Bittlerling

Wildlife filming. Behind the lens.

People often ask me what are the most interesting subjects I’ve filmed.
Up among the top is the Bitterling. It’s a small fresh water European fish which lays its eggs inside a fresh-water mollusc.

After the snow has melted the males turn slightly blue and will guard the biggest healthiest looking molluscs, seeing off all other competitive males.

The female will grow an ovipositor which she will use to squirt an egg deep down in the mussels exhalant tube where the egg will lodge. The male will spray his milt down the inhalant which passes through the mussle and fertilises the egg.

The egg will develop in to a fry with it's own yoke and does not parasitize the mussel in any way. When fully developed the small fry will swim out of the exhalent tube in to the open river. (It is believed that the tiny baby mollusc which has a hook hitches a ride on the fry when it leaves - I personally think this is unlikely.

The male is very choosey about which females he’ll allow to lay her eggs in the mollusc.
If the female laid her egg down in the inhalant, the egg would pass into the stomach and be digested by the mussel whereas squirting the egg down the exhalent the egg is perfectly safe protected by the mussel and provides an exit tube for the fry to swim out of when it’s properly developed.

I filmed this sequence in the studio in Oxford for a series call ‘The Sexual Imperative” which revealed fascinating strategies animals use to reproduce.

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