Sharks Without Fathers, by Rasha Aridi

It seems like everyday we learn about a species that breaks the rules of biology—warm-blooded sunfish, egg-laying mammals, and air-breathing fish, just to name a few. In this case, sharks have been discovered to reproduce asexually. New research has demonstrated that female sharks can produce viable offspring without the contribution of a male, through a process called parthenogenesis. This discovery has prompted new research as to how females are physiologically able to reproduce this way, the consequences and benefits of parthenogenesis, and how it can be used to promote shark conservation.

Up until 2002, female sharks were not known to produce offspring without males. A “virgin birth” of a bonnethead shark occurred at the Henry Doorly Zoo, prompting scientists at the Belle Isle Aquarium to monitor their white-spotted bamboo shark, Chiloscyllium plagiosum, who had been producing oviposited egg cases over a period of six years. C. plagiusum deposited seven eggs that were saved and incubated; four of them developed into embryos (Feldheim, 2010).  This observation set off a series of research projects and case studies on the virgin births of sharks. Now, parthenogenesis has been described in four captive shark species: the bonnethead, blacktip, zebra, and white-spotted bamboo sharks (Feldheim, 2016). 

Parthenogenic zebra shark pup hatched and reared by the Burj Al Arab Aquarium Team. Source. 

The term “parthenogenesis” describes how females can reproduce from an ovum without fertilization. Usually, especially in vertebrates, one diploid gametocyte duplicates its chromosomes and splits them, forming four gametes that would later form a zygote if fertilization occurs. However, parthenogenesis can produce offspring without mating. There are two types: apomictic and automictic. In the first, common in plants, gametocytes undergo mitosis instead of meiosis; offspring are clones of the mother. In automictic parthenogenesis, which is more common in vertebrates, one gamete becomes the ovum and receives cytoplasm and nutrients. The remaining gametes become polar bodies. Typically, the polar bodies dissolve but, in this case, they fuse with the ovum. This sets off embryonic development. Each polar body is genetically different from the other and the ovum because of chromosomal crossing over between the mother’s paternal and maternal chromatids. Therefore, the offspring of parthenogenesis are not clones of the mother (Holtcamp, 2009). As little as half of genetic material is passed from the mother, which can increase homozygosity but also decrease genetic diversity (Portnoy, 2014). This can have positive and negative effects on the offspring.

After the virgin birth of a bonnethead in 2002, it was hypothesized that the female had somehow managed to mate or store sperm. This was disproved using DNA testing and the fact that the females had been isolated from males for the entirety of their lifetimes. Mahmood Shivji and Demian Chapman used amplified fragment length polymorphism (AFLP) fingerprinting to study the pup’s genome, looking for paternal genes. They found that everything in the DNA profile was from the mother (Holtcamp, 2009). Scientists theorized that because the females were not exposed to males, their bodies managed to produce offspring in an alternative way because only isolated females underwent parthenogenesis to produce pups. This demonstrates that elasmobranchs are able to adapt, and completely alter, their reproductive strategy to suit their environmental circumstances (Dudgeon, 2017).

How parthenogenesis occurs in sharks compared to normal fertilization. H. Fletcher.  Source. 

Parthenogenesis can have detrimental consequences on sharks, on a genetic level and a population level. First of all, parthenogenesis is comparable to inbreeding because it reduces genetic diversity. The offspring are at least half genetically similar to their mothers and there is high homozygosity. In fact, elevated levels of homozygosity are the first indication of parthenogenesis. This puts individuals at risk of physiological defects, making them less suited for their environments (Dudgeon, 2017). Additionally, parthenogenesis typically produces much fewer number, usually just 1-3, of offspring instead of the fifteen pups per litter that mothers are able to produce. (Holtcamp, 2009). This can be devastating to populations, especially when they are at risk of harvesting. Inversely, there are positive effects of parthenogenesis on the mother and offspring. The ability for a female to reproduce by herself is helpful in times when males are unavailable or the female is isolated. (Holtcamp, 2009). Unfortunately, this has become a real problem in wild populations because of human interference and the effects of harvesting on populations. Genetically, asexual reproduction can accelerate the loss of deleterious recessive alleles in inbred populations, but this can be dangerous in wild populations because it reduces genetic diversity (Portnoy, 2014).

