After several weeks in the plankton the veliger larva transforms into a special settling form known as a pediveliger, which has two shell valves and a fairly long, pointed foot.  Settling is in 3 main stages, although some authors split these even more finely. 

In the first, the larva sinks to the sea bottom and swims closely over it often dragging the tip of its foot.  The foot is endowed with special sensory regions, and as many as 9 secretory glands that come into play during attachment.  During the second phase of settlement the larva crawls along on its foot, “tasting” and testing the substratum for suitability.  The larva may turn in slow circles at this time or move in a zigzag path over an area of a few square centimeters. In the final stage, the larva homes in on a spot for intense back-and-forth inspection, then attaches and metamorphoses.  General description for the European oyster Ostrea edulis from Cranfield 1973 Mar Biol 22: 203; larval photo courtesy Wallace et al. 2008 Oyster hatchery techniques. SRAC Publ No. 4302, USDA.

NOTE  lit. “foot veliger”

Just before final settlement, the larva may attach a thread, secreted from a special area of the foot known as the byssus gland.  Three general glandular areas participate in secreting the thread: 2 for the core and one for a surrounding sheath (shown in purple/blue shades in the diagram). The secretions harden after release from the byssus duct. 

At settlement the tip of the foot attaches first by means of another secretion (shown in aquamarine colour in the diagram).  The larva then rolls onto its left valve, and pulls its body over its foot.  Two more secretions from the basal part of the foot then combine to cement the left valve to the substratum (glands coloured in yellow and green in the diagram).  The larva then transforms, or metamorphoses, into a filter-feeding bivalve. General description for the European oyster Ostrea edulis from Cranfield 1973 Mar Biol 22: 211.

NOTE  function of the byssus gland in bivalves is considered in more detail elsewhere in the ODYSSEY: LEARN ABOUT MUSSELS: BYSSUS THREADS

NOTE  characteristics of “wet setting” and enormous attachment strength of cementing secretions of oysters and barnacles have attracted great interest because of their potential use in dentistry and other medical applications

Larvae of the Pacific oyster Crassostrea gigas typically spend 2-3wk in the plankton after fertilisation.  During this time the shell grows from 40 to 320µm.  About 1-2d prior to settling the larva develops 2 eyespots, a foot for crawling, and exhibits positive geotaxis (swims to the sea bottom).  Competent larvae can be induced to settle and metamorphose in laboratory culture by exposure to sub-millimolar concentrations of L-DOPA (L-3,4-dihydroxyphenylanlanine).  Exposure to the two catecholamines, epinephrine and norepinephrine, will induce metamorphosis, but not settling.  Bacterial films associated with juvenile oysters will also enhance settlement and metamorphosis of C. gigas.  L-DOPA can be isolated from these films and is effective in inducing the same behaviours.  Apparently L-DOPA wil induce settlement behaviours in >90% of competent oyster veligers and can, in fact, be used as a measure of the competence of larvae to settle.  Coon et al. 1985 J Exp Mar Biol Ecol 94: 211.

Spat of Crassostrea gigas, newly settled

Further studies on the mechanisms of settlement and metamorphosis in Crassostrea gigas by the same research group show that in the absence of chemical stimulation (either L-DOPA or norepinephrine) the larvae are able to delay metamorphosis, but maintain competence, for at least 30d.  After this, the larvae must metamorphose or die.  If delayed, the larvae become increasingly sensitive and may even “spontaneously” settle and/or metamorphose.  The authors incorporate the results of their studies with those of others into a model of oyster settlement and metamorphosis.  They postulate 2 control pathways, a dopaminergic one controlling settlement behaviour, and an adrenergic one controlling metamorphisis. 

