Cross-species matings


Upper: Hypselodoris bullocki and H. apolegma mating. Photo: Phil Slosberg.
Lower: Halgerda carlsoni and H. diaphana mating. Photo: Mitsuo & Ayumi Murakami.

One definition of a species is that it is a group of animals which mate with each other and produce viable off-spring. One consequence of that is that many species have evolved elaborate behavioural and anatomical barriers so that species mate only with their own kind. When two apparently different species are observed mating it is an event of some interest. It means either that our definition of the species involved is faulty, or that the methods these species use to identify their own kind is faulty. A number of such observations have been made by Forum contributors and links to their messages are listed below. See also some general messages below.

Chelidonura amoena & C. electra - Possible hybrid
Chromodoris annae & C. cf. colemani mating?
Hypselodoris bullocki and H. apolegma mating.
Hypselodoris sp and H. apolegma mating.
Risbecia tryoni & R. pulchella trailing. message 1, message 2.
Halgerda carlsoni and H. diaphana mating. [see second message.]
Phyllidia ocellata - colour forms? mating
Phyllidia ocellata & P. babai mating.

Authorship details
Rudman, W.B., 2003 (April 7) Cross-species matings. [In] Sea Slug Forum. Australian Museum, Sydney. Available from

Related messages

Re: Cross-species breeding

April 26, 2006
From: Cynthia Trowbridge

Concerning message #16049:

Dear Jasmine and Bill,
I have been thinking about the same issue of cross breeding but in sacoglossans. I have seen individuals of one sacoglossan species crawl actively over and around individuals of another species while extending and waving the male structure (penis and stylet). I did not witness mutual copulation or even hypodermic penetration.... thus, I would conclude one species was exhibiting sexual signals but the other was not responding. The barriers to cross-fertilization of species differ among taxa but could be behavioral, structural, chemical, etc. I watched the sacoglossans periodically for 24 hours and the slug exhibited sexual interest eventually (many hours) became disinterested, retracted its penis, and crawled away. I would be interested in learning if others have seen analogous behaviors in other opisthobranchs.


Trowbridge, C.D., 2006 (Apr 26) Re: Cross-species breeding. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Cross-species breeding - Sea Hares

March 8, 2006
From: Jasmine Peters

I work at a nature center in south Florida. We occasionally get sea hares in our tanks when visitors bring them from the beach, wondering what they are. Right now we have about 6 breeding and egg laying Ragged Sea Hares (Bursatella leachii pleii) and one Black Atlantic Sea Hare (Aplysia morio ). This morning as I was checking the tanks, one of the Ragged Sea Hares was away from the breeding 'pile' for lack of a better word, and was enveloped by the Black Atlantic Sea Hare. The Ragged didn't seem to be trying to escape, but since the Black is about 3 times larger, it was completely covering the Ragged at times.
Any thoughts? I would love to know what to tell the visitors when they ask.

Locality: Salt Water Tank (2000 gallon, constantly flowing with sea water), N/A, Florida, USA, Atlantic Ocean, 06 March 06, Sandy bottom tank. Length: Black (25 cm), Ragged (10 cm)

Jasmine Peters

Peters, J. S., 2006 (Mar 8) Cross-species breeding - Sea Hares. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Dear Jasmine,
You don't actually say the Aplysia was mating with the Bursatella unless you were using the term 'covering' in the euphemistic sense that horse breeders do. Opisthobranchs are no different from the rest of the animal kingdom in avoiding interspecies breeding as a rule. If you find two closely related species mating then it is worth considering that perhaps they aren't separate species. In this case, Aplysia and Bursatella  are quite distinct genera, so I am sure these two species are not close relatives. Strange things occur in tanks and crowded conditions. The most likely explanation is that the Aplysia just crawled over the Bursatella and decided it was a comfortable, or comforting, place to rest.

Unless you can see the Aplysia with its penis inserted in the Bursatella, I think this is just one of those things that happen. The animals often hide in crevices or aggregate together with their own kind. I guess in unnatural situations, aggregating with relatives is not surprising, but I wouldn't think it is necessarily a reproductive behvaiour
Best wishes,
Bill Rudman

Rudman, W.B., 2006 (Mar 8). Comment on Cross-species breeding - Sea Hares by Jasmine Peters. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Cross-species matings

April 9, 2003
From: Bill Rudman

To accompany Atsushi's message, on two species of Halgerda apparently mating, I have prepared a Page where I will list any messages on the Forum which deal with apparent matings between species. I will also include similar interactions, such as two species of Risbecia trailing, which also suggests some breakdown in either our concept of what is a species, or the species ability to recognise its own kind. There are three in the list at present. If I have missed any, or if anyone as further examples please let me know.
Best wishes,
Bill Rudman

Rudman, W.B., 2003 (Apr 9) Cross-species matings. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Cross-species mating

March 8, 2003
From: Samuel K. Moore

I'm new to the site, but a former Aplysia californica researcher. I've often wondered whether the prevention of cross-species mating had something to do with a chemical attractant exuded during egg-laying. Some research by my former boss, Earl Mayeri (UCSF) showed that contact with an egg-mass led to both egg laying behavior and mating as a female as well as the attraction of other Aplysia to the orgy. We always suspected some pheromone from the atrial gland led to all this. Anyway, I wonder whether cross-species mating might have to do with two species having similar pheremones. In contrast, perhaps speciation could arise by mutations in the pheromone gene. Just a thought.

Does anyone know if anyone has looked at this issue in closely related or geographically colocated species?