The surprising observation for virgin births in chondrichthyes has opened new doors for research and conservation. In this case, viable offspring were produced and studies were done to explain how this occurred and its significance in the sharks’ life history. Both positive and negative consequences were observed, but there was a decrease in genetic variation and an increase in homozygosity. The most significant outcome of this research is how sharks’ reproductive strategies are reflective of their environmental circumstances, which pertains to conservation. Wild sharks are now being studied and observed for signs of parthenogenesis because scientists think that this biological process is more common in nature g than once believed (Holtcamp, 2009). Although it would be fascinating to observe parthenogenesis in wild populations, it is an indicator that the shark population is in trouble. 100 million sharks are killed annually, putting a strain on populations. Furthermore, males and females typically do not interact in the wild, except for mating. Coupled with the fact that they are isolated and shark fisheries typically harvest at a specific location, it is possible for an entire sex to be wiped out in a region (Holtcamp, 2009). When females are ready to mate, they will not have a sufficient group to choose from. If wild females are reproducing via parthenogenesis, it indicates that males are not present in the population or are not suitable. Scientists are now examining if parthenogenesis is an “evolutionary significant alternative to sexual reproduction” (Straube, 2016), instead of a last-resort reproductive means. In conclusion, parthenogenesis provides insight into the environmental circumstances and the sexual history of female sharks. This data can be used to better understand what happens at a genetic and population level, which can improve how sharks are conserved and what scientists can do to better protect them and their ecosystems.

References

Dudgeon, C. L., L. Coulton, R. Bone, J. R. Ovenden, and S. Thomas. 2015. Switch from sexual to parthenogenetic reproduction in a zebra shark. Scientific Reports 7  40537 doi:10.1038/srep40537

Feldheim, K. A., A. Clews, A. Henningsen, L. Todorov, C. Mcdermott, M. Meyers, J. Bradley, A. Pulver, E. Anderson, and A. Marshall. 2016. Multiple births by a captive swellshark Cephaloscyllium ventriosum via facultative parthenogenesis. Journal of Fish Biology 90(3):1047–1053.

Feldheim, K. A., D. D. Chapman, D. Sweet, S. Fitzpatrick, P. Prodhol, M. Shivji, B. Snowden. 2010. Shark virgin birth produces multiple, viable offspring. J Hered: 374-377.

Holtcamp W. 2009. Lone parents: parthenogenesis in sharks.  BioScience 59(7): 546-550. DOI: 10.1525/bio.2009.59.7.3

Portnoy, D. S., C. M. Hollenbeck, J. S. Johnston, H. M. Casman, and J. R. Gold. 2014. Parthenogenesis in a whitetip reef sharkTriaenodon obesusinvolves a reduction in ploidy. Journal of Fish Biology 85(2):502–508.

Straube, N., K. P. Lampert, M. F. Geiger, J. D. Weiß, and J. X. Kirchhauser. 2016. First record of second-generation facultative parthenogenesis in a vertebrate species, the whitespotted bambooshark Chiloscyllium plagiosum. Journal of Fish Biology 88(2):668–675.

Rise of the Mola mola, by Coly Cancino

The Mola Mola, more commonly called Ocean Sunfish, has been a fairly quiet fish when it comes to news. It was recently thrust into the spotlight when a person on a forum called Reddit Rants posted about how worthless it is due to its many deformities. The three deformities called out were that it doesn’t have a caudal fin, it lacks a swim bladder, and its nutrient intake. He went on to say “EVERY POUND OF THAT IS A WASTED POUND AND EVERY FOOT OF IT IS WASTED SPACE.” The redditor could not be more wrong; the Mola Mola is one of the more fascinating fish due to its abnormalities, and it does not let them hold it back.

Common Mola.  Mola mola.  By Mike Johnson, Earthwindow

            The Ocean Sunfish may be considered “half a fish” to some people, due to its obscurities, but it does just fine. The mola are the heaviest of all the bony fish, with large specimens reaching 14 feet vertically and 10 feet horizontally, and weighing nearly 5,000 pounds (Thys 2015). They generally grow and mature at a fast rate due to their short lifespan of 10 years. They are commonly found in temperate and tropical oceans around the world, but that seems to be changing. Mola Mola can seen basking in the sun near the surface and are often mistaken for sharks when their huge dorsal fins emerge from the water. Their teeth are fused into a beak-like structure, and they are unable to fully close their relatively small mouths. Though the fish has many abnormalities, it’s important to consider the organism as whole: more than the sum of its parts.

