Reef Fish Reproduction

Fish differ in where they spawn and when they spawn

Pelagic spawners

-          get eggs away from the danger of the reefs where there are lots of planktonivores

-          Eggs often have characteristics that cause them to float near the surface where there are fewer predators

Spawning rush

-          females swim up in the water column followed by male/males

-                eggs and milt (sperm) released at highest point

Often spawn at the down current edge of the reef because the currents get the eggs away from the reef quickly.  Most spawning occurs at dusk or at night when there are fewer active planktonivores.

They spawn in locations that will eventually return the larvae to the reef.

Bottom spawners

           

build eggs and lay eggs on bottom (demersal spawners)

            triggerfishes

            gobies

            blennies

            pufferfishes

            others

may care for their eggs

            place in protected areas or defend the nest

larvae hatch at night and quicky move up away from the reef

-          tend to be more mature at hatching than pelagic larvae

-          tend to remain near area they were hatched

Migratory spawners

            Move to the downstream edge of the reef to spawn

                        May travel long distances

                                    Alone or in groups

            Groupers

                        Usually live solitarily

                                    Breeding aggregations of 30,00 – 100,000 have been observed

                       

Fish Reproduction

Most bony fish live benthically as adults, but they have a two part life cycle that involves a pelagic stage for the eggs and/or larvae.  Sawning involves the release of eggs and sperem.  All sperm are released into the water column.  Eggs can be laid in the water column (pelagic or broadcast spawning) or on the bottom (benthic or demersal spawning).   Pelagic spawners risk predation while swimming into the water column and although their eggs are not well-cared for, they are at least released where they are at lower risk of predation.  Benthic spawners invest in caring for their young by building nests and defending the eggs.

            Benthic spawning, including damselfish, cardinalfishes, blennies, and gobies, is a relatively rare strategy involving only from 20 – 30% of species.  Females lay eggs, attached to the substrate, in nests that are tended by the males.  The eggs hatch after a period of 6-7 days and the larvae become pelagic.  Larvae often hatch at night and they tend to remain relatively near where they are hatched. 

            Some benthic spawnwers brood their young.

            Pelagic spawners release both eggs and sperm into the water column where fertilization occurs.  Surprisingly, the rates of fertilization can be quite high (80 – 90%).  Spawning often occurs on the down current edge of the reef or in channels in the reef.  One important consideration when deciding where to spawn is choosing a location where the eggs are quickly removed from the danger of the reef.  However, eventually the larvae need to return to the reef so some preferred spawning sites are found at locations where currents tend to eventually return to the reef. 

            Diurnal spawners typically spawn when the current is outflowing which removes the eggs away from the predators on the reef.  Other species spawn at dusk and they don’t seem as concerned about what the current is doing, but instead are focused on the time of day.  In general, most spawning occurs at dusk.  Interestingly, many common species spawn in the afternoon while rare species tend to spawn at dusk. 

            Because there are less planktonivores near the surface, pelagic eggs often have characteristics that cause them to float near the surface.  Pelagic eggs tend to hatch verily rapidly, sometimes hatching after 24 hours.  Not surprisingly, pelagic larvae are not well developed upon hatching, and may still be attached to a yolk sac.  Pelagic larvae look very different than they will as adults.  Larvae often have long spines which aid in floatation and inhibit predation and by be translucent in coloration with black internal linings that make them harder for predators to see.

            It is tough to be a pelagic larva (and not surprisingly it is also difficult to study pelagic larvae). Feeding is difficult because there is not a lot of food floating in the water column.  Pelagic larvae feed on zooplankton.  Not surprisingly, up to 99% of larvae die before settlement.   In addition, larvae are constantly at risk of predation.  The pelagic larval stage can last from 15 – 45 days before larvae return to the reef to settle and switch from a pelagic to benthic lifestyle.  Larvae are able to swim surprisingly well.   Studies have shown that tiny larvae have the potential to swim large distances (45-150 km).  Pelagic larvae appear to be able to locate the reef by hearing (wave breaking and reef noises) or olfaction.  Studies have shown that larvae have a well developed sense of smell.  The settling larvae move into little nooks and crannies in the reef and it appears that there are high rates of predation on larvae during settlement.  Most larvae enter the reef on the night of the full moon so that they can settle when it is darkest making it harder for predators to see them.   After settlement, the larvae metamorphose from their pelagic to benthic stage.  Once they settle on the reef, survival rates may be as high as 50%. 

             Pelagic larvae are by far the most common regardless of spawning strategy. Of 100 families found on coral reefs, only 4 families lacked pelagic breeders.  Out of 96 families of coral reef fishes with pelagic larvae 57 were pelagic spawners (i.e., wrasses, parrotfishes, surgeonfishes, snappers, goatfishes, butterflyfishes, moray eels, among others), 14 were demersal spawners (i.e., damselfishes, triggerfishes, gobies, blennies, pufferfishes, among others), two were live-bearers, six were brooders (e.g., seahorses in body pouches and mouthbrooding cardinalfishes, jawfishes, basslets, and star gazers). 

Mating Systems

Polygamy (many males mating with the same female) is the most common mating system for pelagic spawners. 

            Many species undergo sex change (sequential hermaphroditism).   Sometimes size determines sex reversal.  For example, in Grasbys all individuals are born females and after they reach a particular size, about 8 inches, females switch to males and begin to defend territories.

            Sex reveral is much more complicated in parotfishes and wrasses.  

Sex Change in Nature- Coral Reef Fish

http://www.evolutionfaq.com/articles/sex-change-nature-coral-reef-fish

Sequential Hermaphroditism in Reef Fish
http://academic.reed.edu/biology/professors/srenn/pages/teaching/web_2010/FIshEZ/index.html

Sex Change: Taking it to the Fishes
http://www.science20.com/variety_tap/sex_change_taking_it_fishes_progress

Interesting Recent Article About the Bumphead Parrotfish from the Indo-Pacific

http://news.discovery.com/animals/bumphead-parrotfish-120612.html