딱총새우 집게로 ‘딱’하면 시속 100㎞ ‘물폭탄’
1m 거리 제트기 엔진 소리 비슷…4700℃ 섬광
서·남해안 개벌이나 제주도 바닷가에는 한 쪽 손에만 권투 장갑을 낀 것 같은 특이한 모습의 딱총새우가 산다. 손가락 만한 이 새우는 주로 낚시미끼로 쓰일 뿐 별다른 상업적 용도가 없지만, 물리학자들에겐 <네이처>나 <사이언스> 같은 세계적 학술지에 논문을 발표할 만큼 중요한 생물이다. 이들은 버블제트를 이용해 먹이를 잡는다.
먹이 죽이거나 기절시켜
딱총새우는 물고기나 게 같은 먹이가 나타나면 비대칭적으로 거대한 집게발로 ‘딱’하는 소리를 낸다. 소리를 들은 대상은 기절하거나 죽는다. 왜 그럴까.
딱총새우가 해저에서 아주 시끄러운 소음은 낸다는 건 오래 전부터 알려져 있었다. 심지어 잠수함도 딱총새우 떼가 내는 소음을 방패 삼아 은폐할 수 있을 정도로 소음이 심하다. 사람들은 큰 집게발을 부딪쳐 내는 소리로 알았을 뿐 이 조그만 새우가 어떻게 그렇게 강력한 수중음을 내는지는 수수께끼였다.
그러나 미켈 베르슬루이스 네덜란드 트윈테 대학 교수는 독일 동물학자와의 공동연구 끝에 2000년 <사이언스>에 실린 논문에서 그 비밀을 밝혔다. 버블제트에 의한 수중폭발이 원인이었다.
딱총새우가 큰 집게발을 빠른 속도로 닫으면, 집게발 구조에 의해 압축된 물이 고속으로 분사된다. 여기서 생긴 제트류는 시속 100㎞의 속도를 띠는데, 수류 속 저압부에 진공상태의 기포(캐비테이션 버블)가 생긴 뒤 급격하게 팽창하면서 붕괴할 때 강력한 충격파가 발생한다는 것이다.
충격파는 1m 떨어진 곳에서 제트기 엔진 소리에 맞먹는 190dB로 강력하며, 4㎝ 떨어진 곳에 80㎪(킬로 파스칼, 대기압은 약 101킬로파스칼)의 수압을 발생시키는 것으로 밝혀졌다. 이 정도면 작은 물고기를 죽이거나 기절시킬 정도의 압력이다.
0.006초 뒤 측정해 버블 붕괴 확인
연구자들은 이듬해 <네이처>에 발표한 논문에서 딱총새우가 만든 캐비테이션 버블이 붕괴할 때 섬광이 발생한다고 밝혔다. 버블이 붕괴하기 직전 순간적으로 내부 온도는 태양표면과 비슷한 4700도까지 치솟는다는 것이다. 마치 자전거 타이어에 빠르게 공기를 주입하면 타이어가 뜨거워지는 이치이다.
김봉채 한국해양연구원 동해특성연구부 책임연구원은 28일 이 연구원 웹진에 실은 ‘딱총새우는 어떻게 강력한 소리를 낼까’라는 글에서 집게발을 닫은 뒤 0.006초 뒤에 날카로운 피크 펄스가 발생하는 것을 측정해 캐비테이션 버블의 붕괴를 확인했다고 밝혔다.
김 박사는 “딱총새우는 수중음을 발사해 사냥을 할 뿐 아니라 자신의 영역에 접근하는 경쟁자를 위협하는 등 의사소통에 음파를 활용하는 것으로 알려져 있다”고 밝혔다.
딱총새우는 전세계에 620종 이상이 살며, 우리나라에는 서해, 남해, 제주도 등에 15종이 밝혀져 있다. 주로 수심 60m 이내이고 수온이 연중 11도 이상인 열대나 온도 얕은 바다에 서식한다. 몸길이는 2~7㎝이며 10개의 발을 가지고 있다. 음파를 쏘아 사냥을 하는 습성 때문에 영어로는 ‘피스톨 슈림프’로 일본에서는 ‘철포 새우’로 부른다.
By Andrea Thompson, Senior Writer
posted: 25 February 2009 01:04 pm ET
The armored fish, Materpiscis attenboroughi, may have given birth to its young tail-first, similar to some sharks and rays. Credit: Museum Victoria.
Remains of embryos entombed in their fish mothers' wombs for 380 million years have been found in fossils from an ancient rock outcrop in Western Australia. The finding is a big deal because it suggests that sex goes way back.
The prehistoric fish, called placoderms, are found at the base of the vertebrate evolutionary tree (in a large group we humans also belong to), so it now looks like sexual intercourse, and the mating behaviors that go along with it, were more widespread in these ancient animals than previously thought, said the scientists who made the discovery.
The embryos were found in the body cavities of Incisoscutum ritchiei, an extinct jawed fish that lived from about 430 million years ago to 360 million years ago. Placoderms' heads and parts of their bodies were covered with bony armor. They were also some of the first jawed animals.