One of the deformities the redditor was ranting about was the fact that the Ocean Sunfish does not have a caudal fin, which most fish use to swim and navigate through the seas. Instead of the caudal fin, the Ocean Sunfish has a clavus, which is essentially where the end of the fish folds in on itself forming a rounded rudder type of appendage. The Ocean Sunfish overcomes its lack of caudal fin by swimming with its anal and dorsal fins moving at the same time laterally. This way, it generates a lift type thrust that moves it forward and up at the same time. This type of movement has not been seen in many other fish. It may not move fast at .04-.07 m/s, but the Ocean Sunfish has recently been found to have expanded its migratory pattern (Pope 2010). The Mola mola was recorded sporadically in artic waters throughout the 20th century, but never more than one fish in a given year. Since 2000, however, there has been a considerable increase in both the annual frequency and the number of fish observed in Icelandic waters far off from its usual tropic habitat (Palsson 2017). They have been tracked as early as October in these waters and have been recorded staying in the cooler waters for approximately a month until continuing their migration back toward tropic water. The recent expanded range of the sunfish debunk the myth that the sunfish is not a swimmer and cannot migrate.

About the Mola mola.  Source. 

             The redditor also called out that the Mola Mola does not have a swim bladder, which most fish do have. Instead of a swim bladder, the Ocean Sunfish has subcutaneous gelatinous tissue, which is low in density causing the Mola Mola to be neutrally buoyant (Watanabe 2008). It also uses its abnormal swim style, stated above, to stay afloat. Mola Mola also use this gelatinous tissue to help float at the surface of the water. The fish needs to float at the surface so that birds can land on them to eat off parasites and algae. They tend to collect these organisms on their skin due to the fact that they move slow throughout the water. Even without the swim bladder, the Mola Mola is able to swim throughout the ocean like a regular fish.

            The third obscurity of the Ocean Sunfish that the blogger called out was the fact that it has to eat so much because its diet consists of non-nutritional organisms. Ocean Sunfish are often referred to as obligate, or primary feeders, on gelatinous zooplankton. Large Mola Mola appear adapted for capturing and ingesting large scyphozoan jellyfish. Many also say that the Mola Mola eat a wider variety of organisms including algae, crustaceans, mollusks, and fish (Nakamura 2014). The Ocean Sunfish can get away with this type of diet due to its passive lifestyle not requiring excessive amounts of nutritional benefits.

Close up into the mouth of the Mola mola with fused beak teeth and throat teeth.     Source. 

Overall the Ocean Sunfish is not as helpless as perceived by the redditor and the general public, and are quite the fascinating fish. They can function just like any other fish, but just tend to do it in their own unique way. From their unusual shape to their unusual behavior, there is not a lot known about the Ocean Sunfish. They are still a giant mystery swimming about our oceans.

References 

Thys, T. M., et al. "Ecology of the Ocean Sunfish, Mola Mola, in the Southern California Current System." Journal of Experimental Marine Biology and Ecology, vol. 471, 2015, pp. 64-76doi:10.1016/j.jembe.2015.05.005.

Palsson, J. and Astthorsson, O. S. (2017), New and historical records of the ocean sunfish Mola mola in Icelandic waters. J Fish Biol, 90: 1126–1132. doi:10.1111/jfb.13237

Nakamura, I. & Sato, K. Mar Biol (2014) 161: 1263. doi:10.1007/s00227-014-2416-8

Pope, E.C., Hays, G.C., Thys, T.M. et al. Rev Fish Biol Fisheries (2010) 20: 471. doi:10.1007/s11160-009-9155-9

Watanabe Y, Sato K (2008) Functional Dorsoventral Symmetry in Relation to Lift-Based Swimming in the Ocean Sunfish Mola mola. PLoS ONE 3(10): e3446. https://doi.org/10.1371/journal.pone.0003446

Rio Grande Cutthroat Trout: History, Imperilment, and Management, by Daniel Donahoe

Throughout western North America, 13 colorful subspecies of Cutthroat Trout Oncorhynchus clarki dot the montane landscape. One subspecies of Cutthroat Trout, the Rio Grande Cutthroat Trout Oncorhynchus clarki virginalis, is endemic to southern Colorado and New Mexico (Pritchard et al. 2009). This subspecies of Cutthroat Trout is arguably the gem of the American Southwest, and displays a wide gamut of colors that mimic the picturesque New Mexican sunset.

