Last week Eric and I were exploring the tidal flats in Bodega Harbor when we noticed these patches of orange and white in shallow pools on the surface of the mud:
When we looked closer, we could tell that something had been spawning — releasing eggs (orange) or sperm (white) — but we weren't sure what type of animal was responsible.
Here are two close-ups showing the vast number of eggs:
We kept wondering who was releasing these eggs/sperm, and eventually we came across an animal in the act of spawning:
This is a fairly large polychaete worm. We're not sure which species it is, but after consulting with some experts, it's possible it's in the family Amphinomidae. [I'll update this post if we learn more.]
Many marine invertebrates reproduce by broadcast spawning — releasing gametes into water. The strategy involves some luck in that the sperm need to encounter the eggs. In the situation below, you can see how that might happen (with the sperm drifting over the eggs), but if they were farther apart, it might be less likely:
Although many marine
invertebrates spawn in this manner, you don't get to see it that often
in the field. Because larval forms can sometimes be useful for species
identification, we decided to mix some eggs and sperm together to see if they'd develop into planktonic larvae in the lab. A week later, here's what we
found under the microscope!
This is a larval form called a trochophore. Those long spines (or chaetae) are likely used to defend the larva against predators and might also slow sinking in the water (they can spread the spines out to the side). Many worms might spend weeks developing and growing in the plankton before undergoing metamorphosis into a juvenile worm that will burrow in the mudflats.
Many thanks to Eric for this wonderful photo of a fascinating trochophore larva!
P.S. For a couple of other examples of broadcast spawning, see the posts called "Fortuitous" on 28 June 2013 (sea anemones) and "Broadcast news" on 2 May 2019 (sea stars).
4 comments:
OMG, this is absolutely fascinating!
More proof that paying attention to what we see and wondering about what it is can lead down fascinating paths. Thank you for sharing this with us!
WOW, just WOW!
Hi Jackie,
Curious about the relative sizes of the forms shown, I was about to ask you,
but then thought it more 'responsible' of me to just look up polychaetes myself.
What I found was that, "Polychaete worms can vary in size from a few millimetres to about 3 m (10 ft)."
Sooo, LOL, I'm back to asking if you could provide an idea of the sizes of the egg patch & the worm itself.
Also very clever to 'grow your own', and excellent resulting photomicrograph by Eric!
Vishnu
Hello, all! Thank you for the comments!
Yes, polychaete worms have quite a broad size range. This worm was ~1 cm or so wide, so a sizable worm. You could envision a narrow pinky finger, for example. We didn't see many of the adult worms that day, but we might go back to see if we can find them again to help with the identification.
Jackie
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