Vermeij and Ruch—Miocene barnacles and their hosts
living hosts. Host–barnacle relations at Chancellor’s Point were compared to examples in the Recent fauna, both published and as represented in material from Friday Harbor, Washington, in the Vermeij collection. All material discussed is housed in the Vermeij collection. Each specimen of Conradconfusus parilis was examined for signs of outer-lip damage and repair. We recognized repairs as significant deviations from the normal growth lines on the shell’s last whorl.
Repositories and institutional abbreviations.—All material discussed in this paper is in the Vermeij collection at the University of California, Davis. Figured specimens are at the California Academy of Sciences (CAS).
Results and discussion
Barnacles on shells.—We found 78 individual and basal scars of barnacles (Chesaconcavus chesapeakensis) distributed among 16 of 27 Conradconfusus parilis, giving an incidence of 59% of barnacle-bearing gastropods and an average of 4.9 bar- nacles per colonized shell. Specimens with barnacles range in length from 38mm to 91mm with a mean of 65.7mm; speci- mens without barnacles range from 34mm to 97mm with a mean of 65.5mm. The largest barnacle, on a shell 80.2mmlong, has a diameter of 30mm and a height of 50mm. Most barnacles are bent in the host’s direction of growth, and none encroaches upon the host’s aperture in a way that would preclude the shell from clamping to a hard substrate. No epibionts were found in the apertures of Conradconfusus parilis. Our data therefore indicate that barnacles settled and persisted on living Conradconfusus parilis gastropods. The presence of large barnacles on gastropod hosts as seen
in the late Miocene Conradconfusus parilis is noteworthy in part because so few similar cases are known in the living fauna. Small barnacles of various species sporadically settle and achieve adulthood on intertidal gastropods on productive coastlines such as those in Venezuela, Pacific Panama, New South Wales, Chile, and the North Atlantic, but they rarely affect more than 20% of individuals of a given host species (unpublished observations in the Vermeij collection). Barnacles are extremely rare on gastropods and other hard surfaces on reefs (Newman, 1960); where they do occur, as on some specimens of Menathais armigera (Link) collected by Vermeij near the reef crest in Palau, the barnacles are flat, not extending above the level of the gastropod shell’s large blunt tubercles. The only comparable example in the living fauna known to
the senior author is the association between the sedentary suspension-feeding or kleptoparasitic gastropod Trichotropis cancellatus (Hinds) in subtidal waters of the San Juan Islands, Washington, and the barnacle Balanus rostratus Hoek. Like Chesaconcavus chesapeakensis, B. rostratus has a cylindrical profile and is cemented to the substrate. In a sample of 121 T. cancellatus from Rock Point, dredged
at a depth of 60 to 75 m, 49 (30%) have one or more barnacles on the shell, and one has an attached brachiopod. This 30% incidence is about half the 59% incidence on fossil Conradconfusus parilis and identical to the 30% reported by Schmitt and colleagues (1983) for the subtidal teguline trochoidean Norrisia norrisi (Sowerby) in southern California.
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The high incidence of barnacles on T. cancellatus is surprising in view of the periostracal hairs, which discourage settlement by epibionts (Iyengar et al., 2008). Given the limited data available, we have refrained from
analyzing these differences in frequency statistically. The significant point is that the observed frequency of barnacles on our sample of Conradconfusus parilis is higher than that on any living gastropod species reported in the literature or as observed in the Vermeij collection, but we make no claims about the representativeness of this high frequency in the Miocene of Maryland. In the case of the scallop Chesapecten santamaria,
barnacles occur on seven of 12 left valves (58%) ranging in shell height from 68 to 115mm and with a mean height of 85±12mm; and on two of 12 right valves (17%) with heights of 57 and 134mm. The five left valves without barnacles range in height from 77 to 136mm with a mean height of 102±23mm. Barnacles situated at the valve margin never crossed the margin, but in the single articulated specimen available to us, with a height of 81mm, barnacles are concentrated near the hinge of both valves, perhaps limiting the bivalve’s ability to open the shell. This example is the only case where the possibility of postmortem settlement by barnacles cannot be dismissed. Inner valve surfaces are free of epibionts but do display traces of repaired damage from endolithic organisms boring into the shell. We have five examples of large clusters of Chesaconcavus
chesapeakensis that seem to have lain and perhaps been rolled on the seabed. Similar so-called balanuliths have been described from the living fauna in the North Sea (Cadée, 2007; Donovan, 2007) and from the Pliocene of Chile (Nielsen, 2009). In these cases, the initial substrate was the shell of a mollusk, which after death was completely enveloped by the barnacle aggregation. The only other gastropod host that would have been large enough and common enough to have hosted large barnacles at Chancellor’s Point is the busyconine Coronafulgur coronatus. None of the 12 specimens we examined had epibiotic barnacles. The two hosts supporting abundant Chesaconcavus
chesapeakensis at Chancellor’s Point (Conradconfusus parilis and Chesapecten santamaria) have strongly sculptured shells that would seem ideal for settlement by the barnacle’s cyprid larvae. The deep sutures between whorls and the valleys between spiral cords of Conradconfusus parilis as well as the grooves between radial folds of Chesapecten santamaria offered protection for settling larvae and correspond well to the known preference of living cyprids to settle in furrows and grooves on ridged surfaces (Crisp and Barnes, 1954; Crisp, 1961; Wethey, 1986). Coronafulgur coronatus may have been less suitable because of its semi-infaunal habits and relatively smooth shell despite its deeply channeled sutures. Our data on Chesapecten santamaria are too limited to
make a strong case that the presence of barnacles limits growth, but the greater size of left valves without barnacles compared to left valves with barnacles is consistent with that hypothesis. Among living species, barnacle epibionts have been shown to interfere with the host’s growth and reproduction in the gastropods Stramonita haemastoma (Linnaeus) in Tunisia (El Ayari et al., 2015), Littorina littorea (Linnaeus) in Germany (Thieltges and Buschbaum, 2007), and Batillaria zonalis
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