Because of their size and semipermanent nature, burrows of larger echiuran worms (phylum Echiura: Urechidae, Echiuridae, Bonelliidae, and Thalassematidae) host a remarkable variety of symbionts. Most symbionts appear to be commensals (occasionally with tendencies to kleptoparasitism), although at least 1 echiuran, Bonellia viridis, may in turn be a commensal of the soft rock-burrowing mudshrimp, Upogebia mediterranea. Commensal animals most frequently observed in echiuran burrows are bivalves (11 species in 8 hosts), polychaetes (mostly polynoids; 14 species in 8 hosts), brachyuran crabs (mostly pinnotherids and varunids; 16 species in 8 hosts), alpheid shrimps (6 species in 6 hosts), and copepods (3 species in 3 hosts). Some are facultative commensals (e.g., many polynoids and pinnotherids), but obligate and host-specific associations also occur, particularly among copepods and the loxosomatid entoprocts, Loxosomella spp. (with their associated gastropod predator, Sigaretornus planus). An alpheid shrimp, Alpheus barbatus, may also live in association with non-burrowing, crevice-dwelling echiurans. Other, less-frequent, echiuran commensals include polyclad flatworms (1 species), nemerteans (1 species), nematodes (2 species), entoprocts (2 species), gastropods (2 species), and fishes (2 species). Most echiuran-associated bivalves, entoprocts, and some polychaetes are so-called burrow energy users. The most-diversified communities of commensal animals are found in U-shaped burrows of larger species, such as Urechis spp. and Ochetostoma spp., but also in some relatively small echiurans, e.g., Lissomyema exilii, which builds U-shaped chambers in discarded mollusc shells. Finally, the cosmopolitan echiuran species or species complex, Ochetostoma erythrogrammon Leuckart and Rüppel, 1828 (Echiura: Thalassematidae), is reported here for the 1st time from Venezuelan waters. At least 2 species of pinnotherid crabs, Pinnixa spp., and an unidentified sportellid bivalve were collected from the burrows, together with the echiurans and the upogebiid mudshrimp, Upogebia omissa, suggesting that the crabs and the bivalve may be commensally associated with 1 or both of these burrowing animals. |