The specimens were found in the Gogo Formation of Western Australia by John Long of the Museum Victoria in Melbourne, Australia. At first, he thought that animal remains found in the placoderm fossils were the animals' last meals, but after finding embryos in another group of placoderms, Long and his colleagues took a closer look and found that the new specimens were also the remains of unborn embryos.
"We could see that the new specimens had the same bone structure as the previous embryos, were the same species as the adult, they did not have any broken or stomach-etched features (from digestive acids or from being chomped) and that they were at the same stage of growth as the previous embryos," Long wrote in an email. "All of these facts proved they were embryos, not prey items."
The scientists' work was funded by the Australian Research Council.
Sex evolution
Because the embryos were found inside their mothers, these placoderms must have copulated to produce offspring — instead of laying eggs and fertilizing them outside the mothers, as some species of fish and others animals do.
And since these placoderms sit near the beginnings of the vertebrate tree of life, "it means that complex forms of mating evolved probably about the same time as jaws evolved," Long told LiveScience.
To engage in sex, the fish would have needed the proper, er, equipment. The structure of the mating parts of Incisoscutum are related to the pelvic girdle and hind limb, or leg, as in humans.
"These fish were using the hind limbs for the very first kind of vertebrate copulations," Long said. "We humans have an expression that 'we like to get a leg over,' but placoderms liked to get a leg in."
Exactly where the placoderms fit in on the evolutionary tree is still a key question; fitting them into their proper place will help scientists better understand the evolution of the traits of internal fertilization of embryos and live birth.
The findings of this study, detailed in the Feb. 26 issue of the journal Nature, "may prove to have far-reaching implications for our understanding of early vertebrate evolution," said Per Ahlberg of Uppsala University in Sweden in an accompanying editorial in the journal. Ahlberg was not involved in the study.
Long and other scientists are continuing to turn up and examine placoderm fossils from sites in Australia and are learning more about the ancient creatures and their anatomy and other traits with improved technologies.
"We are learning more about placoderms with each new discovery," Long said.
Small ocean dwellers like krill don’t idly go with the flow. In fact, the water around such animals flows with their go, new research has found, suggesting tiny swimmers may have a big impact on ocean mixing.
Energy put into the oceans by small animals is a significant component of the total contributed by all swimming creatures, adding up to a force comparable to that of winds and tides, scientists report in the July 30 Nature. The research suggests that scientists modeling global climate processes may need to add the contribution of such swimmers to the mix.
The analysis “doesn’t leave a lot to doubt — it’s very convincing,” says oceanographer William Dewar of Florida State University in Tallahassee, who wrote a commentary in Nature on the new research.
Scientists had previously thought that tiny, shrimplike copepods, or even animals such as fist-sized jellyfish, had little influence on ocean mixing. Any turbulence created by these small creatures would quickly dissipate, quashed by the viscosity of the water, which is thick like honey at the copepod scale. But a combination of fieldwork, theoretical modeling and energy calculations reveals that generating a wake isn’t the only way to stir the waters. Swimmers also drag fluid with them as they move, and this effect is especially enhanced in the viscous setting of the small scale, the new research found.
It’s similar to “drafting,” the technique used by bicyclists to catch the draft of a vehicle in front of them, says John Dabiri of the California Institute of Technology in Pasadena, who led the new study. The vehicle or rider in front carries the air that envelops them; similarly, a creature moving through water carries fluid along.
When a swimmer moves through water of different temperatures — as krill do each night when they migrate en masse from deeper waters to the surface — the cold water that goes along for the ride is mixed with any warmer water higher up.
For many years scientists considered an animal’s wake the main biological kinetic energy input to the oceans. But turbulence from a wake wasn’t considered that significant, especially for small creatures or super-efficient swimmers like dolphins, which leave little wake.
But even though wake is diminished at small scales, the amount of water carried with the swimmer is significant, the new work shows. In fact, small creatures carry more water with them per their bodies’ volume than large creatures do, because what for them is “thick” water gets dragged along even more. Charles Darwin’s grandson, physicist Charles Galton Darwin, first described how fluid travels along with a solid body in motion in the 1950s.
In addition to doing theoretical modeling, Dabiri’s team traveled to the island of Palau in the South Pacific to investigate the fluid dynamics of mixing by the local jellyfish Mastigias. By squirting a fluorescent dye in front of the swimming jellyfish, the scientists tracked the flow of water moving along with the jellies. A laser device recorded the jellies’ velocities.
Calculations suggest that the amount of ocean power input from all sea creatures may be as much as a trillion watts, Dabiri says, comparable to the power input from winds and tides. That calculation doesn’t include the contribution of fecal pellets and other debris that drift down through the water column, he adds.
Even small movements may add up to a serious role in the cycling of heat and carbon in the oceans, important factors influencing global climate. “We need to rethink circulation,” Dabiri says. “It’s more than winds and tides.”
