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Many marine organisms have at least a single larval stage. The reproductive adults will release many offspring into a water column. These free-swimming larvae will then become part of the zooplankton, being carried mostly by currents. This life stage is very important to some species, who as adults are benthic. Once the larvae settle, they are there for life. There are three methods of larval dispersion and development:
Direct development or crawl-away larvae have a low dispersal potential. The young usually hatch from the egg looking very similar to the adults of the species.
Lecithotropic larvae have more of a dispersal potential than crawl-away larvae. One thing that characterizes this type is that they are provided with a yolk sac, or some other form of nutrition. This finite source of nutrition only allows for a certain amount of time to disperse and settle before this nutrition source runs out.
Planktotrophic larvae have the greatest dispersal potential. they can survive as pelagic larvae longer than the other types of larvae. This is because they are able to feed on smaller zooplankton and phytoplankton. Most sessile invertebrates have planktotrophic larvae.
There are many settlement cues, all of which vary from species to species.
Chemical cues are common and heavily studied. These cues are biological compounds created by another individual that larvae pick up on and use as a way to tell if a location is safe to settle in. These cues can be from adults of the same species, which ensures it is a safe place to settle, or from predators, which ensures its not a safe place to live. Raymond C. Highsmith studied the induced settlement in sand dollars, Dendraster excentricus. He found that the larvae of D. excentricus showed a preference for adult-associated sand or sand with adults present. He also found that the type of substrate didn’t matter, as long as adults were present, thus proving a chemical cue. The preference for this settlement cue is most likely due to the absence of micro predators that feed on metamorphizing larvae, such as Leptochelia dubai, from areas where adults are present. The reworking of the substrate by the adult sand dollars makes it impossible for these micro-predators to live there, leaving it a safe place to settle.
Physical settlement cues are also important for some species. Some larvae may only settle in areas with certain substrate types. Fish larvae are commonly seen to use physical cues as they prefer certain habitat types. Whalan et al. looked at this largely ignored cue. They studied five types of sessile marine invertebrates found on coral reefs, two species of scleractinian coral and three species of sponge. They found that both of the coral and one sponge species had significantly higher settlement on tiles with microtopography that included divots that closely matched the larval width. This proves that physical cues also play a role in the settlement.
Figure 3 from Whalan et al. [CC BY 4.0] shows the time in hours taken for a percent of coral larvae to settle and metamorphosize. Each line is a different sized divot, and the control was a smooth tile.
Previously there have been observations about where coral reef fish larvae are orientated when they swim offshore. They require orientation cues. A study done by Jack O’Connor andRachel Muheim examined the effects of magnetic fields on the orientation of coral reef fish. During the observations, coral-reef fish larvae revealed remarkably consistent orientation behavior while swimming offshore, requiring large-scale orientation cues. However, the mechanisms causing this behavior are still being investigated. One potential large-scale cue for orientation is the Earth’s geomagnetic field. Here, they examined the effect of magnetic field manipulations on the orientation behavior of coral-reef fish during the pelagic larval phase. In the absence of visual cues, individual larvae responded to a 90-degree shift of the horizontal component of the magnetic field within a Helmholtz coil with a comparable shift in orientation. This shows that they use a magnetic compass for orientation. Their findings suggest that geomagnetic field information guides the swimming behavior of larval fish in the pre-settlement phase. The ability to use large-scale sensory cues allows location-independent orientation of swimming, a behavior that influences dispersal and connectivity of fish populations, which has important ecological implications for anthropogenic development of marine areas.
The information in this chapter is thanks to content contributions from William Trautman
