aplysiids and Phyllaplysia taylori reciprocally copulate, as do other opisthobranchs.  A study on P. taylori collected from Elkhorn Slough, California provides information on sperm storage both in the donor and recipient.  The author uses injections of tritiated thymadine to monitor sperm movements during the copulatory exchange and provides a diagrammatic view of the pattern of movement.  Beeman 1970 J Exp Zool 175: 125. Photograph courtesy Linda Schroeder, Pacific Northwest Shell Club, Seattle, Washington PNWSC.

NOTE this is too detailed to include here

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Research study 2

photograph of anaspid Aplysia californica courtesy Steve Pennings, U Houston, TexasElectrophysiological studies on Aplysia californica collected from Catalina Island and Palos Verdes, California confirm that the rhinophores and tentacles are sensitive to the odour of conspecifics but not, surprisingly, to the odour of food algae.  The authors suggest that formation of large breeding aggregations during summer-autumn may be facilitated by this receptivity.  Audesirk & Audesirk 1977 Comp Biochem Physiol 56A: 267. Photograph courtesy Steve Pennings, University of Houston, Texas.








Sea hare Aplysia californica being held by diver and sea-hare
researcher Steve Pennings. Features visible on the sea hare include
the oral tentacles at the front of the head at Left, the rhinophores
just posteriorad, and the mantle edges complete with numbered tag

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Research study 3

Chemical pheromones are thought to be involved in the formation and maintenance of the breeding aggregations of Aplysia californica described above for populations in Catalina Island.  Observations of reproductive behaviour of A. californica at the Catalina Marine Science Center, Avalon, California suggest that there may be one or more pheromones involved.  The first observation is that mating/egg-laying individuals tend to induce others to engage in the same behaviour; a second, is that individuals depositing eggs are more attractive as sexual partners than non-egg-laying individuals; and, the last observation, is that egg-laying tends to be synchronized among individuals in the same seawater tank.  Audesirk 1977 Behav Biol 20: 235.

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Research study 4

histogram showing frequency of different mating aggregation sizes recorded at Catalina Island, CaliforniaStudies on populations of sea hares Aplysia californica at the Catalina Marine Science Center, Avalon, California reveal that most copulation occurs in Jul-Aug, and the animals die in the autumn.  During summer the animals may form large aggregations.  Sea hares are simultaneous hermaphrodites, with an individual able to act both as a sperm donor and as a sperm recipient at any given time. When copulating in a pair, the individual in back is the sperm donor, while the one in front is the sperm recipient.  Chains of copulating animals may be  formed (up to 19 individuals in this study), with the first in line acting only as a sperm recipient, the last in line only as a sperm donor, and all others inbetween as both. Note that most mating aggregations consist of 2-4 individuals (see histogram). Egg-laying occurs shortly after copulation.  The mating aggregations are often accompanied by masses of recently deposited egg cordons, often laid on top of one another. Audesirk 1979 Biol Bull 157: 407.

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Research study 5

histogram showing mating patterns of sea hares Aplysia californica during the day in Catalina Island, CaliforniaA study of reproductive behaviour of Aplysia californica in laboratory and field situations at Santa Catalina Island, California adds to our knowledge of mating aggregations, sexual roles during copulation, and diel patterns of mating.  Greater than half of the animals in aggregations observed during early morning are engaged in copulation and egg-laying, while most observed in late afternoon and evening are involved in feeding (see histogram on Left). Mixed aggregations of A. californica and A. vaccaria are not uncommon, but no interbreeding is observed.  Mating chains of several individuals are observed and even, occasionally, mating “chains” of 2 individuals. New arrivals generally join a chain at the terminal or “male” end; graph showing size relationship between sperm donors and sperm recipients in copulating pairs of sea hares Aplysia californicadeparting individuals, however, leave a chain from any location.  Few individuals join chains at the front, or terminal “female” end.The sexual role adopted in pair-wise matings is not a function of size, either absolute or relative to a partner (see graph on Right), nor does it seem to relate to feeding status. After laying eggs an individual is more likely to copulate as a sperm donor, perhaps because of the time required to gain enough food energy for the next laying.  The number and size of mating aggregations vary during the day, from about 140 of average size=5 in the morning, to about 40 of average size = 3 during the night (see histogram on Left).  The aggregations may persist for several days, but turnover is relatively high, with about one-third of animals leaving, and perhaps joining, an aggregation each day.  Pennings 1991 J Exp Mar Biol Ecol 149: 249.