The shining gem of New Mexico’s streams, the Rio Grande Cutthroat Trout, is a Salmonid (Family Salmonidae) that inhabits some of the state’s coldest and cleanest waters. With most Cutthroat Trout species diverging around 1-2 million years ago, Rio Grande Cutthroat Trout diverged relatively recently. Rio Grande Cutthroat Trout diverged from the nearby Colorado River Cutthroat Trout Oncorhynchus clarki pleuriticus around 100,000 years ago (Pritchard et al. 2009).

This speciation stems from the rugged geography of the Rocky Mountains. For millions of years, the Rocky Mountains geographically isolated populations of Cutthroat Trout throughout western North America. ultimately leading to the evolution of a variety of colorful subspecies (Loxterman and Keeley 2012). Of the 13 subspecies of Cutthroat Trout located across western North America, the Rio Grande Cutthroat Trout is the most-southerly distributed species (Pritchard et al. 2009).

Historic range of the eight major species of Cutthroat Trout (Loxterman and Keeley 2012).

The first reported sighting of Rio Grande Cutthroat Trout was by Francisco Vázquez de Coronado and the Conquistadores in 1541 during their conquest through New Mexico and southwestern North America (Owen 2012). The Rio Grande Cutthroat Trout was once prevalent throughout rivers in Colorado and New Mexico, but currently occupies only 11 percent of its historic range (Shemai et al. 2007). This southerly subspecies of Cutthroat Trout currently inhabitants three river systems within Colorado and New Mexico. The Canadian River, the Pecos River, and the Rio Grande river are the only rivers that currently harbor populations of Rio Grande Cutthroat Trout (Pritchard et al. 2009). With this information, one might beg to ask: how did the Rio Grande Cutthroat Trout become one of the rarest piscine gems of the American Southwest?

One threat to Rio Grande Cutthroat Trout populations came in the form of a foreign competitor; the Brown Trout Salmo trutta. Brown Trout, originally native to cold-water streams of Germany, were introduced in Colorado and New Mexico throughout the late 1880’s with the intent of spurring the states’ angling opportunities (MacCrimmon et al. 1970). These trout are “hardier” than native Rio Grande Cutthroat Trout in that they can tolerate a wider range of temperatures and consume a diverse range of food sources. Rio Grande Cutthroat Trout found in streams where Brown Trout are prevalent often show signs of being outcompeted and exhibit physical damage from aggressive Brown Trout (Shemai et al. 2007).

To make things worse, Rio Grande Cutthroat Trout are fighting another foreign threat: Whirling disease. This disease, originating from Germany, is the work of a prolific parasite Myxobolus cerebralis. This parasite festers inside aquatic Sludge Worms Tubifex tubifex that eventually release “TAM” spores throughout the water column that attach to trout gills (Ayre et al. 2014). This parasite burrows into the gills, working its way into a trout’s spinal cord and feeding on cartilage along the way. This process causes spinal-deformities in infected trout, which leads to erratic behavior in infected trout (Ayre et al. 2014). Trout infected with whirling disease often exhibit tail-chasing swimming behaviors within the water column, making them easy prey for predators (Ayre et al. 2014). If the trout is not preyed upon, it’s dead carcass will sink to the bottom of the river where the parasite’s eggs are released and ingested by Sludge Worms, thus repeating the parasite’s life cycle. This parasite has been credited for substantial decreases in Rio Grande Cutthroat Trout stocks and other trout stocks in Colorado, Utah, Wyoming, and Montana (Ayre et al. 2014).

