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Examine the specimens as you read the description for each. When finished, complete the
questions for this cluster. The specimens described include up to nine of:
sand dollar
hermit crab
horseshoe crab
The NECTON is distinct from the PLANKTON in that nectonic organisms are able to
direct their own movement and are not restricted to the upper surface. Nectonic creatures
include the dogfish (a type of shark), large shrimp, squid, and marine mammals such as
dolphins, seals, and whales. All nectonic creatures can swim.
Most members of the BENTHOS, however, are unable to swim and are restricted to the
bottom. Examples include numerous species of crabs, urchins, snails and molluscs. A
few, like the lobster and the octopus, are able to swim in emergencies but they normally
travel by walking over the bottom.
These marine creatures interact with each other in a number of ways. The dogfish and
starfish are examples of CARNIVORES or PREDATORS. These animals kill and eat
others using a variety of prey capture techniques. The dogfish chases and bites its motile
prey, whereas the starfish hunts sessile prey. The starfish's strong arms enable it to pry
open shells of oysters and clams (molluscs). Squid and jellyfish are also carnivores but
they use different methods of prey capture. Squid hunt their prey and use sucker-like
discs on their tentacles for capture. Jellyfish do not hunt their prey. Instead, their prey
usually blunders into the almost invisible jellyfish and is quickly stung to death by
poisonous barbs on the jellyfish's tentacles. The relationship between a carnivore and its
prey is called PREDATION (one species benefits, the other is killed).
The horseshoe crab eats almost any available food material including dead organisms. It
is called a SCAVENGER. The limulus leech lives in the gills of horseshoe crabs and
derives its nutrients from the body of the crab. While this harms the crab, it does not kill
the crab. The relationship between crab and leech is called PARASITISM (one benefits,
the other is harmed). The leech is called a PARASITE and the crab is called a HOST.
The hermit crab has interesting relationships with two other marine species. The crab
locates a dead snail and moves into the protective shell. The crab clearly benefits from
this relationship, but the snail neither benefits nor is harmed. This crab/snail relationship
is an example of COMMENSALISM (one benefits, the other is neutral). A tiny species of
polyp called snail fur lives on shells with crabs inside. A hand lens will help you see
these animals. The snail fur camouflages the shell thus protecting the crab inside. In turn,
the crab helps the snail fur with food scraps and transportation. The crab/snail fur
relationship is called MUTUALISM (both benefit). Snail fur never grows on a shell with
a living snail inside. Experiments show that a living snail prevents the snail fur from
growing on its shell possibly by secreting annoying chemicals. The snail/snail fur
relationship is called ANTIBIOSIS because one species prevents the other from growing.
The sea urchin and the sand dollar are both slow-moving echinoderms (spiny skinned
animals). Their strange body structure is arranged in five identical parts in a circular
pattern. The moveable protective spines are obvious in the urchin but are very short and
look like felt in a live sand dollar. Their many feet are also unique. Instead of moving by
muscles, these "tube feet" operate on fluid pressure like a car hoist in a garage. The
animal's body pumps fluid to each "foot" to extend it, and then sucks the fluid back to
withdraw the foot.
All species of shrimp, from the tiny fairy shrimp to the giant krill and prawn, are preyed
upon by larger predators. Shrimp feed on microorganisms, grow rapidly and reproduce in
great numbers. These nutritious animals are the favorite food of many predatory species
of fish, birds, whales and humans.
Review Questions
1. Distinguish between a predator and a parasite. Give examples of each.
2. Name and describe the relationships among hermit crabs, snails and snail fur.
3. Distinguish between motile and sessile.
4. Name three aquatic animals that cannot swim.
5. Distinguish between necton and benthos.
6. What does a scavenger eat? Name some marine scavengers.
7. Why are shrimp so important to the marine ecosystem?