Horseshoe Crabs, Spider Cousins or Something Else?

Horseshoe Crabs represent a long and proud lineage of chelicerate arthropods. Famed for their role as survivors, the order Xiphosura (fancy word for horseshoe crabs) can trace their fossil record back to the Ordovician Period some 445 million years ago. A once diverse lineage of animals, today xiphosura is represented solely by the Atlantic Horseshoe Crab Limulus polyphemus. Traditionally it has been hard to place the Horseshoe Crab into the evolutionary tree of life. Many studies have concluded that the Horseshoe Crab is a distant relative of arachnids. However many of these studies were going in with a biased assumption, arachnids are a monophyletic group of organisms. 

An Atlantic Horseshoe Crab

In evolutionary biology, phylogenies are created using various datasets to reconstruct the evolutionary relationships between different organisms. When a selected group of organisms can trace their heritage back to a common ancestor, we call this group monophyletic. A good example of a monophyletic  group is the Primate Order. All living primates share a common ancestor as consistently indicated by both fossil/morphological datasets as well as genetic datasets and are therefore a monophyletic group of organisms. Things can get messy with recovering the phylogenies of organisms with broad and unclear relationships that originated deep in time. Such is the case with chelicerates.

Simplified Taxonomy of Tetrapods 

Note: You are indeed a fish, no disrespect

You and I are both fish, in a taxonomic sense, as are all terrestrial vertebrates. We can trace our ancestry back to boney fishes in the Devonian and many features we take for granted, such as our ears and Adams-apple are highly derived gill ridges. Obviously though, when I tell you that I'm going to spend a weekend fishing, you the reader aren't confused by evolutionary semantics and envision a set group of animals. Perhaps if I say I'm fishing at a lake, you the reader would envision me reeling in a trout or pike. If I say I'm going to fish at sea perhaps you the reader envision me reeling in a tuna or a shark. You don't however, envision me catching an elephant nor a kangaroo when I say "I'm going fishing" despite both organisms being *fish* in the evolutionary sense as their ancestry is ultimately bound to the water. The same applies for reptiles, most people generally view reptiles and birds as separate categories of organisms. This division between birds and reptiles falls apart when one considers that birds are dinosaurs and dinosaurs are clearly reptiles. Paraphyly refers to when  a large group of animals are given a name/rank but selected subgroups are omitted due to differences in bodyplan or ecological habit. So whenever you call something a "fish" but are failing to include every other terrestrial vertebrate in your referral, you are being what taxonomists call "paraphyletic". 

Simplified Taxonomy of Birds

Note: Crocodilians are the closest living relatives of birds

Chelicerates should be defined before we go any further as things could get confusing quickly. Today the chelicerates include the following; mites, spiders, sea spiders, harvestmen, scorpions, pseudoscorpions, camel spiders, whip scorpions.... You get the point. The exact relationships between the sub-categories of the chelicerates is a bit messy, it was long assumed that chelicerates include a monophyletic "arachnid" sub-group with xiphosura being its sister out-group clade. Genetics has brought into question that assumption. Most morphological studies recovered a monophyletic arachnid group with a sister monophyletic Xiphosura clade. 

Genetic analytical work by Ballesteros et al. has shown however that many of the "arachnid" groups are are not related closely to one another and are instead convergent. More interestingly the study found Xiphosura to be nested smack-dab between Opiliones (harvestmen/daddy longlegs) and Pseudoscorpions both of which are traditionally considered arachnids. This result is interesting as it implies that the traditional definition of "arachnids" is a paraphyly. To recover arachnids as a true monophyly, the definition would need to be expanded to include  Xiphosura, meaning that Horseshoe Crabs are arachnids. 

Phylogenetic tree from Ballesteros et al. 

Melanin in Manta Rays

There are only two species of fish with dark melanin spots on their skin. Both of these species are manta rays. Some manta rays exhibit dark spots on their otherwise completely white underbellies. These dark blotches are caught the eye of researches for that reason. They figured that there must be a reason that melanin is so rare in the ocean and there must be a reason that rays express it.
 
Manta rays have very few known predators. One theory that offers an explanation as to why is that their white underbellies makes them difficult to see against the sky. In that case, the dark spots should be selected against. After following a population of melanistic rays, the researches determined that the dark spots do not affect fitness.

It is also possible that the gene causing this mutation is closely linked with another gene which is improving fitness. Perhaps there is an advantage within the manta rays that we cannot see which has the side effect of these dark splotches. However, as mentioned earlier, initial research shows that these spots do not appear to affect fitness.

Their new hypothesis is that the mutation may be a product of genetic drift. Not all populations of rays have this trait. Some have up to 40% while others have nearly none. It is believed that this neutral trait appeared as a mutation in a population and increased in frequency through random chance. The members of this population mated with other populations, spreading the gene further. If that is the case, this gene can be used to traced back through several generations and better understand migration habits of manta rays. Rays are currently a vulnerable species. Being able to predict where they will go may help the conservation effort. I hope that they find success in the conservation endeavor because mantra rays are an awesome animal that I want to see more of in the future.

