Marine larval ecology
Marine larval ecology is the study of the factors influencing dispersing larvae, which many marine invertebrates and fishes have. Marine animals with a larva typically release many larvae into the water column, where the larvae develop before metamorphosing into adults.
Marine larvae can disperse over long distances, although determining the actual distance is challenging, because of their size and the lack of a good tracking method. Knowing dispersal distances is important for managing fisheries, effectively designing marine reserves, and controlling invasive species.
Theories on the evolution of a biphasic life history
Larval dispersal is one of the most important topics in marine ecology today. Many marine invertebrates and many fishes have a bi-phasic life cycle with a pelagic larva or pelagic eggs that can be transported over long distances, and a demersal or benthic adult. There are several theories behind why these organisms have evolved this biphasic life history:- Larvae use different food sources than adults, which decreases competition between life stages.
- Pelagic larvae can disperse large distances, colonize new territory, and move away from habitats that has become overcrowded or otherwise unsuitable.
- A long pelagic larval phase can help a species to break its parasite cycles.
- Pelagic larvae avoid benthic predators.
Larval development strategies
Marine larvae develop via one of three strategies: Direct, lecithotrophic, or planktotrophic. Each strategy has risks of predation and the difficulty of finding a good settlement site.Direct developing larvae look like the adult. They have typically very low dispersal potential, and are known as "crawl-away larvae", because they crawl away from their egg after hatching. Some species of frogs and snails hatch this way.
Lecithotrophic larvae have greater dispersal potential than direct developers. Many fish species and some benthic invertebrates have lecithotrophic larvae, which have yolk droplets or a yolk sac for nutrition during dispersal. Though some lecithotrophic species can feed in the water column, too. But many, such as tunicates, cannot, and so must settle before depleting their yolk. Consequently, these species have short pelagic larval durations and do not disperse long distances.
Planktotrophic larvae feed while they are in the water column and can be over a long time pelagic and so disperse over long distances. This disperse ability is a key adaptation of benthic marine invertebrates. 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.
Because planktotrophic larvae are for a long time in the water column and recruit successfully with low probability, early researchers developed the "lottery hypothesis", which states that 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 views larval survival and successful recruitment as chance events, which numerous studies on larval behavior and ecology have since shown to be false. Though it has been generally disproved, the larval lottery hypothesis represents an important understanding of the difficulties faced by larvae during their time in the water column.
Predator defense
Predation is a major threat to marine larvae, which are an important food source for many organisms. Invertebrate larvae in estuaries are particularly at risk because estuaries are nursery grounds for planktivorous fishes. Larvae have evolved strategies to cope with this threat, including direct defense and avoidance.Direct defense
Direct defense can include protective structures and chemical defenses. Most planktivorous fishes are gape-limited predators, meaning their prey is determined by the width of their open mouths, making larger larvae difficult to ingest. One study proved that spines serve a protective function by removing spines from estuarine crab larvae and monitoring differences in predation rates between de-spined and intact larvae. The study also showed that predator defense is also behavioral, as they can keep spines relaxed but erect them in the presence of predators.Avoidance
Larvae can avoid predators on small and large spatial scales. Some larvae do this by sinking when approached by a predator. A more common avoidance strategy is to become active at night and remain hidden during the day to avoid visual predators. Most larvae and plankton undertake diel vertical migrations between deeper waters with less light and fewer predators during the day and shallow waters in the photic zone at night, where microalgae is abundant. Estuarine invertebrate larvae avoid predators by developing in the open ocean, where there are fewer predators. This is done using reverse tidal vertical migrations. Larvae use tidal cycles and estuarine flow regimes to aid their departure to the ocean, a process that is well-studied in many estuarine crab species.An example of reverse tidal migration performed by crab species would begin with larvae being released on a nocturnal spring high tide to limit predation by planktivorous fishes. As the tide begins to ebbs, larvae swim to the surface to be carried away from the spawning site. When the tide begins to flood, larvae swim to the bottom, where water moves more slowly due to the boundary layer. When the tide again changes back to ebb, the larvae swim to the surface waters and resume their journey to the ocean. Depending on the length of the estuary and the speed of the currents, this process can take anywhere from one tidal cycle to several days.
Dispersal and settlement
The most widely accepted theory explaining the evolution of a pelagic larval stage is the need for long-distance dispersal ability. Sessile and sedentary organisms such as barnacles, tunicates, and mussels require a mechanism to move their young into new territory, since they cannot move long distances as adults. Many species have relatively long pelagic larval durations on the order of weeks or months. During this time, larvae feed and grow, and many species metamorphose through several stages of development. For example, barnacles molt through six naupliar stages before becoming a cyprid and seeking appropriate settlement substrate.This strategy can be risky. Some larvae have been shown to be able to delay their final metamorphosis for a few days or weeks, and most species cannot delay it at all. If these larvae metamorphose far from a suitable settlement site, they perish. Many invertebrate larvae have evolved complex behaviors and endogenous rhythms to ensure successful and timely settlement.
Many estuarine species exhibit swimming rhythms of reverse tidal vertical migration to aid in their transport away from their hatching site. Individuals can also exhibit tidal vertical migrations to reenter the estuary when they are competent to settle.
As larvae reach their final pelagic stage, they become much more tactile; clinging to anything larger than themselves. One study observed crab postlarvae and found that they would swim vigorously until they encountered a floating object, which they would cling to for the remainder of the experiment. It was 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.
Once returning to shore, settlers encounter difficulties concerning their actual settlement and recruitment into the population. Space is a limiting factor for sessile invertebrates on rocky shores. Settlers must be wary of adult filter feeders, which cover substrate 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.
Larval sensory systems
Although with a pelagic larva, many species can increase their dispersal range and decrease the risk of inbreeding, a larva comes with challenges: Marine larvae risk being washed away without finding a suitable habitat for settlement. Therefore, they have evolved many sensory systems:Sensory systems
Magnetic fields
Far from shore, larvae are able to use magnetic fields to orient themselves towards the coast over large spatial scales. There is additional evidence that species can recognize anomalies in the magnetic field to return to the same location multiple times throughout their life. Though the mechanisms that these species use is poorly understood, it appears that magnetic fields play an important role in larval orientation offshore, where other cues such as sound and chemicals may be difficult to detect.Vision and non-visual light perception
is important to find a suitable habitat. Phototaxis evolved relatively quickly and taxa that lack developed eyes, such as schyphozoans, use phototaxis to find shaded areas to settle away from predators.Phototaxis is not the only mechanism that guides larvae by light. The larvae of the annelid Platynereis dumerilii do not only show positive and negative phototaxis over a broad range of the light spectrum, but swim down to the center of gravity when they are exposed to non-directional UV-light. This behavior is a UV-induced positive gravitaxis. This gravitaxis and negative phototaxis induced by light coming from the water surface form a ratio-metric depth-gauge. Such a depth gauge is based on the different attenuation of light across the different wavelengths in water. In clear water blue light penetrates the deepest. And so the larvae need only to compare the two wavelength ranges UV/violet and the other wavelengths to find their preferred depth.
Species that produce more complex larvae, such as fish, can use full vision to find a suitable habitat on small spatial scales. Larvae of damselfish use vision to find and settle near adults of their species.