Meroplankton are organisms that spend only part of their life drifting in the water column as plankton, typically during their larval or juvenile stage, before settling into a permanent adult habitat. The term comes from the Greek “meros,” meaning part. Unlike holoplankton (jellyfish, krill, and copepods that remain plankton for their entire lives), meroplankton are temporary members of the plankton community. They include the eggs and larvae of animals you’d recognize in their adult forms: crabs, starfish, clams, sea urchins, lobsters, barnacles, and many species of fish.
Which Animals Produce Meroplankton?
The list is surprisingly long. Meroplankton includes the early life stages of both stationary bottom-dwelling animals (sponges, anemones, corals, barnacles) and mobile ones (crabs, lobsters, snails, worms). It also includes eggs and larvae of fish, squid, and octopus, along with the spores of seaweeds. In one survey of the northwest North Sea, researchers recorded 24 species of crab larvae in just two years and 34 species of clam larvae in a single year, compared to only 17 copepod species found over a full decade of sampling. The diversity of meroplankton in coastal waters can be staggering, often exceeding the diversity of permanent plankton.
Larval Stages and What They Look Like
Meroplanktonic larvae look nothing like their adult forms. Each major animal group has its own distinct larval stage with a specific name. Clams and snails produce tiny “veliger” larvae, free-swimming forms with a small shell and hair-like structures for movement. Crabs and lobsters hatch as “zoea” larvae, spiny and translucent creatures that pass through multiple stages before resembling anything like their parents. Barnacles release “nauplius” larvae, which are so different from the hard-shelled adults cemented to rocks that scientists originally classified them as separate species. Sea urchins and brittle stars produce elaborately shaped larvae with long, delicate arms that can spend months drifting in the open water.
These larvae may pass through several distinct stages before they’re ready to settle. Crab larvae, for example, progress through zoea 1, zoea 2, and zoea 3 stages, growing and changing shape at each step. Barnacle larvae similarly advance through multiple nauplius stages before metamorphosing into their adult form.
How Meroplankton Disperse and Settle
For adult animals that are slow-moving or permanently attached to the seafloor, releasing larvae into the water column is the primary way to spread to new habitats. Ocean currents carry these tiny organisms anywhere from meters to hundreds of kilometers from where they were spawned. The longer a larva stays in the plankton, the farther it can travel, which allows species to colonize new territories and maintain genetic connections between distant populations.
The process isn’t random. Current strength, direction, and variability all shape where larvae end up. Stronger currents push larvae farther and spread them over wider areas. Larvae in upwelling zones, where deep water rises to the surface, face particularly dynamic transport. Changes in ocean circulation patterns can shift dispersal distances significantly, potentially pushing larvae toward or away from suitable adult habitat.
Eventually, larvae must find an appropriate spot on the seafloor, undergo metamorphosis, and transition to their adult form. This settlement process is a critical bottleneck. Some larvae feed on plankton during their drift (a strategy called planktotrophy), while others rely entirely on energy reserves packed into the egg by their mother (lecithotrophy). Feeding larvae can afford to drift longer and travel farther, but they also face more risk from starvation and predation.
Survival Rates Are Extremely Low
Life as meroplankton is dangerous. Traditional estimates suggested that for a species with a 30-day larval period, only about 0.25% of larvae survive to settle, meaning roughly 1 in 400. More recent analyses have revised that figure upward to around 1.5% survival over a 30-day period, about 1 in 67. That’s an order of magnitude better than older estimates, but still remarkably low. Mortality rates vary enormously between species, ranging from relatively mild to near-total losses depending on predation pressure, food availability, and how long the larval stage lasts.
To compensate, many meroplanktonic species produce enormous numbers of eggs. A single female crab or oyster can release millions of eggs in a season, banking on the slim chance that a few will survive to adulthood. This reproductive strategy is why meroplankton can dominate coastal waters during peak spawning seasons, sometimes outnumbering permanent plankton entirely.
Role in Marine Food Webs
Meroplankton occupy a dual role in ocean ecosystems. As consumers, larvae feed on microscopic algae and other tiny organisms, competing with permanent plankton for food. As prey, they’re eaten by fish, jellyfish, and filter-feeding animals, funneling energy through the food web. During spring and summer, when spawning peaks, meroplankton can become the dominant food source in coastal and fjord ecosystems.
They also serve as the essential link between the open water and the seafloor. Adult barnacles, mussels, worms, and sea stars form the backbone of seafloor communities, and every one of those adults arrived as a planktonic larva. The supply of settling larvae determines how many adults will populate a reef, a rocky shore, or a patch of seafloor in any given year. This connection between the drifting and bottom-dwelling worlds, sometimes called benthic-pelagic coupling, makes meroplankton a linchpin in coastal ecosystem health.
Vulnerability to Ocean Acidification
Meroplanktonic larvae are particularly sensitive to changes in ocean chemistry. Many of them, including the larvae of clams, snails, sea urchins, and crabs, build tiny calcium carbonate shells or skeletons during their first days of life. As the ocean absorbs more carbon dioxide and becomes more acidic, the water’s chemistry makes it harder for these larvae to form and maintain those structures.
Studies on abalone larvae have found that higher CO2 levels in seawater produce significantly higher rates of shell malformation and smaller shells. When CO2 concentrations rise above a critical threshold (roughly 1,000 to 1,300 microatmospheres), shell defects increase sharply. The effects depend on both the intensity and duration of exposure. Rising water temperatures add another layer of stress, reducing the proportion of larvae that can swim normally. Research on larval bivalves has shown that acidified water combined with low oxygen leads to reduced survival, slower growth, and delayed development. Because these larvae are building their first protective structures during the most vulnerable phase of their lives, even modest chemical changes in seawater can ripple through to adult population sizes on the seafloor.