The Embryonic Development of the Cotylean Polyclad <i>Phrikoceros jannetae</i>

ABSTRACT Polyclads exhibit distinct developmental modes ranging from direct to indirect development, with several transitional stages also recognised. The existence of an indirect developmental mode in polyclads with a planktonic life history stage in the form of a free‐swimming larva is unique among all free‐living flatworms and makes polyclads a compelling subject for comparative studies. Early development is robust across different polyclad species, including direct and indirect developers, stemming from the polyclad flatworms' retention of a stereotyped spiral cleavage pattern, a trait shared with other phyla grouped as the Spiralia. Using a live‐imaging approach, we describe the embryonic development of the cotylean polyclad Phrikoceros jannetae (Pseudocerotidae), documenting all major developmental stages from meiosis to the formation of a three‐eyed, six‐lobed larva. We characterise key events such as cytoplasmic perturbations during meiosis, early cleavages in macromeres, and the appearance of larval morphological features including eyes and lobes. By aligning individual developmental trajectories to the 4‐cell stage, we provide a coherent timeline that facilitates comparisons across embryos. This work expands the developmental knowledge of P. jannetae and contributes to a broader understanding of embryogenesis within Pseudocerotidae. Additionally, our observations reinforce previous findings on symmetry breaking in polyclads, particularly the pivotal role of micromere 4d, a process often overlooked in studies relying solely on static images.