Recruitment to the reef
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Survival during early life

 

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 third topic is considered in this section.  The other recruitment topics are accessible via the icons.

Survival during early llife in sexually reproducing animals is considered here for a few selected invertebrates (jellyfishes, nudibranchs, conchs, and sea urchins), while VERTEBRATES are considered elsewhere.

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 BCCR photograph of fishes above reef taken from a video

"It's risky being a little larva in the plankton, for you can drift far offshore or get eaten by predators. When it comes time to settle, first comes a wall of mouths on the reef front, and then a gauntlet of hungry fishes and invertebrates...then, the risk of being a defenseless juvenile! Only a few queen angelfishes will survive to the juvenile stage" - Cayman Islands 2001. Video of angelfish courtesy Andy Stockbridge, Belize.

NOTE Holacanthus ciliaris

 
 

Survival during early life: invertebrates

 

From the moment eggs or larvae of invertebrates are released from the parent they face a gamut of hungry mouths, both in the plankton and on the reef face. Of the hundreds, thousands, or even millions of eggs released from an invertebrate, only a tiny fraction will eventually settle and metamorphose. After metamorphosis the juveniles must confront a new set of predators. This section begins with a few predators of larval stages of invertebrates, such as pomacentrid fishes and hydroids, and ends with specific examples of risks in the early lives of conchs and lobsters.

NOTE Family Pomacentridae: a taxonomic grouping of fishes that includes damselfishes and chromises

 
 

Fishes that prey on invertebrate larvae

 

Pomacentrids and other carnivorous fishes swarm the reef crest and, as part of their planktivorous diets, intercept and eat settling larval invertebrates.

 

 

 

 

 

 

Brown chromises Chromis multiineata, blue chromises C. cvanea, and several other non-pomacentrid fishes 0.15X

photograph of fishes on reef crest
 

photograph of masses of French grunts on the reef crestShoals of fishes such as these grunts are sometimes so numerous that they will remove virtually all fish and invertebrate larvae attempting to settle on the reef. Even after they settle, the tiny juveniles may be just as vulnerable.

NOTE a shoal is an undisciplined aggregation of fishes. A school, in comparison, is a disciplined aggregation in which all members move in the same direction and exhibit similar behaviours

 

Shoal of French grunts Haemulon flavolineatum 0.1X

 

 


 
photo composite of pomacentrids on the reef crest
 
 

Hydroids that prey on invertebrate larvae

 

photograph of hydroid colonies
Studies on feeding habits of certain Panamanian hydroids show that prey consists mainly of invertebrate larvae and other tiny organisms with a mean overall size of 0.1mm. As each polyp eats 10-20 prey items per day, the impact on survival of settling larvae by even small hydroid colonies is likely to be great. Coma et al. 1999 Coral Reefs 18: 141.

 

 

 

 

 

 

Two different species of hydroid colonies living atop a
gorgonian. The array is a top-down scaling of predator
types, from small- to medium-sized hydroid poyps, to
larger gorgonian polyps, that together represent a lethal
barrier to transit of settling invertebrate larvae 1.5X

 
photo/drawing of hydroid polyp feeding A Panamanian hydroid polyp as featured here would eat a minimum of 16 invertebrate larvae per day. With 100 polyps per colony, the total would be 1600 larvae per day and, with millions of hydroid colonies scattered around even a small area of the reef, the potential depredation by even a single hydroid species would be astronomical!
 
 

Do invertebrate larvae have any defenses?

 


Data for defenses of various coral-reef larvaeOwing to small size and apparent vulnerability, invertebrate larvae are at great risk of being eaten by predators. Although most are transparent and thus hard to spot by a visual predator, here we're concerned with defenses against actually being eaten. The data shown here for a selection of common larvae were obtained in feeding assays using 3 species of fishes and a coral as predators. Bullard et al. 1999 Mar Ecol Progr Ser 191: 153.

Overall, invertebrate larvae with chemical defenses comprise less than 1% of those studied, while those with physical defenses comprise 34%. The remainder, 65%, have no apparent defenses other than possibly transparency. CONCLUSION: most invertebrate larvae are defenseless.

CHEMICALLY DEFENDED: bipinnaria larvae of sea stars are soft and long-lived in the plankton. Many adult sea stars are known to be toxic and so it is assumed that their larvae may be so as well. A planula larva of cnidarians (corals, jellyfishes) is soft and ciliated, but has stinging cells (nematocysts) that likely confer protection of a chemical type. Adult ribbon-worms (nemerteans) are brightly coloured as warning of toxicity. Their pilidia larvae are long-living in the plankton and tend to be colorless even though chemically defended

PHYSICALLY DEFENDED: zoeae larvae of crabs are comparatively large, carnivorous, and have a spiny carapace. The cypris larval stage of a barnacle follows several naupliar larval stages. It has 2 calcareous valves (shells), does not feed, and is the settling stage of development. The larvae of mantid shrimps are aggressive carnivores and, like all crustaceans, have solid spiny exoskeletons for protection. A snail's veliger larva has a shell into which the soft parts can be withdrawn

