How Are Scallops Born? From Spawning to Settlement

Scallops begin life as microscopic eggs and sperm released directly into the ocean in a process called broadcast spawning. Once a fertilized egg forms, it develops through several distinct larval stages over roughly five weeks, drifting in the water column before eventually sinking to the seafloor and attaching to a surface. The journey from spawning to settlement is precarious, shaped by ocean currents, water chemistry, food availability, and predation at every step.

How Scallops Spawn

Most scallop species are hermaphrodites, meaning a single animal produces both eggs and sperm. The gonad is typically divided into two visible sections: a white or cream-colored male portion that makes sperm, and an orange or reddish female portion that produces eggs. In some species, though, sexual identity changes over the animal’s life. The Japanese scallop Patinopecten yessoensis, for instance, is hermaphroditic at one year of age but has not yet reached sexual maturity. By age two, individuals commit to functioning as either male or female, with only a small fraction remaining hermaphroditic from that point on.¹

Spawning is triggered primarily by environmental cues. A rise in water temperature is the most common signal, and in laboratory settings, researchers have successfully induced spawning using both temperature manipulation and serotonin injection.² In the wild, spawning often occurs in late spring or early autumn depending on the species and latitude. Adults release eggs and sperm from slightly open shells near the seafloor, sending clouds of gametes into the surrounding water. A single female scallop can release millions of eggs in a spawning event.

Fertilization in Open Water

Because fertilization happens externally, eggs and sperm need to find each other in the vastness of the ocean. You might expect that scallop populations at low densities would have trouble reproducing, since the gametes would be too diluted. Interestingly, field experiments on sea scallops have found that fertilization success does not always drop as dramatically as models predict. Researchers comparing two populations with a tenfold difference in adult density found no strong density effect on fertilization rates. The low-density group actually achieved higher fertilization success than expected, possibly because scallops tend to cluster into local aggregations even when their overall population density is low.³ This aggregation behavior may act as a built-in safeguard for reproductive success.

Not everything that happens at fertilization is straightforward. The lipids stored in the egg by the mother play a critical role in whether the resulting embryo can develop successfully. Research on noble scallops has shown that disrupting the gene responsible for lipid accumulation in the female gonad leads to fewer eggs being released, lower lipid content in those eggs, and reduced formation of early-stage larvae.⁴ In other words, the mother’s nutritional investment in each egg determines whether her offspring can survive their first days of life, before they are capable of feeding on their own.

The First Days as a Trochophore

After fertilization, a scallop embryo divides rapidly and hatches into its first larval form, the trochophore, within one to two days. A trochophore is tiny, roughly the width of a human hair, and shaped like a spinning top. It grows small bands of cilia (hair-like projections) within a few hours of hatching, which it uses to swim upward toward the sea surface.⁵ At this stage, the larva does not feed. It runs entirely on the lipid reserves packed into the egg by its mother, which makes maternal provisioning so important to early survival.

The trochophore stage is brief. Within about four to five days of fertilization, the larva transitions into the veliger stage by developing a thin, D-shaped shell and a velum, a ciliated feeding organ that replaces the simple ciliary bands of the trochophore.⁵ This is when the larva begins to look and act more like a bivalve.

Growing Up as a Veliger

The veliger stage is the longest phase of scallop larval life, lasting roughly three to four weeks. During this period, the larva swims through the upper water column, using its velum both for locomotion and to capture tiny phytoplankton cells as food. The veliger’s shell, initially a simple D-shape, gradually grows and thickens through the deposition of calcium carbonate. This shell-building process makes the larva sensitive to the chemistry of the surrounding water. Laboratory studies have found that as carbon dioxide levels in seawater increase, fertilization rates and the shell length of early-stage larvae decrease.⁶ Later larval stages seem somewhat more resilient, and in some cases higher carbon dioxide actually enhanced growth, but the early veliger window is a vulnerable one.

As the veliger grows, it develops an increasingly sophisticated set of sense organs. These include an apical organ (a cluster of sensory cells at the top of the larva), rudimentary eye spots, and receptors in the developing mantle tissue.⁷ These senses become essential later, when the larva needs to evaluate potential surfaces for settlement. The veliger also begins migrating vertically within the ocean’s mixed layer, swimming up toward the surface during certain conditions and sinking back down during others. This behavior is not random; it plays a major role in determining where the larva ends up.

