Dance of the Sand Worms

Note: This program first aired July 4, 2015.

A few weeks ago, I got a call from my nearly 4 year old nephew during dinner. I could hear the excitement in his voice as he shouted into the phone about seeing worms in the water at the town dock. I asked him what color they were and he said they were all colors. This brought back a memory from my own childhood, a memory until this moment I wasn’t even sure was real. I remember seeing huge iridescent blue green worms swimming at the surface of the water, and then seeing them washed up dead on the beach the next day. I saw them that one time, and never again, causing me to think I had either imagined it or witnessed something very rare. Hearing my nephew’s excitement a couple fo things became clear, what I saw was indeed real, and that now my family has an official childhood rite of passage: witnessing the spawning swim of the sand worms.

Sand worm, rag worm, clam worm, these common names all refer to various species of polychaete worms found in coastal areas throughout the north Atlantic. Polychaetes are marine segmented worms, meaning their bodies are divided up into hundreds of discrete segments. Believe it or not this body structure represented a big leap forward in terms of evolutionary complexity. Polychaete worms are an incredibly diverse and successful group of organisms, there are over 100 species listed for the Gulf of Maine alone.

The worms I saw as a child, and my nephew saw this spring are Alitta virens, sometimes known as the king rag worm, or sand worm. They live in burrows in low intertidal or sub tidal sand or mud. The burrows are U shaped, with each end of the U an opening  at the surface of the sand. The worms spend most of their lives in these burrows, pumping oxygenated water through them so they can breathe (and a neat side effect is the diffusion of oxygen into otherwise anoxic sediment. The oxygen burrowing worms draw down into the sediment fuels an entire microscopic ecosystem). They stick their fearsome heads and sharp black teeth out of the burrow to feed on various small benthic sea creatures, and are said to sometimes scavenge omnivorously. Rag worms are among the most long lived of polychaetes, some are thought to reach the ripe old age of as much as 7 years. In the end though, regardless of how old they get, their fate is the same. This group of polychaetes practices what is called “semelparity”, meaning they reproduce once, and then they die.

As I said, these worms spend their lives in their burrows, only occasionally leaving to crawl along the bottom to find better digs, otherwise they are in the sediment. Things change however, when it is time to mate. The males undergo epitoky, their bodies change shape and structure in preparation for mating. They turn shimmery blue green and learn to swim, and around the new moon, they swim to the surface at high tide and release their sperm into the water column. That sperm makes its way down to the females who have remained in their burrows. They fertilize the eggs they have spent nearly a year developing in their bodies. Once the eggs are fertilized, the female excretes them on the surface of the sediment outside her burrow, where they develop, hatch and start a new generation of sand worms. After shedding sperm, the males die. After laying the eggs, the females die. Such is the life of a sand worm.

The moon is involved because the worms, both male and female are highly sensitive to photoperiod, and the development of the eggs begins a year before mating in response to changes in day length. Something about the combination of warming water in the spring, lengthening days, and darkness at night tells the males that the females are ready, but as far as I can tell, we can’t know for sure when the magic will happen.  That is why it is such a good idea to be outside as much as you are able. You never know when you will stumble upon a certain flower blooming, a case of tiny spiders hatching, baby birds fledging, a lynx track in the snow, or hundreds of two foot long blue green worms swirling in the water doing their once in a life time dance. 

References:

Alitta virens on EOL http://eol.org/pages/458737/overview

Everything you want to know about clam and blood worms:

http://www.nwrc.usgs.gov/wdb/pub/species_profiles/82_11-080.pdf

All about Annelids, from the Tree of Life: http://tolweb.org/Annelida

Actually, even more that you might want to know about polychaets, from and OLD Woods Hole guide: http://hermes.mbl.edu/publications/biobull/keys/pdf/9.pdf

 On lunar impact of breeding (but not of worms specifically) http://siderealtimes.blogspot.com/2011/12/effects-of-lunar-cycle-on-marine-animal.html

Of Mice and Oogonial Stem Cells

Note: This program first aired on Deecmber 29, 2012.