NOT DEFENDED: a crab nauplius is not free-living. It develops within the egg capsule held on the adult's abdomen; hence, it probably does not have the firm exoskeleton of the zoea larval stage into which it hatches. At the time of settlement, a clam veliger has a bivalved shell. However, this seems not to offer protection against fish and other invertebrate predators. The trochophore larva of a polychaete swims and feeds on phytoplankton. It is soft-bodied and has no apparent defense. Phoronids are small, inconspicuous, tube-dwelling worm-like animals. Their actinotroch larvae are soft, free-living, and feed from the plankton. The mysid larval stage of a shrimp has a typically hard crustacean exoskeleton, so it is surprising that it is readily eaten by fish and coral predators. Hemichordates , or acorn worms, are soft-bodied and live in sand/mud habitats. Their tornaria larvae are also soft-bodied and seem to have no defenses. Barnacle nauplii are free-living and feed on phytoplankton. They have a spiny exoskeleton, buth this seems not to protect against predators, at least not the ones used in the tests

 
 

How vulnerable are coral larvae to predators?

  drawing of yellowtail damselfish in its garden
Recruitment of corals within yellowtail damselfish Microspathodon chrysurus territories may be up to 5 times greater than without, presumably owing to the lesser intensity of grazing within the territories because the damselfish chases away grazing fishes. The tiny coral larvae do have stinging-cell defenses, but these are likely powerless against large fishes like parrotfishes that scrape and ingest large amounts of bottom material with their toughened mouths. Sammarco & Carleton 1981 Proc 4th Int Coral Reef Symp 2: 525.
 
 

photographs of 2mo-old spat of elkhorn coral Acropora palmata on plastic settling platesFew researchers, even those familiar with corals, would likely recognise a 2mo-old juvenile elkhorn coral Acropora palmata in the field (see photograph lower Right). A study in the Florida Keys on larval settlement and juvenile survival of A. palmata shows, in fact, that recruitment is presently poor, and has been so for some time. For several months post-settlement the tiny spat is highly susceptible to rock-scraping predators (chiefly parrotfishes) and overgrowth competitors, such as filamentous red algae as shown in the yellow-circled photograph. Miller 2014 Coral Reefs 33: 1041. Photographs courtesy the author.

NOTE the researcher cultures 2 species of coral in the laboratory, settles the lavae onto ceramic discs, and places these in the field in different orientations. The spat are monitored over period of 6-8wk in 5 separate summers. Survivorship over this period for A. palmata ranges from 12-50%

 

Views of 2mo-old spat of elkhorn coral Acropora palmata
on ceramic settling plates. The 2 spat circled in yellow
are hard to see, in part because of overgrowth of turf
algae.The spat in the lower photo is about 2mm in length

 
 

Juvenile conchs as prey

 

photos/drawings of conchs and their predators
Egg masses of large conchs Strombus may contain up to 50,000 eggs. After a few weeks of floating in the open water eating phytoplankton, the resulting veliger larvae have about one week to find and settle in seagrass nursery areas. These areas are characterised by the presence of certain red-algal species that promote settlement and that are eaten by the young conchs after metamorphosis, when they are about 1mm in length. During their early life at a size of 1-4mm, the young conchs are preyed upon by numerous predators in the seagrass habitat, including crabs, lobsters, worms, filefishes, and shrimps. D'Asaro 1965 Bull Mar Sci 15: 359.

 

 

 

Adult and juvenile milk conch Strombus costatus
surrounded by 3 types of predators when at a young age

 
 

Juvenile lobsters as prey

 

photograph of spiny lobster Panulirus A mature female spiny lobster Panulirus guttatus will produce up to 3 million eggs per year, most of which obviously will not survive to adulthood. Briones-Fourzan & Contreras-Ortiz 1999 J Crust Biol 19: 171.

 

 

 

 

A somewhat larger Caribbean species of
spiny lobster Panulirus argus 0.33X

 

drawing of life cycle of a spiny lobsterSpiny lobsters Panulirus carry their eggs on the undersurfaces of their abdomens until they hatch to a motile feeding stage known as a phyllosoma larva. After a protracted planktonic life of 6 months to a year, a late stage known as a puerulus settles into inshore vegetated nursery areas at a size of 2-3mm. The phyllosoma's transparency provides protection against visual predators and its high surface-area-to-volume ratio enhances its buoyancy. Acosta & Butler 1999 Limnol Oceanogr 44: 494; Lewis 1951 Bull Mar Sci Gulf Carib 1: 89; drawing of puerulus stage from Calinski & Lyons 1983 J Crust Biol 3: 329.

 
 

drawing of puerulus larvae of lobsters ready to settle
Habitats of seagrass and, to a lesser extent, mangroves, are favoured by the settling puerulus-stage of lobsters. The question arises as to what conditions best favour survival of the young lobsters as they swim into the seagrass beds. Shown here are several pairs of conditions. Think about which conditioon of each pair will likely lead to greater survival, then CLICK HERE for explanations.

DAY/NIGHT

DEEP WATER/SHALLOW WATER

EBB TIDE/FLOOD TIDE

FULL MOON/NEW MOON

 
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