The Reproductive and Larval Ecology of Marine Bottom Invertebrates

The study of reproductive and larval ecology in marine bottom invertebrates is crucial for understanding the dynamics of these populations and their ecological conditions. Gunnar Thorson’s research article, published in 1950, focuses on the breeding and larval development stages of these animals, shedding light on the wastage of eggs and larvae, various modes of reproduction and development, and the impact of environmental factors. This article will provide an overview of Thorson’s key findings and their implications in the context of marine ecosystems in 2023.

The Importance of Studying Breeding and Larval Development

Thorson emphasized the significance of analyzing the sensitive stages within the life cycle of animal populations, especially the breeding and larval development period. These stages are crucial because they determine the survival and reproductive success of marine bottom invertebrates. By understanding the reproductive strategies and larval ecology of these organisms, scientists can gain valuable insights into their population dynamics and the factors influencing their distribution.

Effects of Different Modes of Reproduction and Development

Thorson observed that animal populations on the sea bottom maintain a consistent composition of species over long periods of time, despite using different modes of reproduction and development. Species that produce a large number of eggs tend to have higher wastage of eggs and larvae compared to those that produce fewer eggs. This suggests that the wastage of eggs in the marine environment is more significant than in terrestrial or freshwater ecosystems.

Real-World Example: The Pacific Oyster (Crassostrea gigas)

In the context of the Pacific oyster, which is known for its high fecundity, the large number of eggs produced by females contributes to significant wastage of eggs and larvae in natural populations. The Pacific oyster can release millions of eggs into the water column, and only a fraction of them will successfully develop and settle, thus maintaining a stable population. This example demonstrates the trade-off between high reproductive potential and the wastage of eggs and larvae in marine bottom invertebrates.

Factors Contributing to Wastage of Eggs and Larvae

Thorson identified several factors that contribute to the wastage of eggs and larvae in the marine environment. One key observation was that fluctuations in the population numbers from year to year indicate species with a long pelagic larval life, while constant occurrence indicates species with a short pelagic life or non-pelagic development. This suggests that pelagic larvae, which spend extended periods in the water column, experience higher wastage compared to non-pelagic larvae.

Another factor contributing to the wastage of eggs and larvae is the heavy predation pressure from other organisms. Larvae serve as a crucial food source for a range of predators, including other pelagic larvae, holoplanktonic organisms, and bottom-dwelling animals. For example, a medium-sized Mytilus edulis, a commonly found mussel, can filter and kill about 100,000 pelagic lamellibranch larvae within 24 hours in a Danish fjord during the peak breeding season. The impact of predation on egg and larval wastage highlights the challenges faced by these populations in maintaining stable numbers.

Types of Pelagic Larvae and Their Variation

Thorson classified three types of pelagic larvae based on their ecological characteristics. The first type, lecithotrophic larvae, originates from large yolky eggs and exhibits independence from plankton as a food source during pelagic life. This type is absent from high arctic seas but constitutes approximately 10% of species with pelagic larvae in other seas.

The second type, planktotrophic larvae with a long pelagic life, originates from small eggs and relies on the plankton as a food source for growth. This type constitutes the majority of pelagic larvae, ranging from 55-65% in boreal seas to 80-85% in tropical species.

The third type, planktotrophic larvae with a short pelagic life, maintains the same size and organization from hatching to settling. This type represents approximately 5% of species in all Recent seas.

Real-World Example: Sea Urchins

Sea urchins are an example of marine bottom invertebrates that exhibit different types of pelagic larvae. For instance, the common sea urchin (Strongylocentrotus purpuratus) has a lecithotrophic larval stage, while the purple sea urchin (Strongylocentrotus purpuratus) has a planktotrophic larval stage. The variation in larval types within the sea urchin family demonstrates the diversity of reproductive strategies and larval ecology within marine ecosystems.

Impact of Environmental Factors on Larval Life

Temperature and food availability play critical roles in shaping the pelagic life of marine invertebrate larvae. Thorson highlighted that northern and tropical larvae spend a similar duration in the plankton, around three weeks, when exposed to temperatures within their tolerable range. However, individual species may exhibit variations in pelagic life length based on environmental conditions.

Temperature affects larval growth and metamorphosis rates. Low temperatures can delay these processes, allowing enemies to have a longer time to prey on larvae. Similarly, poor food conditions lead to slow growth, prolonging the pelagic life of larvae and increasing their vulnerability to predators. These indirect effects of temperature and food availability contribute to the wastage of eggs and larvae in the marine environment.

Reasons for Non-Pelagic Development

Certain marine bottom invertebrates have a non-pelagic development strategy, meaning their embryos do not undergo a pelagic larval phase. Thorson identified two scenarios for non-pelagic development: (a) large yolky eggs, which result in hatching young of the same species at the same stage of development, and (b) small eggs that feed on nurse eggs, leading to significant size variations among embryos at the hatching stage.