First, environmental cues trigger the release of dopamine, which stimulates neural activity in the “integration center”, and leads to settling and cementation. It is here that sensitivity to environmental stimuli is postulated to increase during a delay of metamorphosis.  During the terminal phases of settlement, norepinephrine (or possibly epinephrine) is released, which triggers metamorphosis. Thus, there are 2 control pathways, one for settlement and one for metamorphosis, and either can be activated experimentally. L-DOPA itself is now thought not to be a natural inducer, but it does play a role after its conversion to dopamine within the larva.  Several lines of evidence, including the fact that soluble bacterial products will induce settlement and metamorphosis, suggest that the inducer is a water-soluble substance.  Coon et al. 1990 Mar Biol 106: 379.

NOTE for other information on factors influencing settlement and metamorphosis in oysters C. gigas see Bonar et al. 1990 Bull Mar Sci 46: 484.  Such factors include light/shade, substrate orientation, surface texture, water flow, temperature, salinity, surface tension, presence of adults or spat, shell-matrix proteins, mantle cavity and tissue fluids, and so on

Later investigations of metamorphosis-inducing substances in oysters Crassostrea gigas provide detail on the effectiveness of various neurotransmitters in inducing settlement and metamorphosis.  Thus, acetycholine at 10^-4 M leads to about 30% metamorphosis, while norepinephrine at the same concentration leads to >80% induction (although most settlers are unattached spat at this concentration). The authors find no consistent effect of water current and only a weak effect of background reflectivity on the success of metamorphosis and attachment.  Beiras & Widdows 1995 Mar Biol 123: 327.

Examples of variable effects of different neurotransmitter substances on metamorphosis success in Crassostrea gigas larvae. Larvae are exposed to the chemicals for 48-h periods

Despite having widely dispersing planktonic larvae, populations of marine invertebrates often show considerable genetic heterogeneity even on small spatial scales.  One underlying assumption in considering genomic variation in a species is that there is broad and uniform reproductive success, but what if this is not true?  What if only a relatively few adults produce larvae in a breeding season, thus making disproportionately large genetic contribution to the recruits?  This idea is tested with a semi-isolated population of Pacific oysters Crassostrea gigas in Dabob Bay, Washington.  After modifying existing techniques to enable quick and efficient genetics studies of single larvae, the researchers sample 877 individual veligers in a 10-d period in August.  Results show significant differences between early and late samples and the rest of the samples, suggesting that the larvae may, indeed, be produced by relatively few adults.  The authors consider, and then reject, the possibility that their results may have come from sampling larvae originating from distant populations, rather than reflecting temporal heterogeneity among spawning populations in the Bay.  The study provides important support for an hypothesis of large variation in reproductive success in a free-spawning species with long-lived planktotrophic larvae.  Li & Hedgecock 1998 Can Fish Aquat Sci 55: 1025.

NOTE enzymatic amplification of DNA by polymerase chain reaction (PCR) with modifications to enable detection of single base-pair substitutions of DNA.  The technique involves designing PCR primers for 3 mtDNA segments totaling nearly 2000 nucleotide base-pairs

Larvae of oysters and most other marine invertebrates are attracted and stimulated to settle by the presence of conspecific adults.  The proximal cue for this gregariousness appears to be a water-borne chemical, but whether this is mainly from the shell or also from water exhaled from the mantle cavity is not clear.  Studies at Friday Harbor Laboratories, Washington on Pacific oysters Crassostrea gigas show that if the larvae are sucked into, or swim into, the mantle cavity, there is a high probability that they will die (>90%).  However, laboratory-flume experiments using a laser-doppler velocimeter to track water-borne particles show that while the adult suspension-feeding currents readily entrain phytoplankton food, they are too weak to capture larvae.  In tests, less than 5% of larvae tested are sucked in, even if they pass within 1mm of the inhalent opening.  When one considers that the “gape” or inhalent-opening represents only about 2.5% of the total shell area, the probability of an oyster larva settling onto a large reef of feeding adults and being killed is low.  In balance, therefore, the risk of cannibalism is greatly outweighed by the selective advantages in evolution of group-living (protection, improved fertilisation success).  Tamburri et al.  2007 Ecol Monogr 77: 255.

NOTE  cultch, or old oyster shells, is commonly used as a settlement substratum for oysters

Reef of Pacific oysters Crassostrea gigas
on Cortez Island, British Columbia