Links:
https://www.nytimes.com/2019/10/14/science/manta-rays-black.html?searchResultPosition=14
https://marinemegafaunafoundation.org/blog/scientists-explore-the-occurrence-of-black-manta-rays-in-the-indo-pacific/

Gene for Coral Bleaching Discovered

Scientists from the University of Texas have identified a gene in coral that is affected when coral becomes heat stressed. Stressed coral release the algae they depend on for energy in a process more commonly known as bleaching. Bleaching results from unusual water temperatures, too warm or too cool, and causes coral to turn white in color. However, coral do not always die from bleaching and the process can be reversed if early enough in the process.

Photo from What is Coral Bleaching? NOAA

"We understood that there is a target gene essentially used as a biomarker for diseased and distressed corals, and that gene is induced by the response pathway that my lab studies," said Pellegrino, assistant professor of biology. Although Pellegrino never studied coral, he specializes in mitochondria and cell biology. 

The team at the University of Texas plan to do testing in a model organism in order to determine if the gene they discovered can protect coral from heat stress and infection. Coral reefs support 25% of all marine species on the plant. They provide habitats and shelter for marine life, aid in nutrient recycling, and protect coastlines from wave action and tropical storms. Laura Mydlarz from the Department of Biology says, “Understanding the existence of this gene means that there is now the potential to identify if corals are experiencing stress before they appear dead or bleached.” Researchers hope to be able to predict coral survival and identify if they are experiencing stress before the bleaching process occurs.

Honestly, I am on the fence about this gene discovery. It’s great because if scientists can prevent coral bleaching and extend the life of coral, I’m all for it. However, I feel like corals bleach because they cannot survive or function properly in the environment and expelling the algae living in their tissues is how they react to the stress. I think further studies need to be done to determine (a) if corals release the algae as a form of survival or response to not being able to thrive in said conditions, (b) if altering the gene will allow coral to continue living as it does in ideal conditions, and (c) how stressful situations affect the functions of coral.

Reason for Flounders being flat found in genomic study

A recent study done by the Julius Maximilian University Wurzburg in Germany has found reason why the flounder is flat in morphology. This has been a topic f discussion for many years and has been a confusing part of flat fishes life cycle. Scientists have had trouble understanding why flounders among other flatfish species would start as a symmetrical larvae, and then reconfigure its whole anatomy in adulthood to be asymmetrical.

The study looked at the genome of two related species to try and uncover the genetic reasoning behind this. The  genomes of related species, the Japanese flounder (Paralichthys olivaceous) and the tongue sole (cynoglossus semilaevas) were sequenced. The researches turned their focus to the genes that were active specifically during metamorphosis when this great change in anatomy takes place. They found a trigger which was a developmental key, Retinoic Acid is the culprit for skin pigment change as well as interacts with the thyroid hormone which is responsible for the transition of the eyes to one side of the head. Through this they discovered that light also has a lot to do with this change as the very same pigments that capture light in the eye are also found in the skin of larval flounder they can sense light differences and increase retinoids acid production based on this.

This is important as flounder among other species of fish are sought after for food as well as economic gain by fisherman and sellers. Flounders sell for a high price on the open market and with overfishing always being a concern as well as just to meet high demand, many people have attempted and are still attempting to raise and farm these fish. Theres been a problem though they have had trouble getting right, as I'm sure you could guess, its metamorphosis! Solving this mystery helps flounder populations as well as the fishing industry as a whole!

Birds Can Be Drawn to the Scent of Plastic

Seabirds typically feed on krill- small crustaceans at the surface of the ocean- but over the last half century or so, they have also been feeding on small bits of plastic (microplastics) that comes from trash that has been broken from exposure to UV radiation and waves. Theories have been made as to why marine animals eat plastic. Some theorized that they mistake it for food because of its small appearance, but two scientists from the University of California, Davis released a study that said otherwise. 

Many people think the birds are stupid and can't differentiate between plastic and their normal food, but Matthew S. Savoca, the paper's lead author, says that these people don't take into consideration that these birds have been trained over hundreds of thousands of years to find tiny pieces of food in the ocean. Gabrielle A. Nevitt, a professor and author of the paper, has been studying seabirds for years. She has found that these birds have a very strong sense of smell and respond to the chemical dimethyl sulfide to find their prey. This chemical is released by phytoplankton as it gets eaten by a predator or breaks down in the ocean or on shore, which signals to the birds to come eat the predator, typically krill or a relative of krill. The two scientists found that this chemical is also released when microplastics are present in the ocean. 

In their study, the scientists used plastic beads that were four to six millimeters in diameter and made of the same plastic used in bottles, bags, textiles, and other objects. After the beads had been in the water for three weeks, dimethyl sulfide was found in the water and air. The birds are using their evolutionary traits and abilities to forage in ways that is harmful to them and is causing obstruction and toxicity. The study claims that by 2050, 99% of all seabird species will have eaten plastic debris. Dr. Nevitt says that the study may have implications for marine animals besides birds, such as baleen whales who eat similar species to the birds, or sea turtles who are also attracted to dimethyl sulfide.  Mr. Savoca said that fifty years ago there was no plastic in the ocean, and now there are hundreds of millions of tons. This number is staggering, and it is scary to know that it will only continue to grow. Humans are changing the world extremely quickly and it is harming the other species that also occupy our planet. The problem with plastic in our ocean is one that may not be able to be solved and eliminated, but it can be slowed down if people are made more aware of what our waste is doing to animals. We unknowingly contribute to the pollution of our oceans every day, even by using products that contain microplastics, so education is the best possible solution to this problem.