Abstract
BackgroundThe phylogenetic status of the aplacophoran mollusk taxon Solenogastres (Neomeniomorpha) is controversially discussed. Some authors propose the clade to represent the most basal branch within Mollusca, while others claim aplacophoran mollusks (Solenogastres and Caudofoveata) to be derived. Larval characters are central in these discussions, specifically the larval test (calymma, apical cap), the ontogeny of the epidermal scleritome, and the proposed absence of larval protonephridia. To date, developmental data are available for five solenogaster species, but most reports are incomplete and need confirmation.ResultsWirenia argentea deposit small batches of uncleaved embryos that are tightly enclosed by a smooth and transparent egg hull. Cleavage is spiral and unequal. The ciliated larvae hatch about 45 hours after deposition and swim actively in the water column. Within 48-60 hours after hatching they become mushroom-shaped with a pronounced apical cap partly enclosing a posterior trunk. The cells covering the apical cap are large and cleavage arrested. On the apical cap there is a prominent prototrochal band of compound cilia and an apical ciliary tuft and the trunk bears a terminal ciliary band (telotroch). Obscured by the apical cap, a ciliary band originates in the stomodaeal pore and surrounds the trunk. As development is proceeding, the trunk elongates and becomes covered by cuticle with the exception of a ventral ciliary band, the future foot. The larvae have a pair of protonephridia. At 5 days after hatching they begin to settle and within the following 7-9 days the apical cap is gradually reduced. Scattered epidermal sclerites form under the cuticle. Wirenia argentea lack iterated groups of sclerites at any developmental stage. At 40 days after hatching, the postlarvae have a fully developed foregut, but the midgut and hindgut are not yet interconnected.ConclusionsSolenogastres develop via a trochophore-like lecitotrophic larva with a preoral apical cap that at least partly represents an enlarged prototrochal area. Homology of this larval type (pericalymma larva) to test cell larvae of other spiralian clades is doubtful. The ontogeny of W. argentea does not provide any evidence for a derived status of Solenogastres within Mollusca.
Highlights
The phylogenetic status of the aplacophoran mollusk taxon Solenogastres (Neomeniomorpha) is controversially discussed
Based on the presence of a comparable larval type in certain polychaetes and sipunculans, the pericalymma larva has been interpreted as the ancestral larval type of mollusks and even of all spiralians [7,11], a view strongly refuted by others [[12], for review]
We studied the ontogeny of Wirenia argentea, a solenogaster species classified within the Gymnomeniidae (Pholidoskepia), a clade supposedly representing an early branch within Solenogastres [5]
Summary
The phylogenetic status of the aplacophoran mollusk taxon Solenogastres (Neomeniomorpha) is controversially discussed. Some authors propose the clade to represent the most basal branch within Mollusca, while others claim aplacophoran mollusks (Solenogastres and Caudofoveata) to be derived. Some authors propose a shell-bearing chiton- (i.e., polyplacophoran)-like ancestor as basal for Mollusca, a hypothesis claimed to be supported by ontogenetic data showing a serial arrangement of dorsal epidermal sclerites in species of Solenogastres [17,18] and Caudofoveata [19]. In larvae of Epimenia babai, by contrast, epidermal sclerites emerge scattered over the entire dorsal surface without showing any seriality [22,23]. To evaluate these contradictory findings, data on additional solenogaster species are crucial
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