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A few days make all the difference

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.

Bald sculpin (Clinocottus recalvus) egg mass, age 19 days.
18 February 2017
© Allison J. Gong

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.

Bald sculpin (Clinocottus recalvus) egg mass, age 19 days.
18 February 2017
© Allison J. Gong

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.

Survivorship curves
Source: Wikimedia Commons, 2017

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.

What do you think?

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