Blue holes of Bahamas provide natural labs to study piscivorous predators in absence of prey

The “blue holes” of Andros Island in the Bahamas look like pristine inland lakes, often teeming with life. They’re actually flooded cave entrances, tall shafts that can connect to cave networks deep beneath the island. Due to poor water circulation, only the top 20-30 meters of a blue hole contain enough oxygen to support life. So while two nearby holes may connect below ground, the organisms living within exist in isolation.

This isolation has led to neighboring populations of the same species developing unique behavior and morphology. A new study published in the Biological Journal of the Linnean Society found that these adaptations are rather predictable, indicating significant potential in our ability to determine population responses to environmental change.

The study, led by Ryan Martin, an assistant professor at Case Western Reserve University, built off earlier research examining evolutionary differences between mosquitofish populations in blue holes with and without the species’ primary predator: the bigmouth sleeper. At the time, Martin was a postdoctoral researcher at North Carolina State University, working under Brian Langerhans, who would go on to co-author the new study.

Martin and the other authors wanted to learn how the bigmouth sleeper, a piscivorous ambush predator favoring the bottoms of lakes and streams, could survive in blue holes with nothing but insects and snails to eat. Andros Island offered the perfect spot to study the sleeper in habitats with and without prey fish, though Martin said that both population types seemed out of place in the holes.

The blue holes of Andros Island offer a chance to study bigmouth sleeper populations in close proximity but isolated by thousands of years. (Courtesy of R. Brian Langerhans)

“In these habitats, A, they’re super different because there is no bottom, and B,[the sleepers] are stuck in these blue holes where there are no fish to eat,” he said. “We wanted to know, how are these predators changing in response to not having any prey at all?”

Studying nine blue holes across the Andros Island archipelago, the researchers used snorkeling gear, minnow nets and good-old-fashioned rods and reels to catch bigmouth sleepers. With a spotter in the water, and most fish living in the first five meters of the hole, capture was simple enough.

“Once we’d got one, we’d bring it up and pretty carefully take photos to get information on their body shape,” Martin said. The researchers also measured the fish, clipped its fin for DNA and flushed its stomach using a water-filled syringe so they could study its diet. The researchers also tagged some of the fish for future identification.

Although Andros is the biggest of the Bahamas’ inhabited islands, it’s also the least-populated. The researchers traveled crumbling logging roads built by the British in the 1960s, slashing their way through thick scrub forest when the trails went no further.

“It’s pretty remote out there, so there were some days where we broke down and were out there for essentially the whole night because we had a flat tire,” Martin said. “Those were bad days — until someone realized we were gone and came out to find us.”

After three seasons in the field, the researchers studied captured fish in laboratory feeding trials to observe how the bigmouth sleeper adapted to a diet of invertebrates. The sleeper usually sucks up small fish swimming within the cone-shaped region in front of its mouth. Sleepers from blue holes without prey fish employed a stronger, narrower “straw” of suction to catch insects and snails.

Two bigmouth sleepers swim with their favorite prey, the mosquitofish, just visible in the background. (Courtesy of R. Brian Langerhans)

Ultimately, the researchers’ predictions proved accurate — good news in a world where climate change could result in many other populations facing adaptation or extinction. The study wasn’t without its surprises, however. In blue holes without prey species, the researchers found more male sleepers than females.

“The strength of that response surprised us. It wasn’t something that we expected to come out as well as it did,” Martin said. He noted that although it’s unclear exactly what caused the divergence in sex ratios, it’s possible that the greater energy requirements of egg production make females less suited to a habitat with less available food.

The researchers were also surprised to find that such drastic changes in morphology could occur over a relatively short period.

“These guys have only been separated for, at most, 15,000 years or so,” Martin said. “That’s a really short time to change how your head is shaped, your muscles and how your jaws are working to actually create these performance differences.”

Funding permitting, the researchers hope to continue studying the bigmouth sleeper and its mosquitofish prey. Future research with lab-reared fish could help determine whether the observed changes are purely environmental responses, or if they’ve altered the sleepers’ genome. Other research might examine how the sleepers and mosquitofish are evolving in response to one another.

Martin speculates: “In populations where the sleeper and mosquitofish have existed for a longer length of time together, are they essentially having an arms race?”

Too bad it’ll take another trip to the Bahamas to solve that mystery.