An uncommon aquarium resident, some spider crabs do find their way into specialized setups and are appreciated for their unique look. Although you won’t see a giant spider crab entering your tank anytime soon, since it can reach a length of 10 feet wide, watching one moult can show what to expect from their smaller brethren you might keep at home.
The prey, in this case, are copepods. Copepods are extremely small crustaceans that are a critical component of the marine food web. They are a favored meal of seahorses, pipefish and sea dragons, all of which are uniquely shaped fish in the syngnathid family.
Copepods escape predators when they detect waves produced in advance of an attack, and they can jolt away at speeds of more than 500 body lengths per second. That equates to a 6-foot person swimming under water at 2,000 mph.
“Seahorses have the capability to overcome the sensory abilities of one of the most talented escape artists in the aquatic world — copepods,” said Gemmell. “People often don’t think of seahorses as amazing predators, but they really are.”
In calm conditions, seahorses are the best at capturing prey of any fish tested. They catch their intended prey 90 percent of the time. “That’s extremely high,” said Gemmell, “and we wanted to know why.”
For their study, Gemmell and his colleague Ed Buskey, professor of marine science, turned to the dwarf seahorse, Hippocampus zosterae, which is native to the Bahamas and the U.S. To observe the seahorses and the copepods in action, they used high-speed digital 3-D holography techniques developed by mechanical engineer Jian Sheng at Texas Tech University. The technique uses a microscope outfitted with a laser and a high-speed digital camera to catch the rapid movements of microscopic animals moving in and out of focus in a 3-D volume of liquid.
The holography technique revealed that the seahorse’s head is shaped to minimize the disturbance of water in front of its mouth before it strikes. Just above and in front of the seahorse’s nostrils is a kind of “no wake zone,” and the seahorse angles its head precisely in relation to its prey so that no fluid disturbance reaches it.
Other small fish with blunter heads, such as the three-spined stickleback, have no such advantage.
Gemmell said that the unique head shape of seahorses and their kin likely evolved partly in response to pressures to catch their prey. Individuals that could get very close to prey without generating an escape response would be more successful in the long term.
“It’s like an arms race between predator and prey, and the seahorse has developed a good method for getting close enough so that their striking distance is very short,” he said.
Seahorses feed by a method known as pivot feeding. They rapidly rotate their heads upward and draw the prey in with suction. The suction only works at short distances; the effective strike range for seahorses is about 1 millimeter. And a strike happens in less than 1 millisecond. Copepods can respond to predator movements in 2 to 3 milliseconds — faster than almost anything known, but not fast enough to escape the strike of the seahorse.
Once a copepod is within range of a seahorse, which is effectively cloaked by its head shape, the copepod has no chance.
Gemmell said that being able to unravel these interactions between small fish and tiny copepods is important because of the role that copepods play in larger ecosystem food webs. They are a major source of energy and anchor of the marine food web, and what affects copepods eventually affects humans, which are sitting near the top of the web, eating the larger fish that also depend on copepods.
COLLEGE PARK, Md – If you’ve owned a pet guppy, you know they often jump out of their tanks. Many a child has asked why the guppy jumped; many a parent has been stumped for an answer. Now a study by University of Maryland biologist Daphne De Freitas Soares reveals how guppies are able to jump so far, and suggests why they do it.
Soares, an expert in the brain circuitry that controls animal behavior, decided to study jumping guppies while researching unrelated evolutionary changes in the brainstems ofPoecilia reticulata, a wild guppy species from the island of Trinidad and the forebear to the familiar pet shop fish. During that 2011 project, a guppy jumped out of a laboratory tank and into Soares’ cup of chai.
“Fortunately it was iced chai and it had a lid on, so he stayed alive,” Soares said. “That was enough for me. I had to use a high speed camera to film what was going on.”
Soares, an assistant professor of biology, and UMD biology lecturer Hilary S. Bierman used high speed videography and digital imaging to analyze the jumping behavior of nine guppies from the wild Trinidadian species.
In a research paper published April 16 in the online peer-reviewed journal PLOS One, Soares and Bierman reported the jumping guppies started from a still position, swam backwards slowly, then changed direction and hurtled into the air. By preparing for the jump – a behavior never reported before in fish, according to the two biologists – the guppies were able to jump up to eight times their body length, at speeds of more than four feet per second.
Soares and Bierman concluded that guppies jump on purpose, and apparently not for the reasons other fish do – to escape from predators, to catch prey, or to get past obstacles on seasonal migrations.
The biologists hypothesize that jumping serves an important evolutionary purpose, allowing guppies to reach all the available habitat in Trinidad’s mountain streams. By dispersing, they move away from areas of heavy predation, minimize competition with one another, and keep the species’ genetic variability high, the researchers believe.
“Evolution is truly amazing,” said Soares, who spent her own money on fish food, but otherwise conducted the study at no cost.
The video above captures a guppy’s high flying technique.
In the April 2013 issue, Jim Benfer profiles stick catfish. For those who are interested, stick catfish can make for a challenging breeding project. These videos and brief descriptions below may help you on the path to spawning your own stick catfish.
Breeding males entice females to spawn with them on vertical aquaria glass near the water surface beginning in the overnight hours.
Usually, the females deposit two adhesive eggs side by side, starting closest to the surface, and working downward until a double chain of eggs has been deposited and fertilized.
We all know that there are people who go to extremes for their pet fish. Some dedicate entire rooms (or backyards, or basements) to them, some raise live foods, others conduct daily water changes, and at least one goldfish keeper built his disabled goldfish a flotation device so it doesn’t have to rest on the bottom.
Check out the video below to see the fish wheelchair in action!