NOTE  this requires much twisting and is facilitated by penii of several cm in length.  By twisting around on on another, each individual simultaneously donates and receives sperm

NOTE  this observation about "front-end" joining has been made for other species of Aplysia; however, the notion is misleading.  An individual does not “join” a chain at the front end; rather, if it happens at all, it is the front-end individual in the chain that does the joining.  The “joinee” could be on its own or be at the tail-end of another chain.  Note that the terminal “female” is the only individual in a chain that is free to feed while copulating and, depending upon species, it often does so.  Also, this leading individual may have a large mass to guide or pull along (up to 6 individuals in the present study), so its potential to glide onto a receptive “female” may be physically limited. Is it possible that the mass of the chain, even if it is only a few individuals, and feeding by the lead individual tends to preclude this first individual adopting another behaviour?  Perhaps it’s like chewing gum and walking at the same time…

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Research study 5.1

Sea hares Aplysia vaccaria in southern California grow large and fast.  Thus, with a 1-yr life span there is considerable variability in sizes of mating individuals.  The question arises as to whether the same “rules” govern the relationship between body size and sexual role in A. drawing of sea  hare Aplysia vaccariavaccaria as in its extensively studied congener A. californica. To investigate this, researchers from UC San  Diego uniquely implant number-coded transponder chips into 19 individuals and monitor mating activities of these and other untagged sea hares in the field every 2-3wk over a 1-yr period.  Results from size measurements on a total of 190 mating pairs show that not only is there a trend (although not statistically significant) toward size-assortative mating, with individuals acting as females being larger than the average size for the population, but as well there is size-assortative spatial clustering.  The paper is a welcome addition to the sea-hare literature, not least because it deals with a species about which so little is known.  Angeloni & Bradbury 1999 Ethology Ecol & Evol 11: 187. Drawing from Beeman 1963 Veliger 5: 145.

NOTE  the implanted chips can be read with a hand-held monitor after the animals are removed from the water

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Research study 6

histogram showing relationship between paternity and mating order in sperm-donating sea hares Aplysia californicaFemales of a species are predicted to invest relatively more of their resources in reproduction than males, and a simultaneous hermaphrodite like Aplysia californica is not excepted.  In this species individuals copulate front to back, with the one in front acting as a sperm recipient and the one in back as a sperm donor.  The one in back is actually on top, between the parapodia of its mating partner and fastened firmly with the anterior part of its foot.  Only rarely, and after considerable twisting, is a pair of individuals able to inseminate reciprocally.  Studies in San Diego, California show that individuals mating as sperm recipients are larger than the mean for the population, providing support for the above investment prediction.  However, sperm-donating individuals are no different in size from the population mean, nor from their mating partners.  By tagging individuals and monitoring their matings, the authors determine that individuals mate repeatedly in both sexual roles, similar to what has been found in other aplysiids.  Multiple matings lead to multiple paternity, with the last sperm donors achieving highest levels of paternity (see histogram).  Of 33 egg masses sampled, 25 have more than 2 paternal alleles and, of these 25, nine have at least 3 paternal alleles.  Overall, the mean number of paternity alleles in all egg masses is 2.1.  Aplysia californica, then, does not appear to specialise in one particular sexual role to the exclusion of the other, a finding noted previously (see Research Studies 5 & 5.1 above).  Angeloni et al. 2003 Behav Ecol 14: 554.

NOTE  determined by microstellite paternity analysis and expressed as the proportion of paternity achieved by a sperm donor in the first egg clutch produced by the sperm recipient after the mating event

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Research study 7

photograph showing spawn of sea hares Aplysia californica, courtesy Steve Pennings, University of Houston, TexasDoes a single mating provide enough sperm for the egg-laying needs of Aplysia californica?  Tests using individuals reared in isolation in the University of Miami, NIH National Resource for Aplysia, Miami, Florida show that, on average, individuals of 240g live mass deposit about 17 million fertilized eggs over 3wk after a single mating encounter.  Each mating is about 94min in duration.  Almost 70% of the eggs develop into veligers.  Given that field animals are known to copulate with multiple partners in large mating aggregations throughout the mating season, the authors conclude that A. californica in the field may rarely experience sperm limitation. Ludwig & Walsh 2004 Mar Biol 145: 727. Photograph courtesy Steve Pennings, University of Houston, Texas.


