Reference: Biol. Bull. 174: 276-286. (June, 1988) Penetrantia clionoides, sp. nov. (Bryozoa), a Boring Bryozoan in Gastropod Shells From Guam MIRIAM J. SMYTH Department of Zoology, University of Maryland, College Park, Maryland 20742* Abstract. Gastropod shells from 1 5 sites around Guam contain specimens of a new species of boring bryozoan in the genus Penetrantia. This is the first boring bryozoan reported from the Mariana Islands and the first represen-tative of the genus in the tropical Indo-West Pacific. The distinctive sculpturing on the upper surface of the oper-culum sets Penetrantia clionoides apart from other spe-cies in the genus. The new species is shown to be most similar to Penetrantia operculata from Hawaii. Scanning electron microscopy reveals previously unreported de-tails of the ultrastructure of recent boring Bryozoa. Evi-dence of the inhibitory effects of crustose coralline algae on these borers is presented. The disputed systematic po-sition of the boring bryozoans is discussed and the situa-tion is shown to be complicated by the discovery here of an unusual form of aperture to the borehole, which displays both ctenostomate and cheilostomate character-istics. Introduction This report describes the first boring bryozoan in the genus Penetrantia from the tropical Indo-West Pacific. It is based on collections made around Guam as part of a larger study investigating the interactions among boring organisms (organisms that penetrate a calcareous sub-strate and live within it), gastropod shells, and crustose coralline algae. An unidentified borer occurred at all 15 study sites, boring mainly in the parietal region of the gastropod shells. Upon investigation, this organism was recognized as being a boring bryozoan. The boring Bryozoa are an ethologically defined group of aberrant species that live within solid calcareous sub-strates in the marine environment (Pohowsky, 1978). Received 16 March 1987; accepted 25 March 1988. * Present address: Life Sciences Division, Mail Stop M888, Los Alamos National Laboratory, Los Alamos, New Mexico 87545. The substrates invaded are gastropods, pelecypods, brachiopods, and crinoids. D'Orbigny (1847) was the first to describe borings in mollusc shells made by bryo-zoans. It was not until 1938 that the anatomy of the zo-oids was presented (Marcus, 1 938). There are approximately 4,000 living species of ecto-proct bryozoans with as many as 15,000 fossil species (Soule and Soule, 1969a). These organisms have diversi-fied greatly and occur in the following forms: soft gelati-nous colonies, fenestrate and squamous colonies, arborescent chitinous or calcareous colonies, single-or multi-layered crustose colonies, colonies boring into hard substrates and those burrowing into soft material. The four genera that burrow through soft material are (after Pohowsky, 1978); Hypophorella expansa Ehlers, 1876 in polychaete tubes (e.g., Chaetopterus and Tere-bella)\ Watersiana paessleri Calvet, 1912 in the tunic of the colonial ascidian, Polyzoa gordiana; Harmeriella ter-ebrans Borg, 1940, in the calcareous walls of cheilostome bryozoa; and Bulbella abscondita Braem, 1951, in rot-ting wood. Pohowsky (1978) reports that certain cheilos-tomes are known to produce shallow pits on encrusted shells (e.g., Hippothoa divaricata Lamoroux, 1821 and Electra monostachys (Busk), 1854). These latter burrow-ing and etching bryozoans are of interest in that they dis-play characteristics intermediate between the true borers and the epifaunal bryozoans. Pohowsky (1978) reports that boring bryozoans penetrate exclusively by chemical means. There are, however, no data in the literature to support this statement. The four genera of Ctenostomata penetrating soft substrates are believed to employ me-chanical techniques. Because of the lack of investigation of boring bryozo-ans, little is known about their biology and ecology. Forty-eight species of borers (assigned to three genera) are known, but the anatomy of only sixteen has been de-scribed. The remaining species are known only by their tracings. 276
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