Shark Evolution

Introduction

The evolution of sharks has been long and complex, beginning between 400 and 500 million years ago and spanning multiple mass extinctions to bring us today’s modern sharks. Sharks have evolved a wide variety of fascinating traits that have allowed them to remain as apex predators of the ocean. These physical adaptations have increased their fitness, allowing them to survive and reproduce at greater rates.

Electron micrograph of the Ebola Zaire virus. This was the first photo ever taken of the virus, on 10/13/1976. By Dr. F.A. Murphy, now at U.C. Davis, then at the CDC.[1].

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Genetics

Phylogeny
Sharks belong to the taxonomic class Chondrichthyes that contains all cartilaginous fish. Specifically, they are one of two branches of the Elasmobranchii subclass, the other branch being skates and rays. The over 500 known shark species are then broken down into nine orders depending on the number of gill slits and dorsal fins, mouth location compared to eye location, snout elongation, and presence or absence of fin spines, anal fins, and nictating membranes.

Evolution
With the earliest known shark fossils dating back roughly 450 million years ago (1), sharks are one of the oldest living groups of animals. They were most prominent between 350 and 250 million years ago when Chondrichthian species made up over 60% of all fish species, before roughly 96% of all marine life was wiped out during the Permian-Triassic mass extinction (2). Certain early types of sharks survived however, with modern sharks and rays beginning to emerge and diversify around 200 million years ago, starting with cow and frill sharks of the Hexanchiformes (2).

Ancient sharks were characterized by weak elongated jaws, crescent shaped tails, caudal keels, and rigid pectoral fins(2). One such example was the Cladoselache genus of cartilaginous fish that preceded modern sharks like the Mako and Great White(1). Another distinct example is the Helicoprion genus with its “tooth whorl” (1) used to grip and saw through prey.

Sharks vs Bony Fish
Over the millenniums, sharks gradually diverged from their bony fish counterparts in several biological and anatomical way. As the names suggest, the most obvious difference lies in their skeletal structure. Bony fish evolved to have endochondral skeletal bone, which forms when bone replaces cartilage during development. However, sharks maintained their cartilage structure, which is lighter and more flexible, allowing faster acceleration and agility when turning.

Sharks are also lighter and thus more efficient swimmers due to their lack of a swim bladder, which is replaced with oil storage in the liver to assist with buoyancy. The only disadvantage of this trait is that sharks must constantly swim to prevent sinking. Another key difference is that shark muscles do not attach directly to the skeleton like bony fish, allowing greater flexibility when hunting or evading predators. This is also true in shark jaws which can protrude to catch prey due to the flexible connective tissue anchoring them to the skull. Shark skin further distinguishes these animals as apex predators due to minute dermal denticles running from snout to tail that reduce drag in the water and thereby increase speed. However, their prioritization of forward acceleration means that sharks cannot swim backwards due to rigid pectoral fins.

Other traits that have made sharks successful predators include their unique teeth structure and electrosensing capabilities. While bony fish have a single row of teeth embedded in the jaw bone with slow replacement rate, sharks have multiple rows of teeth attached to a layer of cartilage above the jaw that are constantly replaced. This allows them to aggressively pursue prey without fear of permanent or lengthy damage. An individual shark may rotate through tens of thousands of teeth within their lifetime. Sharks also have keen senses that enhance their ability to detect and catch prey. In addition to the five senses most animals have, sight, smell, hearing, taste, and touch, sharks have a sixth sense that allows them to detect the electrical field of their prey. For electroreception, sharks have specialized pore-like organs under and around their snout called ampullae of Lorenzini. Each receptor connects to a small canal of conductive fluid that can sense the faint electrical pulses sent when the muscles of prey contract and move ions. Scientists also hypothesize that electrosensing allows sharks to pick up on Earth’s magnetic field or the geomagnetic features in the local seafloor, explaining their incredible ability to accurately navigate across long distances, even at great depths or with no visibility.

Finally, sharks evolved differently from bony fish in their style of reproduction. Bony fish employ R selection, meaning adults mature more quickly and externally fertilize by releasing large quantities of egg and sperm into their environment. However, sharks use K selection, so adults are slow to mature and only produce a few well-developed young through internal fertilization. This method is advantageous because it conserves energy and gives offspring a higher chance of survival from birth.

