Reproductive dynamics, gametogenic development and hermaphroditism of the variegated scallop Mimachlamys varia in the eastern Adriatic Sea

In the shallow, sun-drenched waters of the Mediterranean, a small but resilient shellfish known as the variegated scallop lives among the rocks and seagrass. While larger scallops often dominate the headlines of fisheries and aquaculture, this smaller species, with its colorful, patterned shell, holds a quiet promise for the future of farming seafood in the region. To understand how to raise these animals successfully, scientists must first understand their most fundamental biological rhythm: the cycle of making babies. For shellfish, this is not a single event but a complex process of building up energy, maturing eggs or sperm, and releasing them into the water. This cycle is driven by the changing seasons, the temperature of the sea, and the availability of food. If farmers know exactly when the scallops are ready to spawn, they can collect the best parents to start new generations in hatcheries, ensuring a steady supply of this tasty resource. Without this knowledge, the timing of farming efforts is a guess, often leading to poor results.

A team of researchers set out to map this hidden calendar for the variegated scallop in the eastern Adriatic Sea, specifically within the sheltered waters of Mali Ston Bay. They gathered a group of about 600 scallops and kept them in suspended baskets hanging just two meters below the surface, mimicking the conditions of a real farm. Over the course of a full year, the scientists visited these baskets roughly twice a month. At each visit, they carefully selected adult scallops, measured their shells and weight, and then gently removed their soft tissues to examine the gonads, the organs responsible for reproduction. To get the clearest possible picture, they preserved these tissues, sliced them into microscopic layers, and stained them with dyes that highlighted the cells. This allowed them to see exactly what was happening inside the animal: whether the cells were just starting to grow, fully packed with eggs or sperm, or had already released their contents. They also tracked the surrounding environment, recording the daily water temperature, saltiness, and the amount of microscopic plant life, or chlorophyll, floating in the water.

The story that emerged from the microscope slides was one of a long, drawn-out reproductive season rather than a single, synchronized burst. The researchers found that the scallops did not all mature and release their babies at the same time. Instead, the population operated on a staggered schedule. The females showed a clearer seasonal pattern, with their reproductive organs developing primarily during the cooler months of autumn and winter. By late winter and early spring, many females were fully mature, packed with large, nutrient-rich eggs. However, even as some females were ready to spawn, others were still developing or had already finished. This overlap meant that the bay likely saw a continuous, rolling release of eggs over several months, rather than one massive event. The males told a slightly different story; they were active for much longer, maintaining reproductive readiness throughout most of the year. While the females concentrated their efforts in the cooler half of the year, the males were often ready to fertilize eggs from late autumn all the way through early summer.

This difference in timing between the sexes is linked to the energy required to make babies. Producing large, yolk-filled eggs is a heavy investment for a female, requiring significant energy reserves that take time to build. Males, producing vast numbers of tiny sperm, can afford to stay ready for longer periods with less energy cost. The study also revealed a fascinating detail about the scallops' gender. The smallest scallops that could be clearly identified as male or female were always male. As the animals grew larger, they were more likely to be female. This suggests a life strategy where the scallops start life as males and may change to females as they get bigger and can produce more eggs. The researchers also spotted a few individuals that had both male and female tissues at the same time, though these were rare. These findings confirm that the variegated scallop in this region is flexible, adapting its reproductive schedule to the local environment rather than following a rigid, universal clock.

The environment played a clear role in this biological dance, though not in the simple way one might expect. The researchers discovered that the health of the gonads, measured by how much of the animal's body weight was made up of reproductive tissue, was highest when the water was coldest. As the seawater temperature dropped from the summer highs of around 27 degrees Celsius to the winter lows near 9 degrees, the scallops built up their reproductive reserves. The peak of this buildup occurred in February, right when the water was at its coldest. As the water began to warm up again in spring, the scallops started to release their gametes, and the reproductive tissue shrank. Interestingly, the amount of food available in the water, measured by chlorophyll levels, did not show a direct, immediate link to the reproductive cycle. This suggests that the scallops rely on food they have eaten and stored over time, rather than just what is floating by on the day they are sampled. The saltiness of the water remained mostly stable and did not seem to drive the changes.

For anyone looking to farm these scallops, these findings offer a practical guide. The best time to collect adult scallops to use as parents for a hatchery is from late autumn through early spring, when the animals are most likely to be mature and ready. However, because the population is not perfectly synchronized, farmers cannot expect to gather a batch where every single animal is at the exact same stage. They will need to be selective. The study also highlights that while the water temperature is a reliable signal for when the scallops are maturing, it is not the only factor at play. The animals are responding to a complex mix of their own internal rhythms and the history of food they have consumed. By understanding this prolonged and flexible cycle, farmers in the Adriatic can better plan their operations, moving away from guesswork and toward a more reliable method of cultivating this small but valuable shellfish. The variegated scallop, it turns out, is a creature of patience, spreading its reproductive efforts over many months to ensure its survival in the changing seas.