Where Currents Carry Scallop Larvae

A scallop larva is a weak swimmer by ocean standards, so it goes largely where the water takes it. But the vertical migrations veligers perform in the upper water column interact with currents at different depths, and this interaction has an outsized effect on dispersal. Modeling work covering nearly four decades of sea scallop spawning over the northeast U.S. shelf showed that the interannual variability in where larvae ended up was driven largely by how larvae behaved in the ocean mixed layer. The duration, frequency, and triggers of their vertical migration affected how long larvae stayed over productive habitat like Georges Bank, helping to sustain the persistent scallop aggregations found there.⁵ Ignoring larval swimming behavior in dispersal models led to overestimates of how connected distant scallop populations were, making behavior a surprisingly important factor for predicting recruitment.

Similar patterns appear in European waters. Dispersal modeling of king scallops in the English Channel identified that within a given bay, specific subpopulations act as the primary sources of larvae for the wider area. In the Bay of Saint-Brieuc, eastern subpopulations supplied most of the larvae for the whole bay, while in the Bay of Seine, central subpopulations dominated.⁸ Local hydrodynamics and where the most reproductively active adults happen to be both shape which populations seed the next generation. This has real management implications, since protecting a source subpopulation can matter more for regional stock health than blanket measures across an entire fishery.

The Pediveliger and the Search for Somewhere to Land

Around 30 to 35 days after fertilization, the veliger transforms into a pediveliger.⁵ This is the stage at which the larva develops a muscular foot and the capacity to produce byssus threads, sticky filaments it will use to anchor itself. The pediveliger also gains statocysts, gravity-sensing organs that help it orient in the water column.⁷ With these new tools, the pediveliger can actively swim downward across the thermocline (the boundary between warm surface water and cooler water below) and descend toward the bottom to search for a place to settle.

Settlement is not haphazard. Pediveligers are selective about where they attach, and the cues they respond to are more nuanced than simply finding a hard surface. Experiments with Pacific calico scallops showed that larvae settled in far greater numbers on living black murex snails covered in natural surface growth than on any other substrate. When researchers removed the surface organisms from snail shells, settlement dropped substantially. Bare shell fragments attracted almost no larvae at all, suggesting that the physical complexity of a shell is not what draws scallop larvae. Instead, the biological film and organisms growing on a surface serve as the primary attractant.⁹ There is a grim irony here: the murex snail is a scallop predator, but larvae do not appear to recognize it as a threat. They likely perceive the snail and its encrusted shell as reef structure.

Life as Spat

Once a pediveliger settles and attaches, it undergoes metamorphosis into a juvenile scallop known as spat. At this point, the velum is resorbed and the animal shifts from a swimming, planktonic existence to life on the bottom. Spat secrete byssus threads to anchor themselves to elevated structures: hydroids, filamentous algae, seagrass blades, and polychaete worm tubes are all common attachment sites.¹⁰ By clinging to these upright structures, spat stay above the sediment surface, which may help them access food-carrying currents and avoid bottom-dwelling predators.

Substrate matters even in controlled settings. Studies on great scallop juveniles found that attachment rates were highest on rough natural stone, with up to about three-quarters of juveniles attaching within 24 hours under calm conditions. Preconditioning the surface in flowing seawater for a week (allowing a natural biofilm to develop) significantly increased attachment, reinforcing the importance of biological surface coatings throughout the scallop’s early life.¹¹ Retention dropped as water velocity increased, which means that spat settling in high-energy environments face a higher risk of being swept away before they can establish themselves.

This early attached phase is temporary. Once spat reach a certain size, most scallop species release their byssus and drop to the soft sediment below, where they live as free-moving juveniles and eventually adults.¹⁰ Until they reach that size, however, spat are especially vulnerable. Their shells are thin, they cannot close their valves tightly, and they are largely immobile. Predation during this window can be intense and is considered one of the major bottlenecks in scallop population dynamics.

Bacterial Disease in Hatcheries and the Wild

For wild scallop larvae, the ocean is full of threats that operate below the scale of visible predators. Bacterial pathogens, particularly species of Vibrio, pose a serious risk during the larval and early spat stages. Mass mortality events caused by Vibrio species have plagued hatcheries breeding the Peruvian scallop Argopecten purpuratus, with strains of Vibrio anguillarum, V. splendidus, and V. bivalvicida repeatedly identified as the culprits during die-off events.¹² These gram-negative bacteria can sweep through a larval cohort in a matter of days under warm, crowded hatchery conditions.

