Our Friends the Chondrichthyes

11-20-2019

By Wilson Barrett, Expedition Guide on the Safari Endeavour

I often wonder: if I lived under the sea, would I be a fish or a shark? You might be thinking, what’s the difference? Aren’t sharks a type of fish? And if not, which do I have more in common with?

All these questions and more can be answered by going back in time to the Devonian era, also known as the Age of Fish, a geologic time period famous for the emergence and diversification of many of our slippery swimming friends. Down in the Sea of Cortes aboard the Safari Endeavour, several branches of fish have graced our floating presence.

Over 400 million years ago, backbones started becoming a new trendy thing. They were a flexible trunk to the skeleton where organs and muscles could nestle and share an electrical highway. Over time, several variations came about among swimming animals and today there are two examples that have proven very successful. Our terrestrial lineage stems from bony vertebrate fish, while other branches stayed in the ocean and developed skeletons made of cartilage.

There are over 32,000 known species of fish in the world and over 900 species in the Sea of Cortes. The main category of cartilaginous fish is known as Chondrichthyes. Globally, there are about 510 species in this class and on any given day you can find around 170 species swimming in the Sea of Cortes. Chondrichthyes comprise all sharks and rays, and Baja seems to be a haven for a ton of each.

Sometimes it’s hard to see a connection between the skittish bullseye string ray we see snorkeling and the soaring elegance of a 30-foot whale shark. A closer look reveals they have a lot in common. Their skeleton is mostly a skinny poker-chip-like spine leading to a relatively open skull and fully detached double-hinged jaw. There are little to no limb structures and they don’t even have ribs. The striking superficial differences between rays and sharks become less stark after recognizing that the soft wings of rays are merely expanded pectoral fins. Most rays have a tiny dorsal fin and they all have fairly long pointy noses.

Another defining characteristic that separates Chondrichthyes from other fish is the way they regulate buoyancy. Most fish have air bladders that neutralize them underwater, our cartilaginous friends have developed expansive oily livers to regulate buoyancy.

In Baja, we often get to witness interesting divergent and convergent evolutionary trends within the Chondrichthyes class. For instance, the springy, jovial mobula ray with its stringy tail and protruding mouth lobes, may seem quite different from the large spotted whale shark. However, they both eventually resorted to filter-feeding on plankton in an almost identical way. Long ago they branched away from a toothed ancestor and ultimately developed mechanisms for consuming plankton separately, which are organs like gill-rakers that guide small prey toward the throat and not out the gill slits.

As divergent as these friends have become evolutionarily, they all have good reason to assemble in the waters of the Sea of Cortez. Mobula rays migrate all the way from the Galapagos to the gulf of California where they feed and find romance in large flapping shoals.

On our trips, we gaze at ocean sunsets peppered by the leaping and splashing of these vivacious mini mantas. Whale sharks, which predominantly lead a life of remote solitude, congregate near Bahia La Paz where wind-blown surface waters filled with thick zooplankton, attract second-stage juveniles to slurp and sift near the surface.

Whether you think you could be a shark or ray or fish, there are plenty of reasons to migrate to Baja for a little winter fun!

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