Recruitment to the reef
column spacer Recruitment
 
 

Sexual reproduction: settlement & metamorphosis

 

Recruitment in sexually reproducing animals involves 3 main topics: 1) spawning, larval development, and dispersal of larvae, 2) settlement and metamorphosis, and 3) survival during early life.  The second topic is considered in this section.  The other recruitment topics are accessible via the icons.

Settlement & metamorphosis in sexually reproducing animals is considered here for a few selected invertebrates (jellyfishes, nudibranchs, conchs, and sea urchins) while FISHES can be found in their own section.

title for recruitement to the reef part of BCCR hot-button icon for linking to the topic of spawning, larval development, & dispersal of larvae in BCCR hot-button icon for linking to the topic of settlement & metamorphosis during early life in BCCR hot-button icon for linking to the topic of survival during early life in BCCR
 
 
seahorse dive leader for Biology of Caribbean Coral Reef website photograph of a diver on a reef taken from a video

"As larvae, most or all these fishes and invertebrates spend time in the plankton. As they near the reef and prepare to metamorphose into juvenile forms, they respond to chemical signals from the reef, such as the scent of food or scent of adults of the same species. A larval fish will then descend, alight on the reefor stay close by, and transform into ajuvenile...like this spotted drum but, of course, much smaller in size"- Cayman Islands 2003. Video of spotted drum courtesy Andy Stockbridge, Belize.

NOTE Equetus punctatus

 

 
 

Sexual reproduction: settlement & metamorphosis: invertebrates: jellyfishes

 

Life cycles of reef invertebrates follow a common pattern of production of pelagic larvae, feeding by the larvae in the plankton, then settlement and metamorphosis to a reef-dwelling juvenile. But what if the invertebrate is already pelagic, like a jellyfish...? Jellyfishes employ sexual union of gametes, but also have an asexual-replication component in their life cycle.

In jellyfishes the product of sexual union of gametes is a non-feeding, poorly swimming larva called a planula. The planula drifts for a time then settles to the sea bottom, attaches, and metamorphoses into a polyp. The polyp looks and functions like a small sea anemone. It lives for several years and feeds by capturing small planktonic organisms with tentacles and stinging cells. When it reaches a certain size a ta few weeks of age, it begins to divide asexually to produce small units that will later grow into the adult jellyfish. The division is transverse, as shown in the drawings below, that produces basically an endless supply of small medusae known as ephyrae. Each ephyra is capable of feeding from the plankton and, depending on temperature and food availability, will reach adulthood in several months. Note that sexes are separate in all or most jellyfish species.

NOTE these are sometimes referred to as ephyra "larvae", but this is incorrect. The planula is the larval stage in jellyfishes

 
drawings showing jellyfish development
 

photograph of moon jellies Aurelia auritaphotograph of polyp stage of jellyfish developmentAdult jellies, such as Aurelia aurita, live for just a single season. They are budded off in springtime from the polyp and grow to mature size in summer/early autumn. In comparison, the polyp phase (see photograph above) can last for several years, feeding and budding off young medusae through its lifetime. Polyps and medusae are carnivorous, feeding on crustaceans, invertebrate larvae, and other small planktonic organisms.


An adult moon jelly Aurellia aurita shows its 4 gonads as
horseshoe-shaped structures lining the partitioned digestive
cavity. Judging by their pale colour, this individual is likely
to be male (female gonads are more orange in colour). The frilly structure is the manubrium, used to gather food from the
tentacle-fringe around the bell and pass it to the mouth 0.6X

BELOW: examples of of other jellyfishes that have similar reproductive patterns to that of moon jellies

 
photo collage of different jellyfishes
 
 
seahorse dive leader in Biology of Caribbean Coral Reefs website photograph of jellyfishes taken from a video The video shows a few jellyfishes swimming in a commercial aquarium - Texas State Aquarium, Corpus Christi
 
 

Sexual reproduction: settlement & metamorphosis: invertebrates: nudibranchs

 

photograph of adult nudibranchs Phestilla sp. courtesy Mike Hadfield, University of Hawaii
photograph of veliger larva of Phestilla courtesy Mike Hadfield, University of Hawaii After long periods of floating in the plankton, many species of invertebrate larvae are stimulated to settle by chemical signals of various origins: from their prey, from adult conspecifics, from bacteria associated with conspecifics, and from algal foods of the juveniles. For example, based on our knowledge of Indo-Pacific species of nudibranchs Phestilla, the veliger larvae of the Caribbean Phestilla panamica are likely going to be attracted to chemicals emanating from the coral Porites, a favoured prey of the juveniles and adults. Hadfield 1998 Biofouling 12: 9. Photographs courtesy Mike Hadfield, University of Hawai'i.