Spawn of sea hares Aplysia californica. Several individuals have contributed
to the mass, as evidenced by different colours of spawn. The colours indicate
different ages of the spawn and different algal foods being eaten by the spawning
individuals. A sea hare, possibly one of the layers, is visible in the 5 o'clock position.

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Research study 8

Mate attraction in Aplysia spp. involves long-distance water-borne protein pheromones that are released from egg masses, leading to large aggregations of copulating and egg-depositing individuals.  One of these attractants, named attractin, was previously known from A. californica, while 2 others isolated from extracts of albumen glands of A. californica are described and characterised in this research work.  T-maze choice experiments show that the 3 chemicals in combination not only mimic the attractiveness of egg cordons to sea hares, but appear to act synergistically. The authors state that the newly characterised pheromones may act in concert with attractin to stimulate Aplysia to form and maintain mating and egg-laying aggregations.  Cummins et al. 2004 J Biol Chem 279: 25614.

NOTE  these are designated, presumably with tongue in cheek, enticin and temptin

NOTE  these tests involve Aplyia brasiliana from the Gulf of Mexico and are not considered here.  Other research shows that the attractin “family” of pheromones is separable into 2 groups – one including A. californica, A. brasiliana, and A. fasciata (a Mediterranean species) with over 90% sequence identity, and one including A. vaccaria and A. depilans (also a Mediterranean species) with about 40% sequence identity.  Despite the different gene sequences, the west-coast species A. californica and A. vaccaria are known to be attracted to one another’s breeding aggregations.  Painter et al. 2004 Proc Nat Acad Sci 101 (18): 6929

  Research study 8.1

In a related paper by members of the same research group the authors present amino-acid sequences for the pheromone attractin for 5 species of anaspids including Aplysia californica, A. brasiliana, A. fasciata, A. vaccaria, and A. depilans, and confirm that A. californica attractin is photograph of a swimming sea hare Aplysia brasiliana courtesy Steve Penningsattractive to the 4 other species in T-maze bioassays.  The 5 sequences differ significantly in structure but each contains the same heptapeptide sequence Ile30-Glu-Glu-Cys-Lys-Thr-Ser36, offering the possibility that this conserved region may be the biologically active component involved in mate attraction.  Indeed, a peptide synthesized by the researchers and containing the conserved heptapeptide sequence is found to be significantly attractive to at least one of the 5 species, A. brasiliana.  The authors note that this family of water-borne peptide pheromones is the first to be characterised in invertebrates.  Their unusual feature is that they are not species-specific in their activity, unlike other pheromones known in the animal kingdom.  Cummins et al. 2004 Peptides 25: 185. Photo courtesy Steve Pennings, University of Houston, Texas.

NOTE  this Mediterranean species and its congenor species A. depilans are considered by some scientists to be a single species; however, the different amino-acid sequences in attractin peptides of the 2 species provides another line of thought on the matter

Aplysia brasiliana having a swim. Of the 5 species mentioned above, A. californica and A. vaccaria are strictly west-coast, A. fasciata and A. depilans are mainly Mediterranean, and A. brasiliana is south-eastern N.A. and Gulf of Mexico 0.6X

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Research study 9

For more than 30yr researchers have been interested in mating preferences and order of matings in Aplysia californica.  The ability to grow research subjects to reproductive age in isolation at the University of Miami’s NIH NCRR National Resource for Aplysia has added a new dimension to these investigations because individuals of “virgin” mating status are now available.  All animals used mated at least twice.  As in other studies with A. californica, individuals initially mating as a sperm donor show no preference for second mating role when paired with a “virgin” partner.  However, individuals initially mating as a sperm recipient show significant preference for acting as the sperm donor in subsequent matings with a “virgin” partner.  Singly mated individuals lay about 3 egg masses before accepting sperm from a second donor, suggesting that depletion of their store of allosperm may be a crucial factor in mating decision. The authors provide a thoughtful discussion about maximisation of reproductive fitness in simultaneous hermaphrodites.  Ludwig & Walsh 2008 Biol Bull 215: 265.

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