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There’s gold in the forest

Combine the words "gold" and "California" and you automatically come up with the Gold Rush, don't you? After all, California is the Golden State. And while that nickname may be to honor the golden hills of summer or the poppies that are the state flower, it may also be a tribute to the discovery of gold in 1848. For better or worse, the Gold Rush initiated rapid development of this area, and California eventually became the 31st state in 1850.

For me, and I suspect for many people, gold is one of the quintessential colors of autumn. Yet here we are in the middle of winter heading towards spring, and I saw a lot of gold in the forest the other day. I had taken my Ecology students to Rancho del Oso for the first field trip of the semester and set them loose to saunter through the woods and practice noticing (and recording) patterns in nature. Incidentally, I have adopted the word 'saunter' as a replacement for 'hike' for most of my own outdoor adventures. I have always been a slow hiker, and felt that in order to keep up with other people I had to miss seeing what was going on around me. Not to mention the fact that I'm always stopping to take pictures or examine some weird thing on the ground, or in the trees, or wherever. By giving myself permission to saunter along at the pace at which nature occurs, I have time to slow down and observe more carefully, and come away with a much better understanding of the world I've passed through. It certainly doesn't work for everybody, but I've learned that the journey is as important as the final destination, and that has made hiking sauntering much more enjoyable for me.

So, back to the gold. One of the very first thing I noticed when we hit the trail was this brilliant yellow-orange slime mold growing on twigs on the forest floor. This area is a mixed forest of hardwoods (mostly oaks) and various pines. I can't be certain what these sticks hosting the slime mold are, but they may be some kind of pine.

Slime mold
Slime mold, possibly Leocarpus fragilis, at Rancho del Oso
2019-02-01
© Allison J. Gong

Slime molds are very strange organisms that don't fit into any of the major eukaryotic kingdoms of life (Animalia, Plantae, or Fungi). The current taxonomic position of slime molds is up for debate and far from settled, so I won't go into it here. Like fungi, slime molds feed on dead and decaying plant matter and are part of the decomposer niche of organisms. Also like fungi, most of a slime mold's life is microscopic. In the case of fungi most of the body, called a mycelium, is a network of extremely thin threads called hyphae. The mycelium for most fungi is underground and thus invisible to the casual observer. What we call a mushroom is only the reproductive fruiting body, which pushes to the surface so that spores can be released into the air.

For most of the time, or at least as long as food is plentiful, a slime mold exists as single amoeba-like or flagellated cells that feed on bacteria. These cells are haploid, containing only one set of chromosomes. Sexual reproduction (labelled SYNGAMY in the figure below) occurs when an amoeba-like cell encounters a compatible flagellated cell. I would also be willing to bet that the amoeboid and flagellated cells are triggered to find each other and initiate syngamy when food is scarce, as is the case with many animals.

Life cycle of a slime mold
© Pearson Education, Inc.

The result of syngamy in a slime mold is a zygote which develops into a macroscopic stage called the plasmodium. The plasmodium undergoes nuclear division multiple times but cytokinesis doesn't occur, resulting in a large cell bounded by a single plasma membrane and containing many nuclei. In animal tissues we describe this condition as syncytial; I don't know if the same word is used by slime mold specialists, but the concept applies.

One of the things that makes slime molds truly bizarre is their method of locomotion. Using time-lapse videography, you can actually see how the contents of the cell swash back and forth in a process called cytoplasmic streaming. The net result of all this cytoplasmic streaming is the physical movement of the plasmodium into new territory. It's a process much easier to understand if you can see it, so here's a video from KQED's Deep Look series:

As with many fungi, slime molds are difficult to identify if you don't see the fruiting body. The slime mold that we encountered the other day was an immature plasmodium that hadn't yet produced fruiting bodies. The experts who took a look at my observation on iNaturalist agreed that it is likely Leocarpus fragilis, based on location and time of year, but they cannot be certain.

Continuing with our theme of gold, we saw several small blotches of golden jelly growing on tree trunks. These were the Tremella fungi. There are two species of golden Tremella in our region, T. mesenterica and T. aurantia. It seems that differentiation between the species depends on examination of microscopic structures, so I am unable to tell which species this little blob is. However, I will point out that the species epithet aurantia means 'gold', so I really hope that's the name for this blob.

One of the golden jelly fungi (Tremella sp.) at Rancho del Oso
2019-02-01
© Allison J. Gong

Saving the best for last! Moving away from the creek and into the more enclosed forest we entered the realm of everybody's favorite terrestrial pulmonate gastropod, the banana slug. They were out in full force, chowing down on mushrooms and sliming up the foliage. One of my students picked up a banana slug and let it crawl on her hand for a while, but to my knowledge nobody licked one. All of the banana slugs that I saw were bright yellow with no brown or gray blotches, so I conclude that they were either Ariolimax californicus (the so-called Peninsula banana slug) or A. dolichophallus (the Santa Cruz banana slug, also the school mascot for UC Santa Cruz).

Banana slug (Ariolimax sp.) at Rancho del Oso
2019-02-01
© Allison J. Gong

But this is where things get interesting. According to their mitochondrial DNA these two species, A. californicus and A. dolichophallus, do not have overlapping ranges. And the dividing line between them is Rancho del Oso, with A. californicus occurring to the north and A. dolichophallus occurring to the south. So, if Rancho del Oso is the magic line defining the ranges of these two species, what species are the slugs at Rancho del Oso? I think that answering this question will require a much finer scale study. For now, I'm just going to call them Ariolimax sp., because that seems to be the safest option until things get sorted out.

I've written about banana slugs before, but I've never had a chance to photograph them doing the actual nasty. Luckily for me and the students, banana slugs have no shame. I think the entire class got to get a close look and photos of this copulating pair:

Copulating banana slugs (Ariolimax sp.) at Rancho del Oso
2019-02-01
© Allison J. Gong

This perfect yin-yang symbol is the result of how banana slugs align themselves during copulation. Each hermaphroditic slug has a genital open behind the head on the right side of the body. There's a lot of kinky stuff that happens during banana slug sex, including the chewing off of one partner's penis, but suffice to say that one animal's penis is inserted into the vagina of the other and, well, we don't know how quickly sperm is transferred, but the animals remain locked together for several hours. Yes, HOURS. Ahem. The penis chewing thing doesn't happen every time slugs mate, and biologists are still trying to figure out the function for this unusual behavior.

We have another several weeks (hopefully!) of rainy weather, so there will be lots of time to explore the world of fungi, slime molds, and banana slugs. The combination of rain and lengthening days creates great conditions to revel in the gold of a California winter in the forest.

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