I was asked by a listener recently, if female chickens are born with all of the eggs they will lay over the course of their lives, like human females are. This made me wonder if all females have this characteristic, if that is a distinguishing feature of being female—that the cells that become the gametes form in the embryo, and are not replenished as the individual matures. Conventional wisdom states that we females have a set amount of eggs (more than likely more than we will release in the reproductively active period of our lives), we do not make more as we go along. This is in contrast to males, who are able to continually replenish their supply of gametes.

We need to pause for a moment and return to high school biology, and revisit the concepts of meiosis and mitosis. Our cells have two sets of DNA in them, one from our mother and one from our father. That DNA is in the form of chromosomes. Mitosis is simple cell division, we get two cells when we started with one. The chromosomes in the cell all line up, split in half, and each half then replicates itself in the new cell; each new cell ends up with two sets of chromosomes, just like the parent cell had. Meiosis is cell division that results in the formation of gametes. What makes gametes special is that they have only one set of DNA in them, a mash up of DNA from the mother and the father. In meiosis, the chromosome pairs line up, swap DNA and then get separated from each other into two separate cells. Then the chromosomes themselves split in half, and the halves end up in two more cells, yielding a total of four cells from the original one, each with a single set of genetic information. That is what makes gametes gametes, they have only one set of DNA and they need to join with another gamete cell to get a full set of genetic information, enough to make a new individual. The traditional thinking has been that , at least in mammals, meiosis occurs in the ovaries of the embryo. As a female’s reproductive organs are forming, meiosis is going on, and all the eggs the female is going to have are formed then.

Before we get to the chicken issue, I have to break it to you that the conventional wisdom here has been shown to be wrong. As early as 2004, researchers discovered that in mice, the females seemed to be able to maintain a supply of healthy living eggs, even when fed toxins that would kill their follicles. This meant that the mice could generate new eggs, a discovery that flies in the face of the conventional understanding that mammals are born with the only eggs they will ever have. Since then researchers have identified the stem cells in human ovaries, called oogonial stem cells, that do the same thing for us. These oogonial stem cells are able to undergo meiosis at any point and produce new oocytes or eggs. What isn’t clear is if these oogonial stem cells are just a back up system, or if they share responsibility for egg production with the original meiotic cells in the ovary.


So, on to chickens. It turns out this is a very difficult question to answer definitively. Most of the literature refers to the “all the eggs you’ll ever make” phenomena as occurring in either mammals, or “higher animals ” (which by the way are apparently all vertebrates except fish). There is, thus far, no evidence of oogonial stem cells in chickens, but that doesn’t mean they aren’t there, it just means no one has looked, which is a different thing altogether. Likewise, this conventional wisdom principle has likely been laid like a blanket on top of our knowledge of all “higher animals”, meaning, it is unlikely that scientists have studied the embryos of every species close enough to declare that all higher animal females are always born with all their eggs. To be fair, closely related things usually behave in closely related ways, so this blanket technique may not be as imprecise as I am making it sound. We do know that certain worms continually make new oocytes, so it is not unheard of in that natural world, but chickens are closer to the higher animals than to round worms.

The final word? I suspect that chickens, like humans, make most of their eggs during the development of the embryo, and may, like humans, potentially retain the ability to generate new eggs as needed. This question illustrates nicely, the limits of our knowledge, and way we construct what we know, or think we know about the natural world. Thanks Greg, and keep your questions coming.


References:

From Science Daily, a digest of the original study from Massachusetts General Hospital and the University of Edinburgh, demonstrating that female mice can grow new eggs: http://www.sciencedaily.com/releases/2012/07/120726180259.htm

National Geographic’s take on the same study: http://news.nationalgeographic.com/news/2012/02/120229-women-health-ovaries-eggs-reproduction-science/

Need a review of mitosis and meiosis? Check out this nice slide show from the good folks at PBS: http://www.pbs.org/wgbh/nova/miracle/divide.html

A bit here on the traditional take on female oocyte development: http://anthro.palomar.edu/biobasis/bio_2.htm

Whoa. Here’s a scientific article about what can regulate meiosis in chickens!! http://www.biomedcentral.com/1471-213X/8/85

The truth? Its complicated… http://rstb.royalsocietypublishing.org/content/277/955/201.short