Explanations/summary of of the Periclimenes/sea-anemone experiment, with the true statements highlighted:


The relationship is a mutualism. No. In mutualism the participants do not harm one another. Even when food in the form of brine shrimps is present (Left drawing), the Periclimenes shrimp still clips and eats at least some of the anemone's tentacles, and this behaviour is intensified in the absence of brine-shrimp food (Right drawing).

drawing of anemone with partly-clipped tentacles drawing of anemone with super-clipped tentacles

The relationship is a commensalism. No. Commensalism is defined as a symbiotic relationship in which one partner benefits while the other is unaffected. This is not the case here.


The relationship is a parasitism. Yes. This is probably the best description of the relationship. Even when food in the form of brine shrimps is provided, Periclimenes still clips and eats at least some portions of its host's tentacles. Harm befalls the host while the shrimp benefits.

drawing of anemone with partly-clipped tentacles

The sea anemone is an obligate symbiont, in other words, requires the presence of the shrimp Periclimenes to survive. No. Although 30d was not long enough to test survival under the different treatments, it is evident that the presence of Periclimenes adversely affects the health of their host anemones - a condition made worse when brine-shrimp food is not provided (Left drawing). In comparison, in the absence of Periclimenes (Right drawing) the anemones' tentacles remain intact, implying good health and suggesting that the relationship is not obligate for the anemone.

drawing of anemone with its shrimp drawing of sea anemone without its shrimp

The sea anemone is a facultative symbiont, in other words, can survive in the absence of the shrimp Periclimenes. Yes. This is the conclusion reached by the researchers. The anemone certainly does better in the absence of Periclimenes (Left drawing), judging by the more intact state of its tentacles. As to why the unfed anemone (brine shrimps absent, Right drawing) appears to be as healthy as the fed one, is a question not addressed by the researchers. However, both sets of anemones would have been obtaining nutrients from their zooxanthellae symbionts, and perhaps this is able to tide the unfed ones over the relatively short 30d duration of the experiment.

drawing of unfed sea anemone drawing of fed sea anemone

The Periclimenes shrimp is an obligate symbiont, i.e., requires the presence of the sea anemone to survive. Good guess, but this can't be determined from the results shown here. However, other data presented by the authors show that growth and survival of Periclimenes kept apart from anemones is much poorer than that of ones kept with anemones, suggesting that the shrimp is, indeed, an obligate symbiont. In support of this, under natural conditions on the reef the shrimps are rarely found outside of their host anemones.


The Periclimenes shrimp is a predator of the sea anemone. No. This is akin to saying that a flea on a dog is a predator. A predator is an animal that is commonly larger than its prey, lives separate from it, and attacks from an "outside" position. An attack from a symbiont cannot be defined as an act of predation; rather, it is better described as an act of parasitism