Aquatic Sludge Worm lifecycle by fishpathogens.net. (Source)

These threats underscore the importance of restoring and preserving intact native Rio Grande Cutthroat Trout populations. To save these populations, fisheries managers must first identify genetically-pure populations of Rio Grande Cutthroat Trout. Genetic sequencing has been conducted on multiple Rio Grande Cutthroat Trout populations from a variety of streams and drainages around New Mexico and Colorado (Pritchard et al. 2009). Even though genetic sequencing is becoming a part of normal, everyday fisheries management, researchers will find it difficult to pinpoint a genetically pure population of Rio Grande Cutthroat Trout, as these populations have developed small, but significant differences in their genomes. Populations of Rio Grande Cutthroat Trout that are separated by only a few kilometers have been found to show distinct differences in their genomes (Pritchard et al. 2009).  Historic stockings of genetically dissimilar Rio Grande Cutthroat Trout have diluted the gene pool, effectively creating mongrel Rio Grande Cutthroat Trout. This micro-geographic structuring of Rio Grande Cutthroat Trout populations across the New Mexican country-side has made managing Rio Grande Cutthroat Trout difficult. Fisheries managers must tweak their management strategies depending on the genetic composition of a single population of Rio Grande Cutthroat Trout contained within a single sub-drainage (Pritchard et al. 2009).

Rio Grande Cutthroat Trout by loeflyfishing.com. (Source)

           

Current efforts to conserve Rio Grande Cutthroat Trout are being carried out by a variety of state and federal conservation agencies. One agency, the U.S. Forest Service, has advised the construction of natural and artificial dams to impede the spread of nonnative trout species that can outcompete native Rio Grande Cutthroat Trout (Young 1995). Along with dams, electroshocking has been used to eradicate nonnative trout from pools to eliminate the chance of competition on Rio Grande Cutthroat Trout (Young 1995). Efforts to manually remove invasive trout have also been instituted by conservation agencies. Federal and state conservation agencies within Colorado and New Mexico require fishermen to dispose of nonnative trout species if they are caught while fishing in streams that also harbor Rio Grande Cutthroat Trout (Quist and Hubert 2004). Additional policies have been instituted such as creel limits and protection of Rio Grande Cutthroat Trout spawning areas. Fortunately, these policies have been mildly successful in allowing Cutthroat Trout return to its native range (Quist and Hubert 2004).

    

            The Rio Grande Cutthroat Trout has proven to be an integral part of the American Southwest. This trout, displaying a wide gamut of colors that mirror the picturesque New Mexican sunset, has received a variety of threats from foreign invaders. To counteract these threats, federal and state conservation agencies have instituted the use of dams, creel limits, and other methods to eradicate competitive nonnative trout. These efforts help conserve the Rio Grande Cutthroat Trout for future generations of Coloradans and New Mexicans to enjoy.

References

Ayre, K. K., C. A. Caldwell, J. Stinson, and W. G. Landis. 2014. Analysis of Regional Scale Risk of Whirling Disease in Populations of Colorado and Rio Grande Cutthroat Trout Using a Bayesian Belief Network  Model. Risk Analysis: An International Journal 34(9):1589–1605.

Loxterman, J. L., and E. R. Keeley. 2012. Watershed boundaries and geographic isolation: patterns of diversification in cutthroat trout from western North America. Evolutionary Biology 12:38.

MacCrimmon, H. R., T. L. Marshall, and B. L. Gots. 1970. World Distribution of Brown Trout, Salmo trutta: Further Observations. Journal of the Fisheries Research Board of Canada 27(4):811–818.

Owen, J. 2012. Trout. Reaktion Books, United Kingdom.

Pritchard, V. L., J. L. Metcalf, K. Jones, A. P. Martin, and D. E. Cowley. 2009. Population structure and genetic management of Rio Grande cutthroat trout (Oncorhynchus clarkii virginalis). Conservation Genetics 10(5):1209.

Quist, M. C., and W. A. Hubert. 2004. Bioinvasive species and the preservation of cutthroat trout in the western United States: ecological, social, and economic issues. Environmental Science &  Policy 7(4):303–313.

Shemai, B., R. Sallenave, and D. E. Cowley. 2007. Competition between Hatchery-Raised Rio Grande Cutthroat Trout and Wild Brown Trout. North American Journal of Fisheries Management 27(1):315–325.

Young, M. K. 1995. Conservation assessment for inland cutthroat trout. United States Forest Service General Technical Report RM-256.