GRE Reading Comprehension: JiJing 352-GRE阅读机经352篇 - 5Q2LYFK38PZO3J6RK

Larvae of many marine invertebrate species delay their metamorphosis into juveniles when cues signaling an appropriate juvenile environment are absent, thereby increasing their likelihood of thriving as juveniles and of ultimately reaching adulthood. Nevertheless, delayed metamorphosis has potential costs for juveniles, including reduced growth and increased mortality. Nearly all evidence of such costs involves species whose larvae do not feed but rather subsist on stored nutrients, indicating that insufficient energy reserves may be an underlying cause of these costs. Supporting this hypothesis are laboratory studies showing that in a certain bryozoan, the prolonged larval swimming that results from delayed metamorphosis is associated with size reductions in the juvenile feeding organ (the lophophore) and that one factor influencing the size of juveniles of certain barnacle species is how long larvae delay metamorphosis. However, other studies show that while significantly fewer juvenile Capitella worms survived to adulthood when metamorphosis had been delayed, prolonged larval swimming had no significant effect on juvenile size, suggesting, perhaps, that in some species, factors other than insufficient energy reserves account for the negative effects of the larval stresses that result from delayed metamorphosis.