MALACOLOGIA, 1996, 37(2): 443-511
MORPHOLOGY AND PHYLOGENETIC RELATIONSHIPS OF CERTAIN
PYRAMIDELLID TAXA (HETEROBRANCHIA)
John B. Wise
Houston Museum of Natural Science, One Hermann Circle Drive,
Houston, Texas 77030, U.S.A.
ABSTRACT
The marine gastropod family Pyramidellidae is poorly known. Although numerous and world-
wide, the anatomies of only a few species are known, and our understanding of this family's
taxonomy and systematics is based almost entirely on shell characters.
Eight pyramidellid genera and 12 species were dissected, sectioned, and examined with
SEM. Traditionally used gastropod characteristics are either absent (e.g., radula) or of little use
(e.g., reproductive system minus the penis), because they are undiversified morphologically in
the taxa examined herein. Characters of gut, mantle cavity, and penial complex proved most
useful in developing an understanding of how the taxa in the present study may be defined.
Phylogenetic analysis of 13 taxa and 28 characters yielded six equally parsimonious cla-
dograms of 67 steps and a consistency index of 68%. New systematic standards are proposed
for defining (on the basis of synapomorphies) three of the four traditional pyramidellid subfam-
ilies, the new subfamily Sayellinae, and the new genera Houbricka and Petitella.
To test the hypothesis that protoconch configuration is a reflection of developmental mode
and not phylogeny, protoconch characters were eliminated from a second phylogenetic anal-
ysis. This yielded one cladogram, which when a taxon's developmental mode and host(s) are
known, support the contention that protoconch shape is a result of developmental mode.
' Historically shell characters, to the exclusion of soft-part anatomy, have been used to assign
taxa to the various pyramidellid genera. Therefore, it might be tempting to rely more on ana-
tomical characters and treat shell characters a priori as homoplasious. However, this study
shows that members of the subfamily Pyramidellinae could only be distinguished by concho-
logical characters, whereas in other taxa soft-part anatomy proved the most phylogenetically
useful. The present study indicates that all characters (= total evidence) should be used in any
phylogenetic analysis.
Key words: morphology, phylogeny, Pyramidellidae, synapomorphies, cladograms.
INTRODUCTION
The Pyramidellidae Gray, 1840, are marine
ectoparasitic gastropods that occur in boreal
to tropical waters worldwide, and from the
intertidal zone to several thousand meters.
Pyramidellids feed on a variety of invertebrate
hosts (Robertson, 1957; Ankel & Christensen,
1963; Robertson & Orr, 1961; Fretter & Gra-
ham, 1962; Scheltema, 1965; Boss & Merrill,
1965; Bullock & Boss, 1971; Robertson &
Mau-Lastovicka, 1979; Boss, 1982). They
pierce the host's tissues with a buccal stylet
and remove host body fluids by the muscular
action of their buccal pump (Ankel, 1949a;
Fretter & Graham, 1949; Maas, 1965; Wise,
1993).
The systematic position of the Pyrami-
dellidae has been controversial for over 130
years (Boss, 1982). This controversy, al-
though caused in part by the lack of informa-
tion about this family, is also due to changing
views about gastropod phylogeny (for a
review of the current state of gastropod phy-
logeny and systematics, see Bieler, 1992).
Traditionally, gastropods have been divided
into three subclasses: Prosobranchia, Opis-
thobranchia, and Pulmonata, with the proso-
branchs as primitive gastropods giving rise to
both opisthobranchs and pulmonates. In this
scheme, pyramidellids occupy an intermedi-
ary position between the prosobranchs and
opisthobranchs (Boss, 1982). Because they
have a spirally coiled calcareous shell into
which the entire body is retractable, a foot
with an operculum, a long proboscis, and an
anteriorly oriented mantle cavity, most early
authors placed them in the Prosobranchia,
but because they also have such character-
istics as a palliai kidney, subepithelial eyes on
the median side of the tentacles, an ovotestis,
and a heterostrophic protoconch, later au-
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