Dear Sam,
Welcome to the Forum. I would agree that species identification problems, both in mating and in 'trailing' would seem to be related to an inability to differentiate some chemical marker the slugs produce. However I don't know of any work on this. These 'mistakes' have only been reported a few times. If this means they are uncommon events I guess they are not a good subject for a research project which needs a completion date.

Concerning your work on Aplysia. I remember a paper on the topic (Bernheim & Mayeri, 1995) and a related paper which I saw publicised somewhere as the Sea Hare Love Drug which I'll summarise in a separate message. There is so much work that has been done on Aplysia californica in the last 20 years that we need a new book synthesising the results to put on the shelf alongside Eric Kandell's pioneering works.

• Bernheim, S.M., and Mayeri, E. (1995) Complex behavior induced by egg laying hormone in Aplysia. J. Comp. Physiol. 176: 131-136

Best wishes,
Bill Rudman

Rudman, W.B., 2003 (Mar 8). Comment on Cross-species mating by Samuel K. Moore. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Re: Mating behaviour

March 3, 2003
From: Hans Tibboel

Dear Bill,
Thanks for your comments. The mating nudibranchs were the Risbecia tryoni and Chromodoris annulata and they were located on a wall at Black Rock in Myanmar at approx. 6 meter depth. Our on board photo-pro Mark Strickland took slide pictures of the mating and may be able to post those later on. We're both fulltime working on the boat and do not have much time to check our mail but I will try to keep you updated. Mark discovered in Myanmar a few years ago a new Halgerda that has been named after him (Halgerda stricklandi) and we still regularly come across nudibranchs that we have never seen before. Personally I do not carry any underwater camera gear but have also a great passion for seaslugs and nudibranchs.
Hans Tibboel

Dear Hans,
I would very much like to see some photos as I would like to check the identity of the two animals and just see whether they were actually mating. If you have a look at two earlier messages, one from Stuart Hutchison and the other from Mike Miller, you will that they both report R. tryoni and the yellow-spotted Risbecia pulchella in close contact with each other. R. pulchella is quite similar to C. annulata in colour, and close proximity can suggest mating. Species of Risbecia have this peculiar, and very characteristic, trailing behaviour where one animal follows another - often with the head in actual contact with the tip of the leading animals foot. They seem to be attracted to each other's mucus trail and the photos I mention suggest that sometimes they get confused with the mucus of their close relatives.

Whatever Mark has photographed it is very interesting. It will either confirm that these two species of Risbecia are liable to get confused, or it will show two 'species' mating. In either case it will be worth further investigation.
Bill Rudman

Rudman, W.B., 2003 (Mar 3). Comment on Re: Mating behaviour by Hans Tibboel. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Mating behaviour

February 25, 2003
From: Hans Tibboel

Dear Sir,
On a recent dive cruise in Myanmar I came across two different species of nudibranchs mating with each other. As I've never witnessed this before I was wondering if anybody else has ever noticed this same behaviour and if it will result in actual egglaying and offspring of the participating nudi's.
Hans Tibboel

Dear Hans,
I guess you are not talking about the spotted and unspotted forms of Chromodoris annulata you mentioned in your last message. This question has been raised recently on the Forum when I posted Phil Slosberg's photo showing typical H. bullocki and H. apolegma mating.

In general, matings between different species are very rare, and if they occur, the eggs are either in infertile, or if offspring do occur, the offspring are infertile. There are some strange exceptions amongst the plants but on the whole, mating is considered a good indication that the partners are the same species. I realise that this argument becomes a bit circular, but organisms have usually developed good morphological and behavioural mechanisms to prevent cross-species matings.

The first question I would ask is are you sure that the two animals were actually mating, and if so, how do you know they were different species?
Best wishes,
Bill Rudman

Rudman, W.B., 2003 (Feb 25). Comment on Mating behaviour by Hans Tibboel. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Re: H. bullocki 'colour forms' mating

August 6, 2002
From: Erwin Köhler

Dear Bill,
It looks like nobody is interested in interspecies mating problems ...
Here some words on it: In my youth I collected butterflies, focussing on Sphingidae (Common names: sphinx moths, hawk moths). There were several records of interspecies mating in the field and of course in the lab: Smerinthus ocellata and Laothoe populi.
The next generation was reported to be unfertile.

Köhler, E., 2002 (Aug 6) Re: H. bullocki 'colour forms' mating. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from

Thanks Erwin,
I am glad someone noticed my plea for a discussion. There are examples of interspecies mating, usually resulting in infertile offspring, but in terms of all life on earth, they are a rarity. The question I am trying to get discussed is not whether interspecies mating can occur but whether it is a sensible suggestion in the case of the Hypselodoris bullocki group of species. It seems to me that the argument is starting to look a bit circular. My first thought when seeing two animals mating, is that they are the same species, and if they are distinctly different in shape and colour from one another, it would suggest that there is either sexual dimorphism (in species with separate males and females) or polymorphism within the species. In your butterfly days you must have seen examples of polymorphic butterflies and moths, in which there can be 2, 3, or more, distinct colour patterns within the one species.

The alternative in the case of H. bullocki, is to argue that there are a number of species, very similar in shape and anatomy but different in colour pattern - and by the way, they interbreed with each other! It doesn't sound very convincing to me.
Bill Rudman

Rudman, W.B., 2002 (Aug 6). Comment on Re: H. bullocki 'colour forms' mating by Erwin Köhler. [Message in] Sea Slug Forum. Australian Museum, Sydney. Available from