An unexpected bit of microscopy

by Mike Samworth

Recently I obtained two rather nice swan mussels for my freshwater aquarium.This was another attempt to get my pair of Bitterling to breed. Perhaps I should explain. The Bitterling is a small coldwater fish that lays its eggs inside the swan mussel where they develop into small fry. The female fish has a long ovipositor which develops which she places in the inhalant siphon entrance of the mussel. Following this the male shoots his sperm into this same entrance. Inside the mussel the fry obtain food, oxygen and protection.

There is also a further link between fish and mollusc. The mussels produce eggs of their own which are retained in a special brood pouch for up to 9 months. Here they are also fertilised by sperms from a nearby male coming in with the feeding current. During their time in the pouch the eggs develop into a special larva called a glochidium. These get released into the water where the lucky ones come into contact with a fish. Small shell teeth on both bi-valves allow them to become attached where they burrow into the flesh and form a cyst. Over the next three months a miniature adult mussel develops which then drops off and leads a free existence.

After I had put the mussels into my tank the next morning I spotted some of the glochidia. They were easily taken with a teat pipette so I placed one under the stereo and then high power compound microscope. rather comically they can be seen to open and shut their shells rapidly. Between the shell valves is a long, rather sticky looking thread, called the byssus. Presumably this is helpful in sticking in pond weed and the like to give more chance of attachment onto a passing fish.

The glochidium larva of a swan mussel.

It is always fascinating to find something new that takes your interest and then for that to yield happy times at the microscope. I later found that the young larvae looked nice under crossed polars, and took some photographs. Maybe I can show you some of these later.

If any reader wishes to ask about any of the above, or to comment, please do get in touch by contacting me Mike Samworth.

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