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Four Types of Marine Larval Development

Many marine species produce large numbers of eggs and release them with a limited amount of yolk to reduce the parent animal's energy expenditure per egg. Invertebrate animals, such as corals and molluscs produce more eggs than marine fish, and an oyster can release up to 70 million eggs per spawning. The larvae of certain marine creatures join the "plankton rafts" at the surface of the ocean, while the larvae of other species undergo their development on or near the sea floor. Certain marine creatures produce relatively few eggs that contain a lot of yolk, and their larvae are either not planktonic or are planktonic for only a very short period.

Planktonic Larvae
- Many marine larval animals develop in the plankton community that floats in surface waters of the ocean. As part of this plankton raft, larvae feed on one another and on phytoplankton, which are unicellular algae cells. During this stage of their life, marine larvae develop structures for both locomotion and feeding; they reabsorb these structures once the larvae undergo metamorphosis. When that happens, they return to the coral reef or bottom of the sea, where they continue their lives as young members of their respective species.
Demersal Larvae
- Demersal larvae do not join the plankton raft at the surface of the ocean; they develop on or very close to the bottom of the sea. These larvae eat an assortment of organic detritus that they find either in the sediment, or just above the substrate. The dermersal larvae of certain invertebrate animals do not feed until they undergo metamorphosis. Deep-sea creatures typically produce this group of specialized larvae.
Two Phases of Growth
- The larval stage of many marine creatures is divided into two stages, the "pre-competent period" and the "competent period." Larvae develop toward metamorphosis during the pre-competent phase. These larvae keep their feeding and locomotion adaptations during the competent or "delay period" until they find a suitable area to settle and develop into adults.
Lecithotrophic Larvae
- Lecithotrophic larvae only stay in a larval stage for a few hours; they are completely non-planktonic. The parents of these larvae produce fewer eggs with large egg yolks. Certain bivalve molluscs, which produce lecithotrophic larvae, lay only a few thousand eggs. Another group of marine creatures produce lecithotrophic larvae that develop and go through metamorphosis entirely within their egg case. These larvae emerge from their egg case as benthic juveniles, as in the case of some sharks.

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