The Asexual Life of Brittle Stars

You may have heard the term “asexual” before.  There are two very different directions you can go with the term “asexual”, either asexual reproduction or asexual orientation.  One of these terms people can be, the other… not so much.  The one we are going to talk about here is asexual reproduction, the one humans cannot do. 

Asexual reproduction in its most general sense, means that a living thing (usually plants but some animals too, like many brittle stars) can create new versions of themselves without having sex (i.e. exchanging sperm and eggs) with another living thing.  The living thing just creates duplicates of itself all on it’s own. 

On the other hand, asexual orientation, the one people can be, means that a person is not sexually attracted to other people.  So, if someone tells you that they are asexual, that doesn’t mean they are going to start popping out duplicates of themself without the assistance of another individual’s contribution.  You should probably still give your friend your support though if they are sharing that with you.  And with that, let’s talk about brittle stars and how a lot of them can reproduce both sexually and asexually.

What is a Brittle Star?

Brittle stars are in the family Echinodermata along with sea stars (starfish), sea urchins, sea cucumbers, sand dollars, and basket stars (1).  Many people have not heard of brittle stars, but many have heard of their starfish cousin, which is more accurately called sea stars as they aren’t fish even though they live in the ocean.  Rather, echinoderms are invertebrates, meaning they have no bones.  Brittle stars are similar to sea stars except they tend to be smaller with thin “brittle” arms.  A cool thing about many echinoderms is, that if they lose a limb, it will grow back (2).  This ability to regrow body parts is also the basis that allows for asexual reproduction.

Brittle Star Sex

Not only can brittle stars reproduce asexually, but they can also reproduce sexually with another brittle star.  Sexual reproduction in brittle stars occurs similar to many other marine invertebrates.  Males release their sperm into the ocean, females do the same with their eggs.  This method of sexual reproduction is called broadcasting, because the males just release their sperm into the ocean in hope that they find eggs to fertilize all on their own (3).  This is not the only form of sexual reproduction in brittle stars.  Some echinoderms brood their eggs, which means they will guard them either internally or externally until they hatch into larval babies (4).  Broadcasting is the most common sexual reproduction in echinoderms though.  The babies then go through one of my favorite forms of development: metamorphosis.  Although metamorphosis is the same type of development that amphibians go through the process is very different for these very different animals (5).  I will write on echinoderm metamorphosis in a future article.  Spoiler alert, it is very cool!

Reproduction Without Sex

So the moment we’ve been sitting at the edge of our seats for: sex free reproduction in brittle stars, i.e. asexual reproduction!  There are different forms of asexual reproduction and the type the brittle stars do is called fissiparity (3,6,7).  Fissiparity is common among invertebrates that can reproduce asexually, it is different from how plants reproduce asexually.

Fissiparity basically means breaking into parts.  As mentioned before, if a brittle star or other echinoderm loses a limb it can regrow it (2).  This is just regrowing a limb though, not a new organism.  So in order to reproduce through fissiparity, the location of where the animal breaks apart actually matters and is the difference between creating a new organism and just regrowing a limb.  Brittle stars have a central disc, this is the middle of their body that each of their limbs attaches too (3).  If the brittle star splits apart through the central disc then both halves containing part of the central disc will regrow the missing limbs from the central disc present (3,6,7)!  Mind blowing, it means that one organism basically just created a clone of itself by splitting in half!  The central disc is the key here, if a part of the brittle star breaks off and does not include a section of the central disc then it will not grow into a new brittle star.  If a piece of the brittle star breaks off and does contain a section of the central disc with at least one limb, then it can grow into a new brittle star.  Assuming it is not consumed by a predator first.

Not all echinoderms can do this.  Of all the echinoderms brittle stars have the most species that can reproduce asexually.  Some sea stars, basket stars, sand dollars, and sea cucumbers can also reproduce asexually in addition to sexually.  As for sea urchins, only larval sea urchins can reproduce asexually, once they are adults, sea urchins can only reproduce sexually (6,7).  All echinoderms (to my knowledge) can reproduce sexually as adults.

What did you find most interesting from this article? Anything you would have liked more details on? Comment below and let me know what other cool nature things you want me to write about.

Citations

1. Mulcrone, R.S. (2005). “Echinodermata” (On-line), Animal Diversity Web. Accessed 2021 at https://animaldiversity.org/accounts/Echinodermata/
2. Morris, M. and D. Fautin (2011) “Ophiuroidea” (On-line), Animal Diversity Web. Accessed 2021 at https://animaldiversity.org/accounts/Ophiuroidea/
3. McGovern, T.M. (2002). Patterns of sexual and asexual reproduction in the brittle star Ophiactis savignyi in the Florida Keys. Marine Ecology Progress Series 230: 119-126.
4. Gillespie, J.M. and J.B. McClintock (2007). Brooding in echinoderms: how can modern experimental techniques add to our historical perspective?. Journal of Experimental Marine Biology and Ecology 342(2): 191-201.
5. Mladenov, P.V. (1985). Development and metamorphosis of the brittle star Ophiocoma pumila: evolutionary and ecological implications. Biological Bulletin 168(2): 285-295.
6. Mladenov, P.V. (1995). Environmental factors influencing asexual reproductive processes in echinoderms. Oceanologica 19:227-235.
7. Dolmatov, I.Y., S.V. Afanasyev, and A.V. Boyko (2018). Molecular mechanisms of fission in echinoderms: Transcriptome analysis. PLoS One 13(4): e0195836.