NOTE lit. "of the same species"

Two adult Phestilla sibogae on their favourite food-
coral Porites (Left, 1.5X) and a veliger larva of Phestilla
(Right, 150X). The white lines on the Left photograph
indicate the head ends of the 2 nudibranchs

 

This photo collage shows settlement and metamorphosis of the Indo-Pacific Phestilla sibogae on its coral prey. The images track a veliger from settlement and metamorphosis to the crawling stage, a process that would require only a few hours. Photo sequence courtesy Mike Hadfield, University of Hawai'i.

NOTE settlement behaviour of this species is likely similar to that of the Caribbean Phestilla panamica
photo sequence of metamorphosing nudibranch Phestilla courtesy Mike Hadfield, University of Hawai'i

 
 

Sexual reproduction: settlement & metamorphosis: invertebrates: conchs

 

photograph of queen conch Strombus gigas, juvenile
photograph of red alga LaurenciaVeliger larvae of queen conchs Strombus gigas are strongly stimulated to settle when they contact chemicals released from certain species of red algae in their seagrass-nursery grounds. Boettcher & Targett 1998 Biol Bull 194: 132; Stoner et al. 1996 J Exp Mar Biol Ecol 205: 229.

Juvenile conch
Strombus gigas
1X

 

Red alga Laurencia sp. is an attractant
to settling veliger larvae of queen
conchs and is the foodstuff later
eaten by the juvenile snails 0.3X

 

photographs of conch farm in Turks & CaicosMariculturists of marine snails such as queen conchs Strombus gigas and abalone Haliotis spp. utilise the metamorphosis-inducing property of red algae. The veligers are grown to metamorphic competency on a diet of phytoplankton, then induced to settle and metamorphose on certain red algae. Grow-out of the juveniles can be done in enclosed tanks or in outdoor pens, the latter used to produce queen conchs commercially in Turks & Caicos and elsewhere.





Outdoor grow-out pens in
Providenciales, Turks & Caicos

 
 

Sexual reproduction: settlement & metamorphosis: invertebrates: sea urchins

 

photograph of sea urchin Echinometra viridis
Sea urchins and other echinoderms in tropical regions produce long-lived pelagic lavae that feed on phytoplankton before settling and metamorphosing. Spawning and early develop in sea urchins is covered in another section of BCCR: RECRUITMENT: SEXUAL REPRODUCTION: SPAWNING, LARVAL DEVELOPMENT, & DISPERSAL OF LARVAE.

 

 

 

 

 

Caribbean sea urchins Echinometra viridis are sedentary
and inhabit protective crevices in dead limestone
rocks. Distribution of the species is enabled by
a 3-4wk drifting planktonic phase of the larvae 0.8X

 

diagram showing settlement in a sea-urchin larva
After 3-4wk feeding on phytoplankton the early adult shape, called the adult rudiment, becomes visible within the larva, signifying that the larva is nearing competency. Settlement tends to be a passive activity. As the rudiment grows in size and the larva's mass increases, swimming becomes more energetically costly and the larva sinks to the sea bottom. As it nears the bottom and eventually bumps along it in the current, the larva is attracted to chemicals released both from seaweeds that will be its food as a juvenile and from the adults themselves. When the larva reaches a suitable spot, it metamorphoses to the juvenile form. Given that the rudiment already has the form and features of the juvenile, the metamorphic process is simply one of casting off the larval parts that are no longer useful, and resorbing those that are.

NOTE a term meaning capable of settling and metamorphosing

 

photograph of black sea-urchins Diadema antillarumIn Barbados recruitment of black sea-urchins after a major die-off in 1983 was more than 50% complete on some reefs within 2yr, with greatest recruitment occurring where surviving adults were most numerous. Presumably by then the disease conditions had abated.

Youngsters of 12mm diameter were found sheltering under the spine canopies of adults 3-4mo after peak spawning. Not only would the juveniles benefit specifically from the protection conferred by the spines of adults by this strategy, they would also benefit generally by being in a habitat that is guaranteed to be favourable for their survival. Hunte & Younglao 1988 Mar Ecol Progr Ser 45: 109.

 

 

Black sea-urchins Diadema
antillarum
0.3X

 
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