The evolution of the watering pot shells (Bivalvia: Anomalodesmata: Clavagellidae and Penicillidae)

WA Museum Records and Supplements | Updated 7 years ago

ABSTRACT –  Hitherto, the bivalve watering pot shells (Subclass Anomalodesmata) have been considered to be all contained within one superfamily – the Clavagelloidea – it too possessing but one family and generally comprising two functional clades: (i) the extinct Clavagella and the nestling/boring Dacosta and (ii) the endobenthic Brechites, Penicillus, Foegia, Kendrickiana and NipponocIava (plus the fossil Pseudobrechites and Warnea), and Stirpulina. The cemented Pliocene-Recent Humphreyia can be derived from an endobenthic Brechites-like ancestor (NipponocIava), making a third functional entity. The genus Dianadema also comprises species that are cemented. Recently, the endolithic Blyopa has been proposed as a fourth clade and a unique extension of the clavagellid line of evolution in which the anterior region of the right valve is dissolved as the animal bores deeper. The above studies, in broadening our understanding of the range of superfamilial functional diversity, however, raise questions with regard to the, as currently defined, taxonomic arrangement of the Clavagelloidea.

Species of the extinct genus Clavagella and their extant allies, notably Dacosta, have an internal ligament (but no lithodesma) located between chondrophores, both adductor muscles, a pallial line with a pallial sinus, and no pedal disc. Further, although Dacosta, Bryopa, Dianadema and Stirpuiina cement their left valves to the wall of their crypt or adventitious tube, the right is free inside it. Conversely, both shell valves of Brechiles and its allies, including the cemented Humphreyia, become surrounded by and united marginally with an adventitious tube and to which they are then fused by a calcareous secretion produced from the underlying mantle. Moreover, Brechiles and Humphreyia have an external ligament (the latter with a lithodesma), are functionally amyarian as adults, have no pallial sinus and develop a watering pot anteriorly, internal to which is a highly muscular pedal disc. Most important, however, Humphreyia, at least, has a distinct post-planktonic life history stage in which a free-living, typical bivalve juvenile, metamorphoses into a tube-dwelling adult. At this time, shell growth ceases and only the tube can be extended posteriorly and repaired. This is not the case with Clavagella and its allies although at some stage the juvenile does cement itself to its burrow wall but shell growth, especially of the free right valve, can continue.

All watering pot bivalves possess juvenile shells that in overall form and in the possession of distinct radial striae are reminiscent of representatives of the Lyonsiidae, suggesting a possible ancestry. The clavagellid fossil record shows that Clavagella and its allies (Dacosla, Bryopa, Dianadema and Stirpulina) arose in the Upper Cretaceous and are thus of Tethyan origin. Clavagellids have subsequently radiated into nearly all tropical waters. Conversely, the penicillid Brechiles and its tube-dwelling allies, plus Humphreyia, arose at least 40 million years later, in the Oligocene, and have an Indo-West Pacific distribution.

It is argued, therefore, that the Clavagelloidea, as currently defined, comprises two families. Clavagella sensu slricto and sensu lato (including Dacosla, Brvopa, Dianadema and Ascaulocardium), with Stirpulina, belong to the Mesovic and originally Tethyan Clavagellidae d'Orbignv, 1843. Brechites sensu stricto and sensu lato, with Humphreyia, belong, with the various endobenthic allies of the former, to the Cen%ic and Indo-West Pacific Penicillidae Bruguiere, 1789. Both families appear to have evolved at different times probably from Iyonsiid ancestors. In such a scenario, they have both subsequently radiated into a wide range of marine habitats involving a considerable and remarkable degree of convergent evolution, for example, between the endobenthic tube-dwelling Stirpulina (Clavagellidae) and Brechites (Penicillidae) and the cemented Dianadema (Clavagellidae) and Humphreyia (Penicillidae). Only Dacosta and Bryopa (Clavagellidae), however, have adopted endolithic, nestling/boring life styles, with no penicillid counterpart. The burrow lining was thus essentially exaptive for the evolution of an adventitious tube (Stirpulina) in the Clavagellidae whereas that of the Penicillidae evolved, not just independently, but probably from a burrowing ancestor, like Nipponoclava.