Fine distinctions

Sea urchins have long been among my favorite animals. From a purely aesthetic perspective I love them for their spiky exterior that hides a soft squishy interior. I also admire their uncanny and exasperating knack for getting into trouble despite the absence of a brain or centralized nervous system. Have you ever been outsmarted by an animal without a brain? I have. It’s rather humbling.

Red sea urchins (Mesocentrotus franciscanus) and purple sea urchins (Strongylocentrotus purpuratus) share a common geographic range along the northeastern Pacific but generally live in different habitats. S. purpuratus is the common urchin in tidepools, while reds are almost always subtidal (although I have seen them in the intertidal on very low minus tides). The two species’ habitats do overlap a bit, as the purple urchin can live in subtidal kelp forests alongside the reds. There is a commercial fishery for the gonads of red urchins, which are prized as uni by sushi aficionados. I’ve tried uni once, and it tasted exactly the way I imagined the gonads of a sea urchin would taste. Not a fan. I’d much rather make a different use of urchin gonads.

The other week I collected some urchins from the field, hoping that they’d have nice full gonads. Gametogenesis in many marine invertebrates, including sea urchins, is governed at least partly by annual light cycles. Provided they have sufficient food, purple urchins have ripe gonads and spawn in the winter, from December through March. Reds spawn in the spring, from March through June. In my experience the best time to induce spawning of purps in the lab is December or January, when the urchins have developed gonads but likely haven’t spawned yet. There is no way of knowing the sex of any given urchin or the condition of its gonads, so this exercise is somewhat of a crap shoot even with the best of planning.

Today I shot up my eight field-collected purps, hoping to get at least one male and one female out of the deal. I got lucky with the timing, as one of the smallest urchins was a female and began spewing out eggs. This little female gave a lot of eggs! She was followed by three males and two more females. So out of my eight purps I ended up with three of each sex, and a spawning rate of 75% ain’t bad.

I set up some mating crosses and fertilized all of the eggs. I divided the little female’s eggs into two batches and fertilized them with the sperm of two different males (M1 and M2). Each of the other females’ eggs was fertilized by M1, who gave huge amounts of sperm. When I checked on the eggs about two hours post-fertilization most of them had gone through the first cleavage division and seemed to be developing normally and on schedule.

Just for the hell of it I decided to shoot up some of the red urchins we have in the lab. I didn’t really think they’d spawn, as it’s not the season for them to be gravid. Red urchins are large, heavy animals with long and sharp spines and they are much more difficult to handle. Four of the five that I shot up did nothing, as expected. It took a long time, but just as I was about to give up on them the biggest red began dribbling out a couple thin streams of sperm. I examined the sperm under the microscope and they were very active and healthy. Fortunately I hadn’t returned the purps to their tanks, and two of the female were still putting out some eggs. I rinsed the purp eggs into a clean beaker, pipetted up some of the red sperm, and added it to the eggs.

Sea urchin eggs are covered by a thick jelly coat. In the video you can see many of the red urchin sperm embedded in the jelly coat of the egg. Despite the frantic activity of the sperm, fertilization (as evidenced by the rising of the fertilization envelope off the surface of the egg) took much longer than it does when eggs and sperm come from the same species.

Look at that beautiful zygote! Fertilization success in this hybrid cross was low, only about 50%. The eggs that did get fertilized went through the first cleavage division after about two hours later, which is right on time.

It remains to be seen whether or not the few hybrid embryos I have continue to develop. I have a colleague who has hybridized red and purple urchins successfully in the past, and has raised the offspring to adulthood. I don’t have any expectations of great success with this little experiment, but it would be very informative to raise known hybrid urchins. I’ve seen animals in the field that look like hybrids and there’s no reason to assume that hybridization between these two free-spawning species never occurs. The adults can be found living side-by-side subtidally, and there’s enough overlap in their reproductive seasons that some individuals of each species could very well spawn at the same time. On the other hand, hybridization that can be forced in the lab doesn’t necessarily occur in the field. I dumped a lot of red urchin sperm on those purple urchin eggs, and such high sperm concentration may overcome any mechanisms of reproductive isolation that exist under real-life conditions.

I’ll know more when I check on things tomorrow.