Marine Larval Ecology - Predator Avoidance

Predator Avoidance

One of the major difficulties faced by larvae is the threat of predation. Larvae are small and plentiful, so many animals take advantage of this food source. The situation is particularly dangerous for invertebrate larvae in estuaries; estuaries are nursery grounds for planktivorous fishes. Estuarine species’ larvae have evolved strategies to cope with this threat, including methods such as direct defense and avoidance. Direct defense is usually only evident in species in which larval development takes place entirely within the estuary. Studies have shown that larvae that do not leave estuaries are larger than larvae that develop in the open ocean. Additionally, many estuarine larvae have large spines and other protective structures. These defenses work because most planktivorous fishes are gape-limited predators—what they eat is determined by how wide they can open their mouths—so larger larvae are harder for them to ingest. Morgan showed that spines do indeed serve a protective function by cutting off spines of some estuarine crab larvae and monitoring differences in predation rates between despined and intact larvae. Despined larvae suffered significantly higher predation rates than intact larvae, and were preferentially chosen during feeding trials with both types of larvae present. Additionally, Morgan showed that large-spined estuarine larvae usually keep their lateral spines relaxed, but raise them when approached by a predator. Therefore, predator deterrence in estuarine larvae is not only morphological but also behavioral.

A second strategy to deal with estuarine predators is to avoid them on small or large spatial scales. Some larvae do this at a small scale by simply sinking when approached by a predator. However, a more common avoidance strategy is to become active at night and remain hidden during the day, since most fishes are visual predators and need light to hunt. This strategy is not only evident in estuaries, but is also the main predator-avoidance strategy in the open ocean, since the water column lacks topography and thus hiding places. Most pelagic larvae and other planktonic species 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 their microalgal food source lives. By retreating to areas of low light during the day, marine larvae (and other zooplankton) can significantly decrease their risk of predation. On a larger scale, most estuarine invertebrate larvae avoid predators by leaving the estuary and developing in the open ocean, which has fewer planktivorous fishes. The most common strategy for leaving an estuary is reverse tidal vertical migrations. In this strategy, larvae use the tidal cycle and estuarine flow regimes to aid their departure to the ocean, a process that is well-studied in many estuarine crab species.

The process of reverse tidal vertical migrations begins when female crabs release larvae on a nocturnal spring high tide in an attempt to limit predation by planktivorous fishes. As the tide begins to ebb, larvae swim to the surface waters and are carried away from their site of hatching towards the ocean. When the tide reaches its low and begins to flood, larvae swim towards the bottom of the estuary, where water moves more slowly due to the boundary layer. This prevents them from being sloshed back and forth within the estuary in the surface waters. 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.