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Then share ideas on one large chart. Next, work together to turn students' ideas into questions. Carefully lay the slice of bread on the surface of the water in the pan.

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What happens? Remove the bread and squeeze into a very tight, small ball. Place the bread-ball on the water. Ask students to note what happens. Making Connections. Exploration Fill both glasses about two-thirds full with hot water. Stir vigorously to make the salt dissolve. Place the egg in the glass of water without salt. Remove the egg and place it in the glass of saltwater. Try This! More Activities Create water as salty as an ocean to help you understand saltwater's density relative to fresh water. Here's how: Pass out a cup or glass to each student. To create a solution roughly equal in salinity to ocean water, use these proportions: 4 ounces Have students stir until the salt dissolves.

They can taste but not drink the solution. Negative pitch values indicate the shark was oriented head-downward. Warmer colours in the spectrogram represent stronger signals, whereas cooler colours represent weaker signals. From to , a lack of strong signal in the spectrogram indicates the shark was gliding uphill. The bars above the graph show descent black , horizontal swimming shaded , and ascent with tailbeat gray and gliding white.

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Periodical fluctuations in acceleration were stronger during descent than during ascent i. A cm male prickly shark A , a cm female sixgill shark B , a cm female sixgill shark C; no swimming speed data , a cm female sixgill shark D , a cm male sixgill shark E; with a counter-weight and a cm male sixgill shark F; with a counter-weight. Lines are estimated by generalized linear models with lowest Akaike information criteria. The models indicate swimming speed increased with increasing tailbeat frequency, and swimming speeds at given tailbeat frequencies were slower during descent solid lines than during ascent broken lines in all individuals.

Boxplots in the graphs show amplitude of acceleration signal during ascent and descent. The bottom and top of the box are the first and third quartiles, and the band inside the box is the median. The end of the whiskers indicate the lowest datum still within 1. The circles indicate outliers. All individuals had significantly higher amplitude during descent than ascent. It has previously been widely accepted that sharks are either negatively or neutrally buoyant [ 19 , 20 ], and previous studies have predicted that deep-sea sharks, which floated at the surface, should be close to neutral buoyancy in their natural habitat because cold temperatures in deep water would reduce lipid buoyancy [ 15 ].

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Here we unequivocally demonstrate that some deep-sea sharks are in fact positively buoyant in their natural habitats. Although the presence of strong upwelling currents could theoretically produce similar results i.

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Our observations of uphill gliding also rule out the possibility that hydrodynamic lift generated by swimming forward is exclusively responsible for upward movements by sharks. Hydrodynamic lift requires thrust from tail beats or momentum to sustain forward motion and hence water flow over the lift surfaces. Although short bursts of uphill gliding could reasonably be explained by hydrodynamic lift, positive buoyancy is the only plausible explanation for our observations of sustained uphill gliding over periods of several minutes.

One possibility is that uphill gliding increases stealth during hunting. Many deep-sea fishes have a well-developed lateral line system to detect approaching predators [ 23 ], and positive buoyancy might enable deep-sea sharks to more easily approach such prey undetected from below by near-motionless, upward gliding.

In this study, a camera-equipped sixgill shark only associated closely with the seabed during day, indicating that this species might stay near the seabed during daytime and migrate further up into the water column at night, possibly to forage on bathypelagic prey. Positive buoyancy could also facilitate diel vertical migrations. The metabolic rates of ectothermic deep-sea fishes decrease as temperature declines [ 24 ], thus during daytime, when ectothermic deep-sea sharks are occupying deep, cold water, their metabolic and activity rates are presumably at their lowest.

After spending the night in warmer water, their metabolic rates would be higher, allowing for more energetic swimming down to deeper, daytime habitats. However, at these higher latitudes, they experience uniform temperatures during diel vertical migrations [ 26 ], and seasonal fluctuations in temperature [ 26 ] similar to those experienced during diel vertical migrations in Hawaii. Future, comparative studies of sixgill shark and prickly shark buoyancy between temperate and tropical regions should help to clarify why deep-sea sharks exhibit positive buoyancy in deep water habitats. We note that our findings are preliminary, and more data are required to determine whether this is a widespread phenomenon among all life-history stages of these deep-sea sharks, or whether positive buoyancy is widespread in other deep-sea organisms.

Some deep-sea teleosts may also be positively buoyant in their deep-sea habitats [ 27 ], suggesting that this strategy may be beneficial for exploiting deep-sea environments. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

National Center for Biotechnology Information , U. PLoS One. Published online Jun Meyer , 2 and Katsufumi Sato 1. Carl G. David Mark Bailey, Academic Editor. Author information Article notes Copyright and License information Disclaimer. Competing Interests: The authors have declared that no competing interests exist. Received Sep 19; Accepted Apr This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are properly credited.

This article has been cited by other articles in PMC. Abstract We do not expect non air-breathing aquatic animals to exhibit positive buoyancy. Introduction Buoyancy is a physical challenge common to all mobile aquatic animals that travel vertically through the water column. Table 1 Details of instrument deployments on six deep-sea sharks. Open in a separate window. Data analyses We used Igor Pro Ver.

Results We obtained 36 days of swimming performance data from five bluntnose sixgill sharks Hexanchus griseus TL — cm and one prickly shark Echinorhinus cookei TL cm in their deep-sea habitats Table 1. Fig 1. Diel vertical migration of a bluntnose sixgill shark. Table 2 Summary of daytime and nighttime swimming behaviour.

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Fig 2. Swimming performance during a vertical movement. Fig 3. Relationships between swimming speed and tailbeat frequency during vertical migrations.

Discussion It has previously been widely accepted that sharks are either negatively or neutrally buoyant [ 19 , 20 ], and previous studies have predicted that deep-sea sharks, which floated at the surface, should be close to neutral buoyancy in their natural habitat because cold temperatures in deep water would reduce lipid buoyancy [ 15 ]. Data Availability All relevant data are within the paper.

References 1. Sink or swim: strategies for cost-efficient diving by marine mammals. Sink fast and swim harder! Round-trip cost-of-transport for buoyant divers. J Exp Biol.

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