Temporal Profile of Histological Changes During the Ovarian Cycle, Secretory Cycle of the Colleteric Gland, and Molt Cycle of the Mantle Cavity in the Mature Externa of the Parasitic Barnacle <i>Polyascus planus</i>.

Rhizocephalans are a group of parasitic barnacles that parasitize other crustaceans. The adult parasite consists of an external reproductive sac (the externa), which is connected by a stalk to a system of ramifying rootlets (the interna) that infiltrates the host. The mature externa of <i>Polyascus planus</i> undergoes a cycle, with its external color changes from yellow, transiently to yellowish brown as the embryos are developing inside a brood chamber (the mantle cavity), then to brown, and return again to yellow, upon the peak release of larvae. Hence, this cycle of <i>P. planus</i> is called the Yellow-Brown cycle after the distinct changes in color; the mature externa typically cycles 3-4 times before it becomes detached from the host. The objectives of the present study are to establish, based on histological observations, temporal profile of three cycles-the ovarian cycle, secretory cycle of the colleteric gland, and the molt cycle of the mantle cavity-that occur concurrently in the mature externa of <i>P. planus</i> and register the changes in chronological order on the timeline of the Yellow-Brown cycle. First, about 2 days after oviposition (1.7 ± 0.3 days postoviposition, dpo) during the Yellow stage, secondary vitellogenesis begins - a cohort of early vitellogenic oocytes grows in synchrony with significant and rapid accumulation of yolk bodies inside the developing oocytes. Simultaneously, the follicle and muscle cells undergo large-scale spatial rearrangements. By the time the externa is transitioning from the Yellow to Brown stage (7.7 ± 0.8 dpo), mature follicles with fully developed oocytes tightly enveloped by a single layer of follicle cells are formed and the muscle cells embedded in the inter-follicular tissue. In the colleteric gland, secretory activity of the epithelia begins 3-4 days after oviposition (3.5 ± 0.5 dpo), with the formation of the reticulated inner zone of the ovisac throughout the remainder of the Yellow stage and into the Brown stage, followed by the beginning of the secretion of the outer zone when the externa reaches the mid-Brown stage (9.8 ± 0.5 dpo). Finally, the molt cycle of the mantle cavity is initiated later than the other two cycles, entering early pre-molt (D₁) when apolysis-separation of the cuticle from the underlying epidermis-first becomes visible about 5 days postmolting (4.7 ± 0.3 days post-molting, dpm), and reaches late pre-molt (D₂) with deposition of new cuticle during the transition from the Yellow to Brown stage (8.8 ± 0.8 dpm). By the time the externa returns to the Yellow stage (0 day post-peak release of larvae, dppr), ovarian follicles are rupturing, ovisacs showing signs for imminent detachment, and cuticles in extensive apolysis (very late pre-molt, D_3-4). Subsequently, within a span of about three days after the externa reaches the Yellow stage, the mantle cavity molts (1.4 ± 0.2 dppr), followed by ovulation and ovisac detachment (2.5 ± 0.3 dppr) and finally deposition of the ovulated ova (oviposition) (3.7 ± 0.3 dppr) into the mantle cavity. Probable modes of endocrine regulation of these cycles are discussed in detail.