Alternating sexuality

Marine invertebrate reproduction is a crucial yet underexplored aspect with significant implications for biosecurity and aquaculture. The interplay between temperature, food availability, and gamete production, as well as the synchronized gamete release in Broadcast spawners, presents intriguing mysteries.
Today, we delve into the captivating evolution of alternating sexuality in invertebrates, a research interest of Ocean Wolf's founder since his PhD. Recent literature has speculated on sequential hermaphroditism in the polychaetes subfamily serpulins based on a biased sex ratio, size disparities, and occasional hermaphroditic observations suggest such possibilities. However, conflicting evidence regarding protandry, where individuals transition from female to male, has emerged from different studies, with debates centring on sex ratios and size differences.
Previous studies suggest varying sex ratios, as well as marginal size differences between male and female serpulins, as support for protandry. This challenges previous assumptions and emphasizes the need for further investigation.

The evolutionary journey from asexual reproduction to dioecious individuals began with asexual reproduction, producing genetic clones. The limitations of reduced genetic diversity led to the trend of sexual reproduction, likely starting with simultaneous hermaphrodites. Sequential hermaphroditism evolved to mitigate the risks of self-fertilization and energy consumption, with protandry (males first) favouring smaller, younger males or protogyny (female sex first), highlighting the stronger caregiving role of males.

While sequential hermaphrodites can change sex once, they remain susceptible to self-fertilization and face elevated energetic costs. The move towards dioecious individuals appeared as the more efficient means of ensuring genetic recombination. However, amongst sessile marine invertebrates, a novel form of hermaphroditism, "alternating sexuality", is observed predominantly in bivalves. This phenomenon allows individuals to change sex multiple times based on factors such as population composition, food availability, and energy levels, potentially facilitated by the absence of reproductive organs.
Ocean Wolf proposes that other sessile invertebrates may also exhibit alternating sexuality, enhancing reproductive output and flexibility, especially in species limited in mobility. Further research in this area not only contributes to understanding invertebrate reproductive evolution but also holds relevance for aquaculture and biosecurity, particularly in comprehending the success of non-native fouling organisms. For more information on the reproductive biology of marine invertebrates, contact us at www.ocean-wolf.com and stay tuned for updates.

Below are some pictures from Paul's work,
some sections of hermaphrodites belonging to S. cariniferus