About the Cover
Cover
The images on the cover are a pair of veliger larvae. They developed from fertilized eggs of Tectura scutum, a patellogastropod (true limpet), were maintained at 12 {degree}C, and were photographed at two different stages during their pelagic development. The larva on the left, at 46 hours after fertilization, has neither undergone ontogenetic torsion nor completed the secretion of its protoconch (embryonic shell). The one on the right, at 62 hours after fertilization, has completed both torsion and the secretion of its pre-metamorphic shell. Nonfeeding larvae of T. scutum become competent to metamorphose about 7 days after fertilization [see Fig. 1B (Page, 2002), p.10 this issue].
The apical epidermis of the left-hand larva, located within the ring of locomotory cilia, bears a long apical ciliary tuft. The cells generating this tuft are intimately associated with the neurons of a putative larval sensory structure, the apical ganglion. Shortly after ontogenetic torsion, the long apical ciliary tuft is lost (right-hand larva), and the three large cells that gave rise to it also disappear from the apical epidermis.
Larvae with very long apical ciliary tufts are characteristic of patellogastropods, but this is the only gastropod clade in which such an apical structure appears. But because similar structures do arise from the apical pole of larvae in many other molluscan classes, and because current phylogenies designate patellogastropods as the most basal lineage of extant gastropods, the long apical ciliary tuft is probably a plesiomorphic trait for gastropods.
Rather than an apical tuft, the larvae of other gastropod clades bear, on either side of the midline of the apical surface, a pair of multiciliated cells, each of which generates a relatively short ciliary tuft. In this issue of The Biological Bulletin (pp. 6-22), Louise R. Page describes--through electron microscopy and serotonin immunohistochemistry--the structure of the apical sensory organ (i.e., the neurons plus ciliogenic cells) in the larvae of T. scutum. She reports that these larvae have, not only an apical tuft of very long cilia, but also lateral, multiciliated cells that produce two shorter tufts. These short tufts are probably homologs of those in the larvae of other gastropods. The embryonic derivation of the apical sensory organs appears to be broadly similar among all molluscs, but a clear understanding of the developmental evolution of this structure awaits further comparative data.
(Credits: Photos by L. R. Page, University of Victoria, British Columbia, Canada; other graphics by Beth Liles, MBL.)
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Copyright © 2003 by the Marine Biological Laboratory.
