My bald sculpins have begun hatching! Their egg mass has been disintegrating over the past few days and I couldn't tell if that was because they were dying or hatching. Yesterday I was able to spend some time looking at them and was surprised to see that a few little pink blobs had wiggled their way out of the egg mass while I was manipulating it. Baby fishies! Well, they're still mostly yolk, but each yolk has a baby fish attached to it. They flit around quite a lot and are difficult to photograph. I had to put this trio in a depression slide, the macro photographer's trick of making the universe smaller so the creature can't swim too far away.
This little fish was cooperating with me, so I carefully placed a coverslip on its drop of water and took some video. The first part was shot through the dissecting microscope with epi-illumination from a fiber-optic light, which shows the surface details. The second clip was taken through the compound microscope with trans-illumination; this kind of lighting doesn't show any of the three-dimensional structure of objects but does a wonderful job with transparent objects like larval fish.
I like that the baby fish have spots on their yolk sacs as well as the top of the head. And from the second half of the video it appears that they don't yet have a gut, at least not one that I can see. For the time being they don't need a gut, as they're surviving off the energy stored in the yolk sac, but once the yolk has been absorbed they will have to start feeding. At that point they'll need to have complete guts. I imagine they will be hungry, and hope I have something they'll be able to eat.
How big are these baby fish, you ask? The smallest ones were about 2 mm long, and the biggest one was twice the size, with a correspondingly smaller yolk.
And yesterday I caught some time-lapse video of a baby hatching from its egg. Why have I never played with the time-lapse function on my phone before? It's really cool.
For now I'm keeping the babies in a mesh container, separated from their father so he cannot eat them. I don't think I'll end up with more than a couple dozen hatched larvae, as the egg mass has begun to decompose and many of the embryos have died inside their eggs. And no doubt some of the larvae that I've rescued already will die. I figure I have a few days before I need to worry about feeding the survivors. After that, who knows? Your guess is as good as mine.
It has been a few weeks since I posted about my most recent batches of urchin larvae. Some strange things have been happening, and I'm not yet sure what to make of them. It would be great if animals cooperated and did what I expect; somehow that never seems to be the case. The upshot of all this uncertainty is that there is always something new to learn. I, for one, am not going to complain about that.
One noteworthy thing to report is that my hybrids all died, very quickly and unexpectedly. They had been racing through development and on the dreaded Day 24 they looked great.
And the next time I changed their water, they were all dead. So much for the hybrid vigor I had written about earlier. Teach me to get cocky and think I know what's going on.
Fast forward to Day 52, and some of the cultures are still going strong. I originally set up four matings, and at least some individuals from each are alive. One thing that seems to happen when I start multiple batches of larvae at the same time is that the batch with the fewest numbers does the best. This time my F3xM1 mating was always the least dense culture, but some of them have already begun and completed metamorphosis. And the ones that are metamorphosing are the ones being fed what I expected to be the less desirable food source. As I said, not much of this whole experience is making sense.
The good thing is that I have an opportunity to observe these larveniles in action. As long as they don't get arrested in this neither-here-nor-there stage, they should soon join their siblings as permanent inhabitants of the benthos.
This video contains short clips of three different larveniles. I've arranged the clips from earlier to later stages of metamorphosis. Although these are three separate individuals, you can imagine that each one goes all of these stages.
Having both tube feet (for crawling around the benthos) and ciliated bands (for swimming in the plankton) make these animals unsuited for either habitat. They have gotten very heavy and sink to the bottom, but it doesn't take much water movement to knock them off their five little tube feet. It always amazes me that teensy critters like this, so fragile and easily killed, manage somehow to stick in the intertidal and survive long enough to be grown-up urchins on their own. And yet some of them will. I've seen it happen.
Almost a week ago, my sculpin eggs were doing great. The embryos had eyes and beating hearts and were actively squirming around inside their eggs. A few of them had died but overall they seemed to be developing well. I had high hopes that they would continue to do so, and began to think of what I'd need to do once they hatched.
Today the egg mass is 19 days old, and things aren't going so well.
Many of the embryos on the outer edges have died, and all that remains of them are the tattered remnants of their eggs. Those opaque white eggs have been dead for a while and the pale pink shredded eggs died more recently, in the last day or so. I took a quick peek at the egg mass yesterday, and it looked much healthier than it does today. I'd guess that all told about 30% of the embryos have died since development began.
The embryos that are still alive seem to be fine. Their eyes can now move around independently but I still don't know what, if anything, they can see. Their bodies continue to grow and now they have spots on their tails as well. I can make out where the heart is because I can see it beating, but I can't discern any of the other internal organs. If the lighting is just right I think I can see pectoral fins on some of the embryos, which are too faint and indistinct to photograph. The baby fish are still swimming around inside their eggs, too.
Question of the Day: What caused the eggs' condition to deteriorate so rapidly?
Well, I can think of a couple of explanations.
Explanation #1: Not everybody survives long enough to hatch. Sculpins and other fishes that lay large numbers of eggs are generally described as having a Type III survivorship curve (see right). These organisms have lots of babies, few of which survive to adulthood; probability of death is highest in the youngest age classes. Individuals that do make it to adulthood experience much lower mortality and have a decent chance of surviving into old age. In an egg mass like this, each egg has a very small probability of eventual survival to adulthood. To paraphrase an old saying, if they all survived then the world would be covered in bald sculpins. Obviously that's not the case--and that's a good thing!--so most of these eggs are not going to make it in the long run even in the best of circumstances.
Explanation #2: Crappy water quality. A very strong storm blasted through the area yesterday, complete with wind gusts to about 50 m.p.h. and 1-2 inches of rain, depending on location. All of this rain generates a lot of surface runoff, which carries mud and debris (think bushes and trees as well as garbage) into Monterey Bay. Plus, the high winds and turbulent swell stir up the bottom in shallow areas, resulting in brown, turbid water. This is the water that we use in the lab, and it's our only source of seawater. Today the water was visibly cloudy. At least it seems to be just sediment, though, and not another phytoplankton bloom.
Poor water quality could affect the sculpin eggs if the sediment settles out on the surface of the egg mass, impeding gas exchange between the eggs and the surrounding water. In the field these eggs would be subjected to strong turbulence from the bashing waves, which would keep them clean and the water highly oxygenated. Some species of fish guard their egg masses and blow water on them to clear them of both sediment and fouling organisms. I hadn't seen the parents of my sculpin eggs caring for their offspring at all, but I have been rinsing off the egg mass every day. Maybe I haven't been able to keep the eggs clean enough. It does seem to be the eggs on the outside of the mass that are dying, so cruddiness might very well be part of the problem.
I'll look at them again tomorrow and see if anything has changed. The news could be either good or bad, and I honestly don't know what to expect.
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!