A special mucous coating that serves as a chemical

camouflage allows clown fish to live among the deadly

tentacles of the unsuspecting sea anemone . Utterly

dependent on this unlikely host for protection from

(5)predators, clown fish have evolved in isolated commu-

nities, a pattern that has led to unusual behavioral

adaptations.

The rigidly defined hierarchy of each clown-fish

community is dominated by a monogamous breeding pair

(10)consisting of the largest fish, a female, and the next largest

a male, attended by a fixed number of sexually immature

fish ranging in size from large to tiny. A remarkable

adaptation is that the development of these juveniles is

somehow arrested until the hierarchy changes; then they

(15)crow in lockstep, maintaining their relative sizes. While

the community thus economizes on limited space and food

resources, life is risky for newly spawned clown fish. On

hatching, the hundreds of larvae drift off into the plankton.

If, within three weeks, the defenseless larval clown fish

(20)locates a suitable anemone(either by pure chance or per-

haps guided by chemicals secreted by the anemone), it may

survive. However, if an anemone is fully occupied, the

resident clown fish will repel any newcomer.

Though advantageous for established community mem-

(25)bers, the suspended and staggered maturation of juveniles

might seem to pose a danger to the continuity of the

community: there is only one successor for two breeding

fish. Should one of a pair die, the remaining fish cannot

swim off in search of a mate, nor is one likely to arrive. It

(30)would seem inevitable that reproduction must sometimes

have to halt, pending the chance arrival and maturation of a

larval fish of the appropriate sex.

This, however, turns out not to be the case. In experi-

ments, vacancies have been contrived by removing an

(35)established fish from a community. Elimination of the

breeding male triggers the prompt maturation of the largest

juvenile. Each remaining juvenile also grows somewhat,

and a minuscule newcomer drops in from the plankton.

Removal of the female also triggers growth in all

(40)remaining fish and acceptance of a newcomer, but the

female is replaced by the adult male. Within days, the

male’s behavior alters and physiological transformation is

complete within a few months. Thus, whichever of the

breeding pair is lost, a relatively large juvenile can fill

(45)the void, and reproduction can resume with a minimal loss

of time. Furthermore, the new mate has already proved its

ability to survive.

This transformation of a male into a female, or pro-

tandrous hermaphroditism, is rare among reef fish. The

(50)more common protogynous hermaphroditism, where

females change into males, does not occur among clown

fish. An intriguing question for further research is whether

a juvenile clown fish can turn directly into a female or

whether it must function first as a male.

17.The passage is primarily concerned with

(A) analyzing the mutually advantageous relationship

between two species

(B) comparing two forms of hermaphroditism among

clown fish

(C) describing and explaining aspects of clown-fish

behavior

(D) outlining proposed research on clown-fish

reproduction

(E) attempting to reconcile inconsistent observations

of clown-fish development

18.It can be inferred from the passage that the clown fish

is able to survive in close association with the sea

anemone because the

(A) sea anemone cannot detect the presence of the

clown fish

(B) tentacles of the sea anemone cannot grasp the

slippery clown fish

(C) sea anemone prefers other prey

(D) clown fish does not actually come within the

range of the sea anemone’s tentacles

(E) clown fish has developed tolerance to the sea

anemone’s poison

19.According to the passage, adult clown fish would be

at a disadvantage if they were not associated with sea

anemones because the clown fish would.

(A) be incapable of sexual transformation

(B) be vulnerable to predators

(C) have no reliable source of food

(D) have to lay their eggs in the open

(E) face competition from other clown fish

20.It can be inferred from the passage that sex change

would have been less necessary for the clown fish if

(A) the male clown fish were larger than the female

(B) each sea anemone were occupied by several

varieties of clown fish

(C) many mature clown fish of both sexes occupied

each sea anemone

(D) juvenile clown fish had a high mortality rate

(E) both male clown fish and female clown fish were

highly territorial