Reproduction of benthic organisms

Additional to annual cycles and longterm effects, egg production has a structuring influence on the benthic community composition. We can discren three development types for benthic invertebrates.

Direct development

Without larval stage (sometimes with breeding care, e. g. Astarte species). Eggs with relatively much yolk. These eggs are laid at the sea floor and develop directly into offspring.

(a) Planctonic development

Eggs or larvae stay in the watercolumn, they develop into a meroplactonic stage with generation cycle. Hydro- and Scyphozoans are the main members in this group with nonsexual, polype based reproduction. This group spatially expands by drift distribution. A member of this group is Aurelia aurita. Larvae are subjected to currents to find new hard substrate and new places to life.

(b) Planctonic development

Very small eggs and planktotrophic larvae. The larvae remain in the water column for 2 - 6 weeks feeding on planktonic algae. If they reach a new colonization area they start a metamorphosis to become an adult. Sometimes the larvae are capable of substrate evaluation within a few seconds of contact with the substrate. It is a rule of thumb that from 1 Mio. eggs only one individual becomes an adult.

Lecithotrophic development

Relativly large yolk rich eggs with short pelagic stage (Oyster, Ostrea edulis). The members of this group produce only a little amount of eggs

Caused by the strong saisonal signals in the Baltic Sea the benthic community shows also an annual cycle. This cycle has important impact on the demersal fish species since these mainly feed on benthic organisms. Species like Diastylis rathkei and Byglides sarsi  are quite strong influenced by this predation pressure but it seems that this is of minor importance for the inter annual population fluctuations. Oxygen depletion plays much more of a role for populations sizes as specially seen during the last years. Far spreading depletion in the Kiel Bay durig 1981 caused dramatic population shifts.