Sulcichnus, Martinell & Domenech, 2009

Martinell, Jordi & Domènech, Rosa, 2009, Commensalism in the fossil record: Eunicid polychaete bioerosion on Pliocene solitary corals, Acta Palaeontologica Polonica 54 (1), pp. 143-154 : 150-151

Sulcichnus tracemaker

Generally speaking, traces of bioerosion offer palaeontologists a first−hand source of ecological information. Despite of this, information about such traces is not commonly reported in studies of modern biota. However, in the specific case of Sulcichnus neontologists have shown particular interest in identifying the trace and attributing it to examples from the fossil record ( Zibrowius et al. 1975).

Fage (1936) was the first to describe these grooves on Recent Flabellidae , and he attributed them to the activity of the worm Lumbrineris flabellicola ( Fage 1936) ( Polychaeta: Eunicida : Lumbrineridae ). Zibrowius et al. (1975) undertook the study of biotic relationships between L. flabellicola and various contemporary species of deep−water ahermatypic corals. However, they went a step further and attributed to this same annelid similar traces observed in numerous Neogene specimens, both via the literature and museum collections (see previous section). Given the small number of changes undergone by Polychaeta over time, Zibrowius et al. (1975) concluded that the same association currently observed between Lumbrineris and certain ahermatypic corals could be extended as far back as the Miocene.

In contemporary seas this association has been mainly, although not exclusively, observed on solitary skeletons of deep−water ahermatypic corals (azooxanthellates) ( Fig. 5 ). Zibrowius et al. (1975) described this association on the basis of samples from the NE Atlantic, the SW Indian Ocean ( South Africa), Madagascar, the China Sea and Japan. Samples were dredged at depths between 125 and up to 1,100 m. As observed by these authors, the eroding polychaete L. flabellicola inhabits a self−secreted membranous tube exteriorly attached to the host and causes a superficial to deep groove on the coral skeleton, a Recent equivalent of Sulcichnus being described here. Following the terminology of Taylor and Wilson (2002), Lumbrineris can be considered an episkeletozoan.

The worm seems to be almost eurytopic, following the distribution of its potential hosts. Thus, it has been detected as far afield as the Philippines, Japan, and the NE Atlantic. Zibrowius et al. (1975) report several Recent species that act as hosts for the worm, belonging to the genera Caryophyllia , Flabellum , Rhizotrochus , and Balanophyllia (all of which are solitary), as well as ahermatypic colonial species of the family Dendrophylliidae .

At present, the relationship established between L. flabellicola and the coral host is clearly obligatory and permanent for the worm ( Zibrowius et al. 1975; Cairns and Zibrowius 1997; Martin and Britayev 1998). It may correspond to a case of parasitism (− +), commensalism (0 +), mutualism (+ +) or amensalism (00). In the former the relationship would be negative for the host, while the others would not. Studies on Recent material ( Martin and Britayev 1998) strongly suggest a relationship based on commensalism. In the current specimens the anterior part of the worm is always found close to the calyx of the coral, and may even stretch beyond it. This would enable the eunicid to place its proboscis inside or around the mouth of the cnidarian and obtain food. It would then return to its tube ( Miura and Shirayama 1992, Martin and Britayev 1998) with no detriment to the host. However, much remains to be known about symbiotic relationships between worms and other groups, and commensalism may have received undue emphasis.

Martin and Britayev (1998) described 292 species of commensal polychaetes belonging to 28 families. Therefore, commensals would be found in 31% of the known families of polychaetes. In addition, these authors state that majority of commensal polychaetes (67%) are obligatory symbionts, and among these, 59% are found exclusively on a single type of host. This figure rises to 87% when considering those which appear on two or three hosts. These data fit well with our observations in the fossil record.

Maeandropolydora and Caulostrepsis are mainly attributed to the activity of spionids ( Polychaeta : Spionida : Spionidae ), which infest a variety of skeletal and non−skeletal substrates. Among the most recent papers, Wielgus et al. (2002, 2006) report numerous genera of colonial corals infested and bored by spionid polychaetes in the Red Sea. Worms activity modify the colonial morphology in polluted water, thus acting as parasites. Rodrigues (2007) and Rodrigues et al. (2008) discuss the ecological significance of spionid traces from an example of infestation in Recent brachiopod shells. They conclude that these traces may represent either comensalism or parasitism, so further studies are needed to determine the dominant symbiotic interaction.

Figures

Fig. 5. Recent polychaete Lumbrineris flabellicola (Fage, 1936) infesting alive caryophyllids. Pictures by Alan and Eve Southward, from the Marine Biological Association, UK; reproduced with permission.