Facetotecta, or crustacean "y-larvae," occur in all the world's oceans although the adult forms remain completely unknown. At the metamorphic molt from the last naupliar instar to the terminal cypris larval stage a free carapace, six pairs of natatory thoracopods, and a segmented thorax and abdomen all develop anew. Unlike in earlier molts, the cephalic shield and the so-called "faciotruncal integument" usually remain together at this last naupliar molt, and the posterior "trunk" portion of the exuviae, while hollow, is not empty. In mounted preparations examined by phase contrast or differential interference contrast (DIC) microscopy, a ghost-like image of part of the cypris thorax, particularly the thoracopods and even their setae, is commonly visible inside the naupliar exuviae, and may be universally present in the Facetotecta. To investigate this "ghost," we used DIC and digital photographic stacking, and also scanning electron microscopy, on slide or stub-mounted final naupliar exuviae of an assortment of undescribed species of Facetotecta that had been reared from planktonic lecithotropic nauplii to the cypris stage at Sesoko Island, Okinawa, Japan, and at Keelung and Green Island, Taiwan. These techniques showed that the "ghost" is a delicate, three-dimensional, fibrous structure, essentially a sling-like mold or matrix with struts attached to the outer cuticle and pairs of deep pockets that previously held the thoracopods of the developing cypris y. Whether it is endoskeletal in nature, the (partial) exuvia of an additional instar, remnants of apoptosis, or something else is currently unknown. Nothing similar has been reported in other thecostracans, or in other crustaceans that undergo a similarly abrupt metamorphosis at the last naupliar molt.
Keywords: larval development; metamorphosis; naupliar morphology; y-cypris; y-nauplius.
© 2019 Wiley Periodicals, Inc.