Shell-boring versus tube-dwelling: is the mode of life fixed or flexible? Two cases in spionid polychaetes (Annelida, Spionidae)

Free crawling was an original life style, and the ability to build tubes and bore into shells evolved independently in various annelids and more than once within the family Spionidae. Absence of morphological differences between the shell-boring (SB) and tube-dwelling (TD) spionid worms and the innate ability of borers to build tubes raised a question whether the mode of life is fixed or flexible. Sequence data of three gene fragments of the mitochondrial 16S rDNA and nuclear 18S and 28S rDNA (in total 1,677 bp) have shown that SB and TD individuals of Dipolydora carunculata from the Sea of Japan were genetically identical, whereas those of Polydora triglanda from Taiwan were genetically distant and not conspecific. These data indicate that some species are constrained to a certain mode of life, while others are flexible and individuals can be either SB or TD depending on the place of settlement in the end of their larval development.