Larval Dispersal and Migration in the

Marine Environment

Chapter 5

From spawning to larvae…

Larval Ecology

Many marine animals reproduce by spawning and then release offspring into ocean waters. These young must find food, protection, and a suitable home.

Most marine communities are composed of species that reproduce by producing various larval types. Therefore, an understanding of larval ecology is central to understanding how such communities persist.

• Larvae: independent morphologically different stages that develop from fertilized eggs and must undergo a profound change before assuming adult features.

•Settlement: when larvae fall out (leave) the water column, select habitat, and undergo metamorphosis. This is usually brought on by cues from the environment.

•Recruitment: The larvae that have (1) dispersed and settled, (2) metamorphosed successfully, and (3) survived to be detected by the observer.

Larvae and adult of goose neck barnacle species.

Palthyoa species larvae

Hairy Triton and its larval form

Christmas Tree worm (polychate worm species) and its larval form

Sea star larvae and and newly settled sea star (juvenile)

Larvae and their adults

Dispersal Types in Benthic Species

• PLANKTOTROPHIC DISPERSAL - female produces many (103 - 106) small eggs, larvae feed on plankton, long dispersal time (weeks)

• LECITHOTROPHIC LARVAE - female produces fewer eggs (102 - 103), larger, larvae live on yolk, short dispersal time (hrs-days usually)

• DIRECT RELEASE - female lays eggs, or broods young, juveniles released and crawl away

• Teleplanic Larvae: larvae that live in transoceanic currents (are always planktotrophic larvae)

Lecithotrophic larva: tadpole larva of the colonial ascidian Botryllus schlosseri

Planktotrophic larva ofsnail Cymatium parthenopetum

teleplanic larva of an urchin

Veliger Larvae: a larval type only found in Mollusks. Torsion, twisting of the body that produces the spiral shell, is already taking place or will very soon after hatching.

Many marine mollusks produce veliger larvae, which are lecithotrophic larvae. This type of larvae is usually ready to settle within days of hatching, and is unique to mollusks.

PROBLEM OF SWIMMING LARVAE:water motion carries them away fromappropriate habitats

Shore Population

Longshore drift

Loss to offshore waters

Self-seedingeddies

Wind-drivenrecruitment

onshore

Internal waves, tidal bores

Dispersing of larvae

Some helping hands in dispersal• Winds that wash larvae to shore• Internal waves - bring material and

larvae to shore• Eddies that concentrate larvae in spots• Behavior – rising to the surface at certain

times of day, or with the rise of the tide. Most larvae have some ability to control the direction in which they disperse.

Estuarine larval adaptations - retention

Larvae rise on the flooding tide, sink to bottom on theebbing tide: results in retention of larvae within estuary

Estuarine larval adaptations - movement of larvae to coastal waters, return of later stage larvae

Blue crab, Callinectes sapidus

Effect of local eddies on larval retention in a patch reef on the Great Barrier Reef, Australia

larva of coral

Newly settled coralDistance from reef perimeter

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Gamete production and larval life must often be timed precisely to allow settlement and promote dispersal, to avoid being swept from suitable habitat, and to counter predation. These factors influence reproductive success (R in population models).

Planktonic Post-settling stagelarvalstage

Examples of timing for Reproductive Success:

a) larvae in intertidal areas that move at night are selected for (avoid predation).

b) Grunion spawning on beaches; timed with highest spring tides of Spring.

c) Coral reef damselfish lay eggs at dawn; larvae hatch 4 sunsets later (avoid predation). Egg production timed so that hatching is with the ebb tide which sweeps larvae offshore (lowers predation).

Settling problems of planktonic larvae

• Presettling problems:

Starvation

Predation in plankton

Loss to inappropriate habitats

Settlement Cues

Larval settlement is an active choice, aided by chemical and physical cues.

Photopositive and photonegative behaviors exist, sometimes within same individual.

Almost all larvae prefer settlement sites with bacteria present.

Gregarious settling is where larvae settle on adults (oysters, barnacles). (What are the costs and benefits??)

Larvae frequently use chemicals produced by other animals and plants to determine settlement locations.

After all this, metamorphosis takes place and the animal becomes a juvenile, and looks like a small adult).

Recruitment:

The larvae that have (1) dispersed and settled, (2) metamorphosed successfully, and (3) survived to be detected by the observer.

Larvae and adult of goose neck barnacle species.

Why disperse when mortality is thought be 90% or higher for most

species??

Biogeography and Function of Larval Dispersal

Why disperse?

• High probability of local extinction; best to export juveniles

• Spread your young (siblings) over a variety of habitats; evens out the probability of mortality

• Maybe it has nothing to do with dispersal at all; just a feeding ground in the plankton for larvae

Biogeography and function of larval ecology

Most marine inverts have planktonic feeding larvae.

However, planktonic feeding larvae are rare in high latitudes (Fig. 5.28)

As you more South, planktonic feeding larvae dominant.

Allows species to invade new areas

Avoids crowding (areas where space is the limiting factor for population growth).

The End