Category Archives: Wildlife Wonders
This is an aerial photograph of whale sharks feeding at the Afuera aggregation in August 2009. (Credit: Oscar Reyes)
ScienceDaily (May 30, 2011) — Whale sharks (Rhincodon typus) are often thought to be solitary behemoths that live and feed in the open ocean. Scientists at the Smithsonian Institution and colleagues, however, have found that this is not necessarily the case, finding that whale sharks can be gregarious and amass in the hundreds to feed in coastal waters.
Aggregations, or schools, of whale sharks have been witnessed in the past, ranging from several individual sharks to a few dozen. However this new research, which involved both surface and aerial surveys, has revealed an enormous aggregation of whale sharks — the largest ever reported — with up to 420 individuals off the coast of the Yucatán Peninsula in Mexico. What brings them together is food.
“Whale sharks are the largest species of fish in the world, yet they mostly feed on the smallest organisms in the ocean, such as zooplankton,” said Mike Maslanka, biologist at the Smithsonian Conservation Biology Institute and head of the Department of Nutrition Sciences. “Our research revealed that in this case, the hundreds of whale sharks had gathered to feed on dense patches of fish eggs.”
While whale sharks may seem conspicuous as the heaviest and longest of all fishes, growing more than 40 feet long, there is still much that is unknown about them. They have a very widespread distribution, occurring in all tropical and sub-tropical regions of the ocean around the world. Understanding this filter-feeder’s diet is especially important since food sources determine much of the whale shark’s movement and location.
During the dozens of surface trips that team members made to the aggregation, called the “Afuera” aggregation, they used fine nets to collect food samples inside and immediately outside the school of feeding whale sharks. Scientists then used DNA barcoding analysis to examine the collected fish eggs and determine the species. They found that the eggs were from little tunny (Euthynnus alletteratus), a member of the mackerel family.
“Having DNA barcoding is an incredibly valuable resource for this research,” said Lee Weigt, head of the Laboratories of Analytical Biology at the Smithsonian’s National Museum of Natural History. “It not only allowed us to know what exactly this huge aggregation of whale sharks were feeding on, not readily done from only physical observations of eggs, but it also revealed a previously unknown spawning ground for little tunny.”
The team of scientists also examined a nearby, less dense aggregation of whale sharks, known as the Cabo Catoche aggregation, off the northern tip of the Yucatán Peninsula. They found that the prey of this group mostly consisted of copepods (small crustaceans) and shrimp. Increased sightings at Afuera coincided with decreased sightings at Cabo Catoche, and both groups had the same sex ratio, implying that the same animals were involved in both aggregations.
“With two significant whale shark aggregation areas and at the very least one active spawning ground for little tunny, the northeastern Yucatán marine region is a critical habitat that deserves more concerted conservation effort,” said Maslanka.
The whale shark is listed as “vulnerable” with the International Union for Conservation of Nature and Natural Resources. Populations appear to have been depleted by harpoon fisheries in Southeast Asia and perhaps incidental capture in other fisheries.
The scientists’ findings were published in the scientific journal PLoS ONE, April 2011. In addition to the Smithsonian Institution, team members were from the Comisión Nacional de Áreas Naturales Protegidas in Cancún, Mexico, the Center for Shark Research in Sarasota, Fl., project DOMINO and the Georgia Aquarium, Inc. in Atlanta, Ga.
- Rafael de la Parra Venegas, Robert Hueter, Jaime González Cano, John Tyminski, José Gregorio Remolina, Mike Maslanka, Andrea Ormos, Lee Weigt, Bruce Carlson, Alistair Dove. An Unprecedented Aggregation of Whale Sharks, Rhincodon typus, in Mexican Coastal Waters of the Caribbean Sea. PLoS ONE, 2011; 6 (4): e18994
Loligo squids approaching and touching the egg capsules, which harbor the contact pheromone that initiates high-level aggression among males. (Credit: Roger Hanlon)
ScienceDaily (Feb. 11, 2011) — Scientists have identified a pheromone produced by female squid that triggers immediate and dramatic fighting in male squid that come into contact with it. The aggression-producing pheromone, believed to be the first of its kind discovered in any marine animal, belongs to a family of proteins found in vertebrates, including humans. Results of the study appear in the February 10th issue of Current Biology.
Male-male aggression is a complex process that involves neural, hormonal, physiological, and psychological stimuli and is widespread in the animal kingdom, where it is associated with the acquisition or retention of shelter and food, and in sexual competition. “The identification of this pheromone as a key component of this signaling system is highly unusual because the male squids need only to come into contact with these protein molecules to initiate the complex cascade of behaviors that we term aggressive fighting,” says Roger Hanlon, senior scientist at the Marine Biological Laboratory (MBL) and study co-author.
Squid are highly advanced marine invertebrates with a complex mating system rivaling that of vertebrates. Most mating and egg laying in the longfin squid (Loligo pealeii), the focus of this study, occurs in the spring when the animals migrate from deep offshore waters to shallower waters along the eastern seaboard, from North Carolina to Maine. Females mate multiple times with multiple males, who compete fiercely for females.
In field studies, the scientists observed a visual attraction by male squid to eggs laid on the sea floor followed by an escalation from calm swimming to the highest level of aggressive fighting — even in the absence of females — when they physically contacted eggs. Seeking to identify what was triggering the behavior, Hanlon and his colleagues, including Scott Cummins of The University of the Sunshine Coast, Australia, Bernard Degnan of Queensland University, Kendra Buresch of the MBL, Jean Boal and Johanna Holm of Millersville University, and Gregg Nagle of the Medical College of Georgia/University of Georgia Medical Partnership, conducted laboratory experiments at the MBL.
They discovered a protein pheromone produced in the female reproductive tract and embedded in the outer surface of eggs. After purifying the pheromone and presenting it to male squid in the lab, they found the same extreme aggressive responses, even when the protein was “painted” translucently on a glass vial that contained squid eggs. “The contact pheromone was incredibly resistant to degradation,” says Nagle. “It appears to remain intact for an extended period of time until the eggs are seen and contacted by male squid.”
“Our lab experiments show that the male squid that touches the eggs first becomes aggressive faster than other males who have not yet touched the eggs,” says Hanlon. “This leads to dominance by the males that encounter the pheromone. Dominant males pair with the females and mate more often, and they gain greater fertilization success so the extremely competitive aggression has a payoff.”
“It was exciting to map an important behavior back to the molecular level,” adds Boal. “The research involved careful teamwork among people with very different backgrounds and skills. It was great to be part of such a far-flung and interesting group of scientists.”
While there are multiple discoveries of pheromones that can elicit aggression in land-based animals, the scientists maintain that this discovery in squid will help them to understand the critical signaling beneath our oceans. “Squid may have revealed a more direct way of stimulating aggression,” says Hanlon. “We doubt that many researchers have thought that contact with molecules in the external world could stimulate such complex and extreme aggressive behavior.”
Interestingly, the protein pheromone found in squid has some similarity with beta-microseminoproteins (ß-MSP), a family of proteins found in humans and other animals. “The functions of ß-MSP in vertebrates have not been determined, but our findings in squids may inspire other researchers to consider similar functions in higher vertebrates,” says Hanlon.
- Scott F. Cummins, Jean G. Boal, Kendra C. Buresch, Chitraporn Kuanpradit, Prasert Sobhon, Johanna B. Holm, Bernard M. Degnan, Gregg T. Nagle and Roger T. Hanlon. Extreme Aggression in Male Squid Induced by a β-MSP-like Pheromone. Current Biologoy, (in press)