Back in mid-December I collected a couple of small intertidal fishes and brought them back to the lab for observation and identification. Then the female laid a batch of eggs, which I've been watching ever since. Today the eggs are 15 days old. They are developing pretty quickly, I think, at ambient seawater temperatures of 12-13.5°C. Some of the changes can be seen with the naked eye, while others are visible only with some magnification.
Here's a timeline of development for the first couple of weeks in the earliest life of bald sculpins.
Day 4: The egg mass is clean and the eggs are clear and pink. The very young embryo can be faintly seen as a paler pink strip lying on top of the darker pink yolk, which fills most of the internal volume of the egg. There are also some oil droplets associated with but not part of the yolk.
It wasn't until this day that I was convinced the eggs were alive. Until then they looked like undifferentiated pink blobs with not a lot going on.
Day 7: Today they had eyes! And they were swimming around inside their eggs!
Day 10: Today the eyes look more like fish eyes and are taking on a silvery sheen. Black pigment spots are forming along the dorsal surface of the embryos, and the yolk is noticeably smaller. The eggs are starting to look dirty to the naked eye, due to the darkening eyes and pigment spots.
Today was the first day I could see their heartbeats! It was surprisingly difficult to capture the beating hearts with the camera.
Day 15: Some of the eggs have died, becoming opaque and hard. A few have broken open and are empty. The overall color of the egg mass is paler, as the larvae are consuming their yolks. The black pigment spots are becoming more prominent and seem to be concentrated on the top of the head.
They look like baby fish now! They're still flipping around inside their eggs and I think may be responding to light. They don't seem to like it when I shine the light on them.
I've put together a short video of the eggs at various stages of development so far.
Back in mid-December I collected some urchins at Davenport Landing. Some of these urchins are the parents of the larvae that I'm culturing and observing now. Towards the end of the trip I flipped over some surfgrass (Phyllospadix torreyi) and saw two fish, obviously sculpins, huddled together; they had been hiding in the Phyllospadix and waiting to be submerged when the water returned with the high tide. I have a probably inordinate fondness for intertidal fishes, and love catching sculpins. These were too big to be fluffies (Oligocottus snyderi) but I couldn't pin down an ID any closer than that. I brought them back so I could take a closer look at them in the lab.
Trying to key out the intertidal sculpins in California is an activity fraught with danger. There are about a dozen species that are likely, plus more that are occasionally encountered in the intertidal. When identifying fishes ichthyologists use meristics, or counts of things such as scales along the lateral line or hard spines in the dorsal fin, to differentiate species. Since you can't very easily count the number of spines in the dorsal fin while observing a fish thrashing around in a ziploc bag, I needed to get them under the dissecting scope.
Here is a picture that I took of the fish this morning. This is the same posture they had when I first saw them in the field. I think the male (paler fish on the right) is guarding the darker female. Oh, and while I'm at it, I should say that skin color is an unreliable characteristic to use when IDing sculpins. Their skin color can and does change very rapidly, depending on the surroundings and the fish's emotional state.
See those little tufts on the top of the head of the fish on the left? Those are called cirri. When I was keying out these guys I narrowed down the options to either bald sculpin or mosshead sculpin, and the distribution of the cephalic cirri was the final determining factor. Mosshead sculpins (Clinocottus globiceps) have cirri densely scattered over the entire head, while in balds (Clinocottus recalvus) the cirri extend forward only to just behind the eyes; in other words, bald sculpins have no cirri between the eyes or anywhere anterior to the eyes. In my fish the cirri clearly do not extend forward of the eyes, making these bald sculpins.
It usually takes animals a week or two to settle in after being collected from the field. After a couple of weeks the fish were eating regularly and hungrily. Sculpins don't have an air bladder, which helps keep them from getting washed out of their home pools as the tide moves in and out, and tend to sink if they aren't swimming. They can, however, swim very well. Once they got used to the idea of food coming at them from above they would start looking up when I removed the lid to their tank. When they're really hungry they will swim up and attack the food, ripping it from my forceps. Otherwise I dangle food in front of their faces and they take it a little more gently. Now they are both eating well.
One of the sculpins went off its feed last week and then surprised me by producing a mass of pink eggs. She had deposited the eggs on the underside of the cover instead of on the surfgrass I have in the tank. No wonder she hadn't been eating; with all those eggs inside her there would be no room for food! I decided to keep the eggs and see what, if anything, would happen with them.
Each of the eggs is about 1mm in diameter, and they are indeed pink. They are stuck together in a pretty firm mass. I peeled it off the cover of the tank and the whole mass remained intact. I can easily pick up the mass and put it into a bowl for viewing under the dissecting microscope. At first I could see that the eggs contained a large yolk and some smaller oil droplets but I couldn't tell whether or not they were alive. I cleaned them off to remove any dirt or scuzz, then returned them to the tank, hoping the parents wouldn't eat them. Over the first several days I couldn't see any change in the eggs except some of them became opaque and white, obviously dead. And it looked like maybe the stuff inside the eggs was shifting around a bit, but I wasn't sure if that was something good going on or the beginning of decomposition. The egg mass continued to stick together, though, which I took as a positive sign.
Then yesterday when I looked at the eggs I was able to convince myself that, yes, something is happening inside them. I saw tiny little fish bodies, complete with bulbous rudimentary heads, developing on the yolks!
Each egg is a pale pink sphere containing a darker pink yolk. At this early stage of development the yolk takes up most of the interior space of the egg. Lying across the yolk, with a swelling at one end, is the developing fish embryo. The swelling is the head. Even at this stage the three body axes (anterior-posterior, dorsal-ventral, and left-right) have been established for quite a while. The yolk will shrink as the energy stores within it are consumed by the developing embryo. I don't know if sculpins hatch as larvae (i.e., with a yolk sac still attached) or as juveniles (after the yolk sac has been completely consumed). I hope I get to watch these eggs and see!