Current Research on Shark Genetics
Recent studies on elephant sharks (Callorhinchus milii) have given insight into the emergence of gnathostomes, jawed vertebrates, in the evolutionary timeline. Elephant sharks have been identified as the slowest known evolving vertebrate, with synteny conservation that makes them ideal candidates for comparison with tetrapod genomes. Data suggests that jawed vertebrates diverged from their jawless ancestors around the same time that paired fins and immunoglobulin-based adaptive immunity evolved. From there, gnathostomes divided into bony fish and cartilaginous fish. Through genomic sequencing, it was determined that cartilaginous fish, including sharks, lacked the genes to encode secretion of calcium-binding phosphoproteins, an essential component in bone development, driving their separation from bony fish.

Elephant sharks, as well as most cartilaginous fish, also differ from bony fish and other vertebrates in their adaptive immune system. Despite having polymorphic major histocompatibility complex class II molecules, they lack the canonical CD4 co-receptor and cytokines and cytokine receptors related to the CD4 lineage. Because elephant sharks are the most distant relatives to humans that still have an adaptive immune response that relies on T-cell receptors and immunoglobulin antibodies, scientists analyze their genome sequences to identify differences in immune response that could benefit human medical treatments.

Microbiome

Like most animals, sharks constantly interact with microbes in their environment as well as in their own bodies. Although some microbes are harmful, disease-causing agents, many others are essential to gut health and overall survival of individuals. Unlike many terrestrial vertebrates, sharks are uniquely susceptible to the bacteria in their environment due to skin lesions from parasites, lacerations from prey or mates, the influx of seawater during mating, and the exposed capillaries of gills. As a result, they have developed complex defense mechanisms to protect themselves from harmful pathogens, while still maintaining healthy levels of beneficial microbes.

When the bacterial flora of five common shark species were examined, isolates of the genus Vibrio made up the vast majority of the samples. Vibrios were found thriving in many different organs and tissues, as well as the surrounding sea water, indicating they are autochthonous or native to sharks. Certain vibrios in the mouth and stomach helped breakdown starch, protein, and chitin, and vibrios in the gut improved digestion. Sharks also have a short intestine with a spiraled valve, giving them more surface area than bony fish to absorb nutrients. However, vibrios were often absent from the kidneys, liver, and spleen, suggesting a bacterial-host relationship that may impact shark physiology, specifically urea regulation. This was corroborated in a study that found increased toxin levels and high mortality rates in sharks with vibrios in these regions, especially in the kidneys.

Researchers have also drawn comparisons between different shark species to determine how feeding behavior affects intestinal microbiome diversity and metabolic function. After comparing great white sharks with filter-feeding whale sharks, they found that diet actually had minimal influence on the diversity and abundance of fecal microbes and that microbial genera varied little between individuals of the same and different species. It was discovered, however, that the microbiomes of external body niches like the eyes and skin were far more taxon-rich than internal niches. In fact, the gut microbiomes of many shark species displayed significantly less richness than most bony fish. This again suggests that evolved defensive traits prevent microbes in the water column from heavily influencing the internal anatomy of sharks.

One such example of unique defense mechanisms is the microbiome of a shark's skin. Among others, the Psychrobacters bacteria inhibits aquatic fungal pathogens, allowing sharks to heal at far more rapid rates than other animals, including humans. Despite being elusive and notoriously adverse to captivity, sharks have garnered more attention in recent years for their potential application to the study of human disease prevention and microbiome health.

Uniquely Adapted Sharks

Certain shark species have evolved unique traits that better equip them to survive and reproduce within their specific habitat and niche. Their traits distinguish them from the rest of the shark population and allow for unusual behavioral advantages. The below examples highlight particularly unique adaptations, while demonstrating the diversity in size, diet, habitat, and behavior that characterizes sharks worldwide.

Hammerhead Sharks Hammerhead sharks are perhaps the most recognizable type of shark with their distinct lateral head extension, called a cephalofoil, which evolved roughly 20 to 35 million years ago. This makes the Sphyrnidae family, which contains eight species of hammerhead sharks, the most recent modern species to evolve. Over the years, many hypotheses have been proposed to explain this feature, including increasing vision for hunting, providing additional lift for swimming, and even allowing them to pin stingrays to the seafloor. However, the most widely accepted hypothesis is that the cephalofoil provides extra surface area for the ampullae of Lorenzini, enhancing their ability to detect the electrical fields of prey, thereby increasing hunting efficiency and success.

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Edited by Madeleine Campbell, student of Joan Slonczewski for BIOL 116 Information in Living Systems, 2022, Kenyon College.