Marine Larval Ecology - Dispersal and Settlement

Dispersal and Settlement

Probably the most widely accepted theory explaining the evolution of a larval stage is the need for long-distance dispersal ability. Sessile organisms such as barnacles and tunicates, as well as sedentary species like mussels and crabs, need some mechanism to move their young into new territory, since they cannot move long distances as adults. Many species have relatively long pelagic larval durations—the amount of time a larva is in the water column before it is competent to settle—on the order of weeks or months. During this time in the water, larvae feed and grow, and many species move through several stages of development. For example, most barnacles molt through six naupliar stages before molting to a cyprid, the stage at which they seek an appropriate settlement substrate. This allows the larvae to use different food resources than the adults and gives them time to disperse.

This strategy, however, involves a certain degree of risk. While some larvae have been shown to be able to delay their final metamorphosis for a few days or weeks, few if any species are able to delay metamorphosis indefinitely, and most species cannot delay it at all. If these larvae metamorphose too far from a suitable settlement site, they perish. Due to the imperative of finding a suitable settlement site within a certain timeframe, many invertebrate larvae have evolved complex behaviors and endogenous rhythms to ensure their successful and timely settlement, which will be explained below.

Many estuarine species exhibit swimming rhythms of reverse tidal vertical migration to aid in their transport away from their hatching site, however, the same species can exhibit tidal vertical migrations to reenter the estuary when they metamorphose and are competent to settle. This process is similar to the reverse tidal vertical migrations described in the section discussing predator avoidance above, but instead of swimming down on flood tide, settlers remain in the surface waters, allowing themselves to be transported into the estuary.

Another change that many larvae undergo after they reach their final pelagic stage is to become much more tactile, clinging to anything larger than themselves. For example, Shanks observed crab postlarvae in the lab and found that they would swim vigorously until they encountered a floating object. Postlarvae would then cling to the object for the duration of the experiment. Shanks hypothesized that by clinging to floating debris, crabs can be transported towards shore due to the oceanographic forces of internal waves, which carry floating debris shoreward regardless of the prevailing currents.

If they are able to successfully return to shore, settlers encounter a new suite of problems concerning their actual settlement and successful recruitment into the population. Space is a limiting factor for sessile invertebrates on rocky shores, and larvae might not find any open habitat. Additionally, settlers must be wary of adult filter feeders, which usually cover the rocks at settlement sites and eat particles the size of larvae. Settlers must also avoid becoming stranded out of water by waves, and must select a settlement site at the proper tidal height to prevent desiccation and avoid competition and predation. To overcome many of these difficulties, some species rely on chemical cues to assist them in selecting an appropriate settlement site. These cues are usually emitted by adult conspecifics, but some species cue on specific bacterial mats or other qualities of the substrate.