UCSB researchers have discovered something fishy about the phrase, “What doesn’t kill you makes you stronger.”
Following their studies of the bluehead wrasse – a carnivorous reef fish common in the Caribbean Sea – UCSB researchers found that these underwater creatures are better equipped for survival in the future when they face various threats early on in their lives. The researchers – post-doctoral student Scott Hamilton, marine biology professor Robert Warner and seven others – appeared in last week’s edition of Proceedings of the National Academy of Sciences online.
Transgender Fish
According to Warner, the bluehead wrasse, like various other reef fish, has the ability to change sex. Warner, who has studied the fish since the 1970s, said the wrasse most likely change sex in order to produce more offspring.
“Females are always guaranteed reproduction,” he said. “If they become bold enough and large enough to change into a male, then they will have more babies than if they were only female or male.”
All bluehead wrasses are born as females and can later mature into males, Hamilton said. Generally, reef fish change sex when they grow to about 72 percent of their maximum body size.
However, Hamilton said some bluehead wrasses turn into males at younger ages. Those males are small and look identical to females, which are yellow with a white underside and a black line in their midsection, he said. Since male bluehead wrasses are very territorial, Hamilton said young males pose as females to trick the head male. When the alpha male is not paying attention, the smaller male sneaks by and mates with the other fish.
Additionally, the sex change is a competitive task. The schools of wrasse fish range in size, with ratios of one male to five females to as large as one male to 80 females, Hamilton said.
“[Usually,] when one big male dies, the next largest female in the group will undergo the sex change,” he said.
The large female will act aggressively to let the other females know she will change, and within one week she no longer has eggs and instead produces sperm, he said.
Delving into the Deep
Hamilton and Warner’s study was conducted in one summer on the island of St. Croix in the U.S. Virgin Islands, during three recruitment pulses – periods when large numbers of young fish flood to the reef, Hamilton said. The fish were analyzed at two sites on the island: Butler Bay, which is in the northwest corner of St. Croix, and Jack’s Bay, which is in the southeast side of the island. Of the two sites, Hamilton said Butler Bay contained more local fish and Jack’s Bay had more foreign fish.
Warner said St. Croix was an ideal location to analyze the fish because it provided easy access to the reef, which made it less difficult to tag fish. It housed a well-established laboratory onshore and was full of well-protected patch reefs – areas that act as aquariums as small as a tennis court.
“It’s like having a whole bunch of different aquariums to work with,” Warner said.
In addition, the fish in this region were easier to study because they sleep at night and their young arrive onshore in large numbers at predictable times. On average, Warner said males mate 25 times a day. However, in a study he conducted in the late 1980s, he found one male had sex with 80 females in one day.
“Knowing a lot of the reproductive patterns of the fish is extremely helpful in interpreting the arrival of the young [onshore],” Warner said.
He said it is important to know where the young come from to determine how their early life affects their adulthood.
Originally, scientists thought that fish staying near the reefs where they were spawned would live longer than fish that developed offshore. They believed this because the reef areas are richer in nutrients and provide shelter, Hamilton said. However, he said they found that fish that developed in reef water died at about double the rate of fish that had to travel long distances to the reef. One explanation is that long-distance fish are healthier and have a lot of fat stores in their body compared to local fish, he said.
On average, bluehead wrasses only live two and a half to three years and spend the first 45 days of their life in a development stage. During this period, the fish drift in ocean currents until they reach a reef.
Counting the Rings
To analyze the bluehead wrasse, Hamilton said his team used a chemical analysis of the fish’s ear bone, which is called an otolith. Otoliths are hard structures composed of calcium carbonate located behind a fish’s eyes and below its brain. Like tree rings, otoliths keep a daily record, Hamilton said. For each day the fish is alive, a new layer is added to the otolith.
Otoliths work like tracking devices because they show the history and location of the fish, Hamilton said. The chemicals found in the ring reveal the type of water where the fish was located on any given day, he said. For example, when fish are in areas with high concentrations of lead, their otoliths permanently retain the lead. Based on the lead found in the rings, scientists can infer on a particular day that the fish was onshore, because high amounts of lead are due to run-off from land, Hamilton said.
In addition, each area of the ocean has a particular chemical associated with it, Hamilton said. Other chemicals include magnesium, barium and strontium – all of which are common trace metals that are enriched in different waters due to pollution. However, for this particular study, Hamilton said he and his team focused only on whether the fish was onshore or offshore.
To analyze the chemicals, Hamilton said his group used laser ablation/mass-spectrometry.
“The laser vaporizes a hole in the otolith,” Hamilton said. “Then the material is sucked into the mass spectrometer and the instrument analyzes the concentration of different elements.”
Implications
Hamilton said this study helps scientists understand how different populations are connected.
“Our results showed we cannot just measure connectivity from numbers,” Hamilton said. “We have to follow populations.”
Connectivity means the exchange of young between fish populations, he said. In other words, it is the percentage of fish born on St. Croix that make it back to the island and the percentage of fish that go somewhere else, he said. He said this concept is important to understand when determining where to place marine reserves.
