Larval Development
Marine larval development can be broadly classified into three categories: direct development, lecithotrophic, and planktotrophic. Direct developers are characterized by a larval stage that has very low dispersal potential and usually looks like the adult form of the animal. These larvae are also known as “crawl-away larvae,” since numerous marine snails exhibit this type of development, and their larvae crawl away from the egg mass. Lecithotrophic larvae generally have greater dispersal potential than direct developers. Many fish species and some benthic invertebrates have lecithotrophic larvae, which are provided with a source of nutrition to use during their dispersal, usually a yolk sac. Though some lecithotrophic species are capable of feeding in the water column, many, such as tunicates, are not, and must settle before depleting their food source. Consequently, these species have short pelagic larval durations and do not disperse long distances. Planktotrophic species, on the other hand, generally have fairly long pelagic larval durations and feed while in the water column. Consequentially, they have the potential to disperse long distances. This ability to disperse is one of the key adaptations of benthic marine invertebrates. During their time in the water column, planktotrophic larvae feed on phytoplankton and small zooplankton, including other larvae. Planktotrophic development is the most common type of larval development, especially among benthic invertebrates. The relatively long time most planktotrophic larvae spend in the water column and their apparently low probability of successful recruitment led some early researchers to develop a “lottery hypothesis” that states animals release huge numbers of larvae to increase the chances that at least one will survive, and that larvae cannot influence their probability of success. This hypothesis, though, views larval survival and successful recruitment as chance events, and numerous studies on larval behavior and ecology have shown this to be false. Though it has been generally disproved, the larval lottery hypothesis does represent an important understanding of the difficulties faced by larvae during their time in the water column, particularly because it recognizes the low probability of larval survival.
All three types of marine larvae face two major problems: avoiding predation and finding an appropriate site to settle.
Read more about this topic: Marine Larval Ecology
Famous quotes containing the word development:
“The man, or the boy, in his development is psychologically deterred from incorporating serving characteristics by an easily observable fact: there are already people around who are clearly meant to serve and they are girls and women. To perform the activities these people are doing is to risk being, and being thought of, and thinking of oneself, as a woman. This has been made a terrifying prospect and has been made to constitute a major threat to masculine identity.”
—Jean Baker Miller (20th century)