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University of Delaware marine biologist Ana Dittel answers questions about Bythograea thermydron (pronounced "Bith-oh-gray-uh ther-me-drawn"), the top predator at hydrothermal vents. |
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Crabby Questions
QUESTION:
Where does this crab live?
ANSWER:
At vent sites in the eastern Pacific Ocean among dense clusters of tubeworms at an average depth of 2.7 kilometers (1.7 mi).
QUESTION:
What does it eat?
ANSWER:
Like other crabs, the vent crab seems to have a very good sense of smell. This is important in locating food. We’ve observed it feeding on several species of deep-sea worms, as well as clams and mussels. Also, some studies have suggested that the adult crabs feed on bacterial mats. These are colonies of bacteria so dense they are visible to the naked eye.
QUESTION:
How abundant is this crab?
ANSWER:
It’s present in such high numbers at vent sites that Alvin’s pilots actually use it as an indicator that we are approaching an active vent field.
QUESTION:
It appears that this crab has eyes. Can it see in the dark? How does it find prey?
ANSWER:
The growth stage beyond the larval stage is called the megalopa. At this stage, the crab has well-developed eyes that can sense light levels expected at depths around 1,000 meters in the water column. In contrast, once the megalopae develop into adult crabs, they basically have night vision as can be found in night-vision goggles.
QUESTION:
Why do you think the megalopa stage is red, and the adult stage is white?
ANSWER:
The bright red coloration seen in the megalopa (post-larval stage) is due to high concentrations of carotenoid pigments. In contrast, the adult crabs have low concentrations of carotenoids.
QUESTION:
Why are you studying it?
ANSWER:
What most intrigues me about this crab is how it colonizes new vents. Vent fields may be separated by many hundreds of kilometers, so how do new sites become colonized? To find out what strategies or mechanisms this crab uses, we are studying how its early life stages (larvae) disperse and where they develop.
QUESTION:
Do these crabs inhabit vent sites in other oceans in addition to the Pacific?
ANSWER:
There are several genera and species of vent crabs. For example, Bythograea thermydron, Bythograea microps, and Cyanograea are found along the East Pacific Rise, while Austinograea williamsi is common in the western Pacific. Segonzacia mesatlantica is found in the Mid-Atlantic Ridge in the Atlantic Ocean.
QUESTION:
Are these crabs good swimmers?
ANSWER:
When we did our laboratory studies at atmospheric pressure (which is more than 250 times less than the pressure of the deep-sea environment), we found that megalopae are very good swimmers. Speeds in warm water were comparable to those of the fastest-swimming megalopae of shallow-water forms like the blue crab Callinectes sapidus. Even at low temperatures typical of bottom waters near the vents (2–5°C), megalopae are capable of sustained swimming speeds of 4 centimeters per second. That’s fast!
QUESTION:
How do you maintain them in the lab?
ANSWER:
We are able to maintain the larval stages and small juveniles at room temperature and atmospheric pressure. However, the adults are pressure-sensitive and do not survive long at atmospheric pressure. Therefore, we must keep them in pressure chambers.
QUESTION:
Is there anything else we should know about vent crabs?
ANSWER:
There are many interesting and important aspects of the biology of these crabs that we do not know about. For example, we do not know how long it takes for the larvae to develop or where these larvae develop. We do not know how long they live, and more importantly we still do not know how they colonize new vents.
The crabs that inhabit hydrothermal vents have developed a form
of eyesight similar to that of night vision goggles used by the
military, according to University of Delaware researcher Charles
E. Epifanio.
Epifanio, a professor in UD’s College of Marine and Earth Studies who has done extensive work with the larvae of the blue crab that inhabits the shallow waters of the Mid-Atlantic coast, was part of a collaborative team involved in a three-year National Science Foundation study of the larvae of the vent crab, Bythograea thermydron.
The project was designed to look at the dispersal of crab larvae from one vent site to another.
“The vents are extremely unique and the community of organisms that lives there has evolved to live there and only there,” Epifanio said. “We wanted to look at how the communities are established and maintained. We had done similar work on the larval distribution of the blue crab in shallow waters and, with NSF support, wanted to look at the vent crabs.”
In the course of the study, the scientists found something unusual about the vent crab larvae--unlike almost any other creature in the vent community, they could survive the extreme changes in light, temperature and water pressure on the trip from the vents to the research ship.
“Generally when we bring organisms up to the surface, they die of depressurization or temperature shock,” Epifanio said. “These larvae were not susceptible to that, so we were able to handle them in the laboratory.”
The UD team found it could culture the vent crab larvae through the first several juvenile stages, keeping them alive for several months. “We found we could get through five stages, each punctuated by a molt,” Epifanio said.
Discovering this, the UD researchers put out the word to colleagues. “We have these unique animals, and you can’t get them anywhere else,” Epifanio said. “Other researchers jumped at the chance. We took the samples and cultured the larvae, and they did the physiology.”
Robert N. Jinks headed a research team at Franklin and Marshall College in Lancaster, Pa., and Thomas W. Cronin did work at the University of Maryland Baltimore County. Assisting Epifanio in the UD laboratories were Gina Perovich, a graduate student who earned a master of science degree in 2001, and associate research scientist Ana I. Dittel.
What the physiologists found was that in the early larval forms, the eyes of the vent crabs appeared normal externally and were much like those of shallow-water crabs. But upon metamorphosis, the eyes were highly degenerative and, in the end, became naked retinas.
“The eyes changed dramatically to deal with different wavelengths of light,” Epifanio said of the crabs' vision. “Vents produce light in the infrared wavelengths and the bottom line is, they basically have night vision as can be found in night vision goggles.”
Epifanio and Dittel were joined by UD’s Craig Cary as co-investigators for the NSF grant that supported the study, which was reported in Nature in 2002.
