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.

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