Question

4. A species of crab lives in estuaries where fresh water from a river drains into the salt water of the ocean. The mix of fresh and salt water is variable with tidal movement, so the salt concentration in the crab’s habitat varies quite a bit. Predict how this crab uses a negative feedback mechanism to prevent cells in its gills from bursting during times when salt concentrations drop below the internal salt concentration of the crab’s cells.

Answer 1

Crabs breath through gills, they are able to take in oxygen and transport it into the animal's bloodstream. The gills of crabs are located under the carapace near the first pair of walking legs. The oxygen that crabs need is taken into the gills through water or moisture in the air.

Marine Crabs work on the concept of Osmoregulation when breathing in estuaries which have different salinity levels at different Tidal levels.

Crabs become hyperosmotic (during high sea tide) regulators at salinities below 24 ppt and became hypoosmotic (during low sea tide) at higher salinities. Upon a salinity decrease from 20 to 1 ppt, crabs adjust their haemolymph osmolality to a stable hyperosmotic level in 8 hr. Crabs

The fowllowing is the explanation

1. A stimulus is initiated when the salinity changes.
2. Sensors - Sensory nerves on Gills detetct the salinity chnages.
3. The Integrator (Brain) -- It send signals to activate haemolymph to hyperosmotic or hypoosmotic level and adjust their haemolymph osmolality to a stable level depending on osmotic condition.
4. Effector - The Salt levels in the Gills are maitained by osmosis either dilution or increase of salt in gills.
5. The water flows over gills and suppplies oxygen to the body.