Some researchers have found that certain larval batches survive Vibrio exposure better than others, and the difference appears to relate partly to the communities of bacteria the larvae themselves carry. The idea that a larva’s own microbial passengers might protect it from pathogens is an active area of research, with implications for how hatcheries might manage disease without relying solely on antibiotics.

Ocean Acidification and the Future for Scallop Larvae

Rising carbon dioxide levels in the atmosphere do not just warm the ocean; they also lower its pH, making the water more acidic. For an animal that builds a calcium carbonate shell during its most vulnerable life stage, this is a problem. A meta-analysis compiling results from published studies on larval bivalve responses to acidification found that larvae are the most susceptible life stage. When the resulting relationship was applied to a dispersal model for Atlantic sea scallops in the Mid-Atlantic Bight, the simulations showed that acidification-sensitive larvae experienced a 17% lower settlement success rate and over a 50% reduction in larval transport between major fishing grounds compared to larvae unaffected by acidification.¹³

The interaction between temperature and acidification adds another layer of complexity. In the modeling study, water with higher aragonite saturation (the mineral form scallop larvae use to build shells) could compensate for temperature-induced mortality in some scenarios, while in others the two stressors compounded each other.¹³ Where larvae are spawned, when they are spawned, and how fast they grow all influence whether they encounter water chemistry that helps or harms them. The geographic and seasonal patchiness of acidification means that some scallop populations may feel the effects much sooner than others.

Why So Few Survive

The sheer number of eggs a single scallop produces, sometimes millions per spawning event, is itself a clue to how many larvae die. The attrition is enormous at every stage. Eggs that go unfertilized are lost. Trochophores that fail to reach productive surface waters starve. Veligers get eaten by zooplankton, jellyfish, and filter-feeding animals. Pediveligers that settle on unsuitable substrate or in high-energy environments are swept away or picked off by predators. Spat that survive settlement face crabs, sea stars, snails, and flatworms. The few individuals that make it through all of these filters become the next generation of adults.

This brutal selection is not a flaw but a reproductive strategy. By flooding the water with larvae, scallops ensure that at least some offspring land in favorable conditions, even though the parents have no control over where any individual larva ends up. It also means that small changes in larval survival rates, whether from warmer water, shifting currents, acidification, or disease, can translate into large swings in adult population size years later. For fisheries managers and aquaculture operators alike, understanding each stage of this journey is what makes it possible to predict recruitment and manage scallop stocks before problems become visible on the seafloor.

1
PubMed Central. Sex change in scallop Patinopecten yessoensis: response to population composition?
2
Aquaculture. Induction of spawning by temperature and serotonin in the hermaphroditic tropical scallop, Pecten ziczac
3
Ecosphere. Fertilization success in scallop aggregations: reconciling model predictions and field measurements of density effects
4
PubMed. Srebp-1 bridges gonad development and lipid accumulation by regulating lipogenesis in noble scallop Chlamys nobilis
5
Progress in Oceanography. Impact of larval behaviors on dispersal and connectivity of sea scallop larvae over the northeast U.S. shelf
6
PubMed Central. Mixed effects of elevated pCO2 on fertilisation, larval and juvenile development and adult responses in the mobile subtidal scallop Mimachlamys asperrima (Lamarck, 1819)
7
Developments in Aquaculture and Fisheries Science. Biology and Ecology of Scallop Larvae
8
Ocean Dynamics. Modelling larval dispersal of the king scallop (Pecten maximus) in the English Channel: examples from the bay of Saint-Brieuc and the bay of Seine
9
Journal of Shellfish Research. Settlement of Pacific calico scallop larvae (Argopecten ventricosus, Sowerby II, 1842) on their predator, the black murex snail (Hexaplex nigritus, Philippi, 1845)
10
ICES Journal of Marine Science. Early post-settlement mortality of the scallop Pecten fumatus and the role of algal mats as a refuge from predation
11
Aquaculture. Factors affecting byssus attachment in juvenile scallops, Pecten maximus (L.)
12
Aquaculture. Scallop larvae resistant to a pathogenic Vibrio harbor host-associated bacteria with probiotic potential
13
Estuaries and Coasts. Meta-Analysis of Larval Bivalve Growth in Response to Ocean Acidification and its Application to Sea Scallop Larval Dispersal in the Mid-Atlantic Bight