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<p>Facetotecta, or “y-larvae”, are mysterious planktonic crustaceans that are known only from their larval instars, but which are often assumed to be endoparasitic as adults. Dozens of mostly undescribed forms occur in the shallow-water plankton over a fringing reef at Sesoko Island, Okinawa, Japan. Recently, it has become possible to discriminate the different forms of their nauplius-stage larvae (“y-nauplii”) in a replicable way. A large year-to-year overlap in morphospecies recovered during fieldwork at Sesoko Island in 2018, 2019, and 2023 suggests that a full inventory is close at hand. To date, 49 morphospecies of y-nauplii (8 planktotrophic, 41 lecithotrophic) have been recognized in the area, among which three have been formally described. A detailed analysis of the temporal fluctuations in abundance during October 2023 showed that most morphospecies were rare, but two were particularly common: Type A*, a planktotroph with a long (&gt; 3&#xa0;weeks) period of naupliar development allowing for long-distance dispersal, and Type AG*, a lecithotroph with a short (3&#xa0;days) period of development that implies rapid local recolonization. During the survey, both types showed distinct, largely non-overlapping peaks in abundance, related perhaps to their different dispersal/feeding strategies. An examination of the morphology of the swimming/feeding appendages in y-nauplii of Types A* and AG*, together with a mapping of feeding versus non-feeding nauplii on a recent comprehensive phylogeny of Facetotecta, suggests that broader taxonomic coverage of naupliar feeding structures in this group may provide useful information regarding the evolutionary direction of planktotrophy versus lecithotrophy in marine larvae.</p>

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Abundance survey of crustacean “y-larvae” (Thecostraca: Facetotecta) over a fringing reef in Okinawa, Japan, with special reference to the two dominant planktotrophic and lecithotrophic naupliar types

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  • 发表日期：

    2025年1月20日

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  • Jørgen Olesen,
  • Mark J. Grygier,
  • Ferran Palero,
  • Niklas Dreyer,
  • Danny Eibye-Jacobsen,
  • Yoshihisa Fujita

摘要

Facetotecta, or “y-larvae”, are mysterious planktonic crustaceans that are known only from their larval instars, but which are often assumed to be endoparasitic as adults. Dozens of mostly undescribed forms occur in the shallow-water plankton over a fringing reef at Sesoko Island, Okinawa, Japan. Recently, it has become possible to discriminate the different forms of their nauplius-stage larvae (“y-nauplii”) in a replicable way. A large year-to-year overlap in morphospecies recovered during fieldwork at Sesoko Island in 2018, 2019, and 2023 suggests that a full inventory is close at hand. To date, 49 morphospecies of y-nauplii (8 planktotrophic, 41 lecithotrophic) have been recognized in the area, among which three have been formally described. A detailed analysis of the temporal fluctuations in abundance during October 2023 showed that most morphospecies were rare, but two were particularly common: Type A*, a planktotroph with a long (> 3 weeks) period of naupliar development allowing for long-distance dispersal, and Type AG*, a lecithotroph with a short (3 days) period of development that implies rapid local recolonization. During the survey, both types showed distinct, largely non-overlapping peaks in abundance, related perhaps to their different dispersal/feeding strategies. An examination of the morphology of the swimming/feeding appendages in y-nauplii of Types A* and AG*, together with a mapping of feeding versus non-feeding nauplii on a recent comprehensive phylogeny of Facetotecta, suggests that broader taxonomic coverage of naupliar feeding structures in this group may provide useful information regarding the evolutionary direction of planktotrophy versus lecithotrophy in marine larvae.