Non-pelagic development is likely favored in environments such as the Arctic and Antarctic seas, where temperatures and food availability are limited. By hatching at a more advanced stage of development, these species increase their chances of survival in harsh conditions. The ability to support non-pelagic development through smaller eggs is more prominent in arctic species compared to their Antarctic counterparts.

Real-World Example: Antarctic Krill (Euphausia superba)

Euphausia superba, commonly known as Antarctic krill, exhibits non-pelagic development as a survival strategy in the Antarctic seas. This species relies on a brood protection and viviparity mechanism for the development of its embryos. By carrying their eggs until hatching and providing nourishment, female krill ensure the survival of their offspring in a harsh environment with limited food availability and short periods of phytoplankton production.

Finding Suitable Substratum for Settling

The process of finding a suitable substratum for settling is crucial for larval survival and successful recruitment. Thorson observed that young pelagic larvae exhibit photopositivity, crowding near the surface. As the larvae approach the metamorphosis stage, they become photonegative, seeking shelter in deeper waters. Several marine invertebrates, including polychaetes, echinoderms, and prosobranchs, prolong their pelagic life until encountering a suitable substratum.

These larvae rely on their negative phototactic behavior, combined with ocean currents, to transport them over wide bottom areas. By intermittently testing the substratum at intervals during their descent towards settling, they increase their chances of finding a suitable place to develop further. This strategy enhances larval dispersal and colonization in marine ecosystems.

The Ecological Implications of Continuous Currents

Continuous currents, particularly those from the continental shelf towards the open ocean, can have significant impacts on the composition of marine fauna. Larvae transported by these currents from coastal areas to the deep sea may perish due to the absence of suitable settling substratum or adverse environmental conditions. However, the significance of continuous currents varies in different regions.

For example, in the Gulf of Guinea, continuous currents deeply influence the fauna composition due to the extensive transportation of larvae away from the coast. In contrast, their impact is relatively small along the European western coast and southern California. The interplay between continuous currents and larval dispersion shapes the distribution and abundance of marine bottom invertebrates in different ecosystems.

Main Sources of Waste Among Larvae and Their Impact

The wastage of eggs and larvae among marine bottom invertebrates primarily occurs due to predation by other organisms. Thorson highlighted that the toll levied by enemies, including other pelagic larvae, holoplanktonic animals, and bottom-dwelling organisms, is the most essential source of waste among larvae. This high predation pressure contributes to the regulation of population sizes and shapes community dynamics.

The high numbers of eggs produced by some species help compensate for the wastage caused by predation, ensuring the persistence of populations. However, the heavy predation pressure during the free-swimming pelagic life stage remains a critical factor influencing population dynamics and reproductive success.

Influence of Reproductive Strategies on Species Distribution

Reproductive strategies significantly impact the distribution of marine bottom invertebrate species. Thorson highlighted that most groups of marine invertebrates exhibit only one mode of reproduction and development, which restricts their area of distribution. In contrast, polychaetes demonstrate a remarkable lability in their mode of reproduction and development, enabling them to compete in wide areas of the sea.

For example, compared to other Western European marine invertebrates, a higher percentage of polychaete species has been found in the Indian Ocean and along the Californian coast. This flexibility in reproductive strategies contributes to the broader range of habitats occupied by polychaetes and their ability to adapt to different environmental conditions.

Real-World Example: Ragworms (Nereididae)

Ragworms, a diverse group of polychaete worms belonging to the family Nereididae, showcase the lability in their reproductive strategies. Some ragworm species reproduce sexually, while others can reproduce through fragmentation or fission, allowing them to colonize various marine ecosystems. Their adaptive reproductive strategies contribute to their wide distribution across different seas and continents.

Determining Larval Development through Adult Shells

Thorson discovered that the shape of the top whorls, or apex, of adult shells in prosobranchs (a group of marine gastropod mollusks) can provide insights into their larval development. The differences in shell shape can indicate whether a species undergoes a pelagic or non-pelagic development. This finding is significant not only for studying the larval development of prosobranchs but also for understanding the ecological conditions of earlier geological periods through fossil species.

Real-World Example: Conus Mollusks

Conus mollusks, commonly known as cone snails, demonstrate the potential for using adult shell characteristics to infer larval development. The presence of a prominent spire in the adult shell suggests a planktotrophic larval stage, while a short spire indicates a non-pelagic development. Understanding the larval ecology of conus mollusks has practical implications for conservation and management efforts, as some species are ecologically important and possess venomous properties.

Takeaways

Gunnar Thorson’s research on the reproductive and larval ecology of marine bottom invertebrates provides valuable insights into the dynamics of these populations in marine ecosystems. By understanding the modes of reproduction and development, wastage factors, and the ecological implications of various larval characteristics, scientists can gain a deeper understanding of the factors influencing the distribution and abundance of marine bottom invertebrates. This knowledge is essential for conservation efforts and for effectively managing these fragile ecosystems.

To access the original research article by Gunnar Thorson, please click here.