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The accessory nidamental gland (AN gland), a reproductive organ of the mature female squid Loligo opalescens, harbors a dense culture of bacteria of unknown function. A multilayered sheath surrounding the L. opalescens egg case is similarly colonized by bacteria that presumably originate in the AN gland, as evidenced by their presence in the egg case at oviposition. This study investigates how these bacteria are transmitted to juvenile squid and examines some morphological consequences of bacterial colonization of AN gland tissues. By observing the structure of the AN gland in adults and the development and bacterial colonization of the gland in juveniles raised in captivity, we determined that the AN gland was absent in newly hatched squid and did not appear until 87 days post-hatching. At 129 days posthatching, the organ displayed tubules composed of a single layer of epithelial cells and expressing numerous cilia and microvilli. These tubules were not yet fully formed and thus were open to the mantle cavity and external seawater, possibly to aid in the acquisition of microorganisms. Since the AN gland developed a considerable time after hatching, it most likely acquires its symbionts horizontally from environmental seawater and not vertically from the egg case sheath. The switch from expression of cilia to production of microvilli on the epithelial cell surface may dictate the competence of the tissue for bacterial colonization. Electron microscopic examination of juvenile and adult AN glands revealed that an analogous process occurs during the development of the related light organ of other cephalopod species that harbor symbiotic bacteria.

Bacterial Symbionts Colonize the Accessory Nidamental Gland of the Squid Loligo opalescens via Horizontal Transmission