Did you know that you belong to a species that exhibits gonochorism? That in fact, you yourself are a gonochorist?

Ominous as that sounds, in biology the term only refers to individuals being born with just one of at least two distinguishable sexes.  Typically, this is either male or female.

Now you might think that this is just overstating the obvious.  After all, aren’t most things unisexual?  It may surprise you to learn that there exist several groups of organisms, each containing thousands of species, that vehemently disagree.  Without having to resort to being asexual, they have evolved some of the most fascinating ways to reproduce.

The major innovators are flowering plants, which can have individual flowers that are hermaphrodite (having both male and female parts, or stamens and ovaries) or gonochorous (either only stamens, or ovaries).   In a positively tongue-twisting development, plants can be, for example, polygamomoenoecious!  Staggeringly, a single plant can simultaneously have male, female and perfect hermaphrodite flowers.

There are also some elegant examples of these adaptations in the animal kingdom, most of which happen to live in the sea.

The nudibranchs (derived from nudus, or Latin for naked, and brankhia, or Greek for gills) are marine gastropod molluscs (or mollusks, there’s some disagreement here).  Although I don’t commonly associate beauty with slugs, I cannot deny that the world of sea slugs is the closest thing to Suzanne Collins’ exaggerated Capitol that I have ever seen.

Chromodoris willani at the 'Drop Off' divesite, Verde Island, Puerto Galera, the Philippines
Chromodoris willani at the ‘Drop Off’ divesite, Verde Island, Puerto Galera, the Philippines                                                  Photo by Alexander R. Jenner, available from a Creative Commons Share Alike 3.0 Unported License

 

Nudibranches (Hermissenda crassicornis) in California tide pools
Nudibranches (Hermissenda crassicornis) in California tide pools                                                                    Photo by Brocken Inaglory, available from a Creative Commons Share Alike 3.0 Unported License
Chromodoris lochi
Chromodoris lochi                                                                    Photo by Alexander R. Jenner, available from Creative Commons Share Alike 3.0 Unported License
Berghia coerulescens
Berghia coerulescens                                                                   Photo by Parent Géry, available from Wikimedia Commons

These delightfully dressed creatures are simultaneous hermaphrodites, containing both male and female genitalia.  Although such an adaptation was suggested to have evolved due to low population density (“It’s so hard to find a date around here”) and low mobility (“I see him, but it’ll take me a year to get within kissing distance at this rate”), it appears that a lot of simultaneous hermaphrodites break these rules.

Some actually live in higher densities than ever known before, which brings me to another interesting part of their anatomy –  the sperm storage organs.  Combine this with the special adaptations they have to digest excess sperm, and you know that these guys are doing a lot more than just kissing.

The pinnacle of innovative reproduction among molluscs was recently discovered by Sekizawa and colleagues (2013).

We already knew that they both produced and accepted sperm during copulation, thus performing the roles of both traditional sexes.  They store sperm within the body in two structures, named the bursa copulatrix and the seminal receptacle.  The bursa can actually digest external sperm (or allosperm) if needed, to make space for more.  What the researchers could hardly believe they were seeing was the autotomization of penises after copulation, in which penises literally drop off the animal’s anatomy.

In a further staggering development, they then proceed to grow a new penis.  Moreover, regrowth occurs within a mere 24 hours!

Researchers first acquired the organisms for this study off the coast of Okinawa in Japan, by scuba diving for them during their reproductive season (from April to June).  In a new spin on ‘long-term research’, they observed 31 reciprocal nudibranch copulations (yawn!)

Each animal mated with the other simultaneously, by inserting their ‘male’ genitalia into their partner’s ‘female’ counterparts.

A pair of Nembrotha milleri mating at The Drop Off divesite, Verde Island, Puerto Galera, The Philippines
A pair of Nembrotha milleri mating at The Drop Off divesite, Verde Island, Puerto Galera, The Philippines                           Photo by Alexander R. Jenner, available from a Creative Commons Share Alike 3.0 Unported License
Mating nudibranchs
Mating nudibranchs                                                                                                                                             Photo by Barry Peters, available from Creative Commons Attribution 2.0 Generic License

After an average of nine and a half minutes (± ~ 5 mins), they push away from each other, eventually extricating their genitalia from their partner’s body.  Their penises remain elongated for a little while, and ~ 21 minutes after stopping to mate, they do something that is extremely rare.

They simply excise their penises and continue on with life.

Researchers discovered a spiral structure within the organ that appears to be necessary for copulation.  Once their disposable penis drops off, this spiral structure with ‘undifferentiated’ cells then transforms into the next penis.

The most they have to wait to copulate again is 24 hours, which, I think we can all agree, isn’t that bad.  And if that isn’t good enough, while waiting for their penises to regrow, they sometimes compromise and copulate as just females!

Sources

Sekizawa A., Seki S., Tokuzato M., Shiga S. & Nakashima Y. (2013). Disposable penis and its replenishment in a simultaneous hermaphrodite, Biology Letters, 9 (2) 20121150-20121150. DOI:

More about nudibranchs

National Geographic takes on the nudibranchs!

Get your fill of nudibranch beauties here

Got a picture of something you suspect is a nudibranch? Identify it here using just your photograph

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