Glimpses of the giant spionid Spio aequalis

The beachworm Spio aequalis is a large polychaete unique to New Zealand. However, it has not been seen for fifty years.

Spio aequalis Ehlers, 1904 remains an enigmatic polychaete known only from the original discovery by Schauinsland somewhere on the Chatham Islands coast. Two subsequent unpublished South Island sightings have come to light recently, but the length of time expired since the last observation fits one definition of an extinct animal, and it has yet to be refound, despite resampling attempts at all three of its now known locations.

What is Spio aequalis? Spio aequalis, at around 15 cm long and 7 mm thick, is one of the largest spionids known. The original type specimens are lost, so we depend on Ehlers' description in old German and his figures. Possibly they were bad representations of one of the other large New Zealand spionids? We know now that Ehlers described the animal fairly accurately; his drawing of the head (below) with its odd bulbous pleated peristomium is recognisably from the same spionid as in the photo here. However, the blunt prostomium is somewhat misrepresented in Ehler's figure.

Is the worm really a Spio? That is questionable. It is distinctive in several features, including the hooks and branchiae. Crucially it may lack the characteristic anal cirri of the genus, and it has eggs differing from other Spio.The egg envelope is thickened and alveolated, usually an indicator of planktonic early development, and a character thought to be important in the phylogeny of the family.

Hugo Schauinsland of Bremen visited the Chatham Islands during January-February 1897 whilst travelling around the Pacific. He was an energetic collector and took back to his native Germany a very wide range of animals and fossils from the Chathams. The place where he found Spio aequalis is not known. It might have been near Waitangi.

Below are the S. aequalis figures of Ehlers.

Waitangi (Chatham Island), Moeraki (North of Dunedin), and Squally Bay (Banks Peninsula) are the known localities of Spio aequalis.

William Benham collected polychaetes in November 1899 at Moeraki north of Dunedin. Amongst them were one or more Spio aequalis, and, although Benham never published the record, he recognised it was the same as Ehlers' species. Benham's worm is partly in the British Museum, mistakenly recorded as one of Schauinsland's originals, and partly in the Otago Museum!

Fifty years later George Knox did some collecting on Banks Peninsula in 1949. He too found a large Spio species; this time obtaining several specimens at Squally Bay. He never published the record. It was Spio aequalis again, and these are the animals photographed here. There are no other occurrences known. The species has never been found in offshore samples.

Scolelepis and Scolecolepides species are the other large coastal NZ spionids. However, they occur in sands, the typical spionid habitat and are quite commonly seen.

Spio aequalis habitat. The known habitats of other species collected at the same time suggests that Schauinsland, Benham, and Knox had made routine intertidal or rock pool collections. Probably Spio aequalis lives on more exposed coasts, but protected under stones. It is probably a surface deposit-feeder and also may need trapped sediment in which to burrow. Can it be found again? There is no obvious reason for it to disappear since its rocky shore habitat is untouched by the effects of coastal urbanization. It may be always relatively rare, or perhaps it varies in abundance and geographic range from year to year. If you have the luck to discover Spio aequalis somewhere on the New Zealand coast please preserve the specimens, and please contact Geoff Read.

References

Ehlers, E. 1904: Neuseeländische Anneliden. Abhandlungen der Königlichen Gesellschaft der Wissenschaften zu Göttingen. Mathematisch-Physikalische Klasse. Neue Folge 3: 1-80.
Blake, J. A. 1984: Four new species of Spionidae (Polychaeta) from New Zealand, with comments on a syntype of Spio aequalis Ehlers from Chatham Island. Proceedings of the Biological Society of Washington 97: 148-159.