17 days old

What a difference a week makes! The Pisaster larvae have grown and developed quite a bit since I looked at them a week ago. Here they are as little space ships again.

Early brachiolaria larvae of Pisaster ochraceus, age 17 days. 19 June 2015. © Allison J. Gong
Early brachiolaria larvae of Pisaster ochraceus, age 17 days. 19 June 2015.
© Allison J. Gong

Since they are getting so big, Scott and I decided to redistribute the larvae from four jars into six. This will give them room to grow and ensure that they aren't overcrowded. To do this we first concentrated them all into a single beaker, then divided the entire population into two jars, then subdivided each jar into three jars, for a total of six. See all the larvae in the beaker?

Brachiolaria larvae of Pisaster ochraceus, aged 17 days. 19 June 2015. © Allison J. Gong
Brachiolaria larvae of Pisaster ochraceus, aged 17 days. 19 June 2015.
© Allison J. Gong

The largest larvae are ~1200 µm long, getting big enough to fill up the field of view under the lowest magnification of the compound microscope. The most noticeable difference from last time, aside from the overall increase in size, is that the ciliated band is becoming more lobed. These lobes will eventually be elaborated into the long arms of the mature brachiolaria larva ('brach-' is Greek for 'arm'). See below:

Brachiolaria larva of Pisaster ochraceus, age 17 days. 19 June 2015. © Allison J. Gong
Brachiolaria larva of Pisaster ochraceus, age 17 days. 19 June 2015.
© Allison J. Gong

The other rather obvious development is that the left and right coeloms from the previous observation a week ago have fused together in the anterior (top of the picture) and posterior (bottom of the picture) region of the body.

From here on out the larvae won't get too much bigger; if I remember correctly they'll grow until they're about 1500 µm long. Their brachiolar arms will get really long and pretty, though, greatly increasing the length of the ciliated band. Eventually their juvenile rudiments will form . . . but that's a post for another day. More on that when it happens.

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