Clown Fish

If you are one of the millions who saw the hit movie Finding Nemo, you know what a clown fish is.What you don’t know from watching the movie is that the cute orange and white-striped marine animals have a special talent: They can switch their sex!

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    Clown fish live in small groups inside sea anemones, scientific studies have shown. A sea anemone is an animal that attaches itself to a rock and uses the stinging cells on its long, brightly colored tentacles to kill prey.


    Because clown fish are immune to the stings, they can live within the tentacles of an anemone, where they are also safely out of reach of predators.

    Each group of clown fish has a strict hierarchy, or social order. At the top of the hierarchy is a
    breeding female. Next in line is a male. Third, fourth, fifth, and sixth are nonbreeders. Each fish is 80 percent as large as the fish above it in the hierarchy.

    Scientists also discovered the gender-bending ability of clown fish. One study of 97 groups of clown fish living in anemones on coral reefs off the coast of Papua New Guinea. They discovered that when the top female dies, every other fish in the group takes a step up the social ladder. The male that was below the female grows bigger and switches sex to become the breeding female. The nonbreeding member that was next in line grows bigger too and becomes a male. The other fish also expand in size and move up the ladder. A new nonbreeder is recruited to occupy the vacant rung at the bottom.

    Scientists are not sure how clown fish switch sex. It is believed that the fish live in a strict
    hierarchy to keep the peace. Knowing one’s place helps the fish avoid conflict and dwell in harmony for decades.

    What Determines Who Fish Live With (the Queue)?

    In group-living species, the distribution of reproductive success among group members is often highly skewed towards a socially dominant individual. A group member’s lifetime reproductive success is then dependent on the social rank it achieves. A ‘queue’ analogy has been used to describe systems in which social rank, and the reproductive opportunities associated with rank, are acquired passively.

    In queue-structured groups, individuals do not attempt to displace more dominant group members through direct competition.

    Clown Fish Sometimes Rest With Coral

    Instead, subordinates wait, and move up their group’s social hierarchy as more dominant individuals die. After joining a group as its least dominant member, an individual only achieves dominant social rank in that group if it outlives all those that arrived before it.

    Therefore, individuals living in queue-structured groups should adopt tactics that maximize their probability of acquiring dominant social rank. To that end, individuals recruiting into a population may seek to join shorter queues. 

    Likewise, if some degree of intergroup movement remains possible for an individual after the initial joining decision, then the relative probabilities of achieving social dominance in two groups should influence the decision to move from one group to the other.

    Social groups in many species have queue-like elements, but only recently have the implications of queuing begun to be considered. Often, the role of queuing is difficult to assess because groups cannot be treated exclusively as queues. For example, cooperatively breeding family groups are queues, in that the offspring of a breeding pair may inherit the parent’s territory.

    Theoretical models that incorporate queuing have been developed with just such groups in mind. But in most cooperatively breeding groups, patterns of relatedness among group members and the presence of separate male and female queues within each group complicate interpretations of subordinates’ options and behaviors. Moreover, because the groups result from delayed dispersal, there can be a causal connection between a queue’s length and the quality of the resource (a breeding territory) for which the queue formed.

    In these cases, it can be difficult to determine whether individuals are in line for a single territory or for any of a set of contiguous territories. The extent to which social ranks correspond to order of arrival (i.e. the degree of queue discipline) and the mechanism through which queue discipline is enforced can also be unclear.

    Anemonefish (Amphiprion spp.) are particularly well suited to studies of queuing’s implications because many of the complexities associated with queues in other group do not apply. Individuals occur in small groups occupying various species of tropical Indo-Pacific sea anemones.

    Groups are queues for social dominance and, with dominance, the opportunity to reproduce.

    Anemonefish ecology and life history have been extensively: a fish is not stung by its host anemone, and relies on its host for its own protection and to protect clutches of eggs laid adjacent to the anemone. An anemone is occupied exclusively by a single anemonefish group, typically consisting of a large dominant female (a), a smaller functional male (b), and a series of increasingly smaller, subordinate, sexually immature (c) a strict, size-based, linear dominance hierarchy.

    Baby Clown Fish

    What Do Juvenile Clown Fish Do?

    Subordinates do not act as reproductive helpers. Juvenile fish settle individually to anemones
    following a planktonic dispersal period, and join existing groups as the smallest, least dominant members. Therefore, group members are not relatives, and a breeding pair’s reproductive success cannot directly affect group size. Within each group, aggression directed by more dominant individuals towards their subordinates suppresses growth and sexual maturation of the latter. If the female is removed, the male grows and changes sex, and the largest of the immature fish matures as the new male.

    Juvenile fish settle individually to anemones following a planktonic dispersal period, and join existing groups as the smallest, least dominant members. Therefore, group members are not relatives, and a breeding pair’s reproductive success cannot directly affect group size.

    Within each group, aggression directed by more dominant individuals towards their subordinates suppresses growth and sexual maturation of the latter. If the female is removed, the male (the new a) grows and changes sex, and the largest of the immature fish (formerly g) matures as the new male. Scientists determed this is ‘female-controled protandrous hermaphroditism’.

    Clown Fish Change Sex

    Sex change and social inhibition of maturation greatly simplify the interpretation of social group structure: a group can be treated as a single queue with a terminal reproductive reward. Within a queue, social inhibition of growth enforces queue discipline. The risk of eviction offers a convincing explanation for subordinates’ willingness to accept their circumstances. The result is that a new settler’s only prospect for reproduction is to outlive more dominant groupmates. The longer the queue, the longer a settler’s expected wait, and the lower its likelihood of surviving to maturity.

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