Phormosoma rigidum A. Agassiz, 1881

Anderson, Owen F., 2016, A review of New Zealand and southeast Australian echinothurioids (Echinodermata: Echinothurioida) — excluding the subfamily Echinothuriinae — with a description of a new species of Tromikosoma, Zootaxa 4092 (4), pp. 451-488 : 484-485

Phormosoma rigidum A. Agassiz, 1881

Material examined. None.

Remarks. This species is included here because the two known specimens, the type material now in the collection of the British Museum of Natural History, were trawled from a single station in 1260 m off New Zealand’s East Cape during the Challenger expedition of the mid- 1870 s (Station 169). According to Mortensen (1935) these were split by Alexander Agassiz, one being lodged in the British Museum and described as P. rigidum and the other taken by him to the United States where, forgetfully, he re-described it 23 years later as P. z e l a nd i ae A. Agassiz 1904. Having synonymized these species, putting the slight differences in the gill slits and spines down to the difference in size between the specimens, Mortensen (1935) remained unsure of even the genus. Only fragments of spines were preserved and no pedicellariae at all, and Koehler (1926) considered it may have been a small Tromikosoma australe , as it was near to the type locality of that species (although he was mistaken, it was in fact over 4500 km away). In any case Mortensen (1935) recommended awaiting further material from the type locality to help confirm their identity, and perhaps now that time has come.

The essential differences between P. rigidum and P. b ur s ar i um are in the aboral tube feet and pores. In P. rigidum the pores and tube feet of the primary plate and outer secondary plate are not fully developed and the poriferous zone has the appearance of a single column of tube feet, consisting of the well-developed tube feet on the inner secondary plate. After examining specimens of New Zealand Phormosoma species from well over 100 lots, from a wide variety of localities including some near the type locality of P. r i g i du m, and from a depth range of 200 m to 2500 m, 6 specimens were distinguished which appeared to exhibit this feature to varying degrees. This little collection comprised almost all of the smallest specimens examined (less than about 50 mm TD), matching roughly the size of the type specimens of P. rigidum , and the closest being within 100 km or so of the type locality. However, a comparison of genetic material (cytochrome c oxidase subunit 1, CO1) between five of these specimens and four specimens of P. bursarium showed identical sequences (author’s unpublished data). It seems unlikely now that P. r i g i du m (or P. zelandiae ) are valid species, especially given the intensity of sampling of deepsea benthic habitat around New Zealand in the 130 years since the Challenger ’s visit. Although it seems doubtful if examination of the type specimens of P. r i gi d um would resolve their identity, given their small size, age, and previous confusion surrounding their provenance, this should be attempted.

The depth of the type locality of P. rigidum may be significant; 1260 m being at the very deepest end of the range for P. bursarium , deeper than Sperosoma obscurum , and shallower than most S. nudum and all T. australe , T. rugosum , and K. asterias . The only species outside of the Echinothuriinae that is common in this depth is Hygrosoma luculentum , which can be easily distinguished from all these species, even in small specimens, by the shape of the distal end of the teeth.

The species has been reported from Australia “ Off Danger Point to off Sydney NSW, off Lord Howe Is; west Pacific Ocean. Depth range 117– 1260 m ” (Rowe & Gates 1995: p 285) without referring to any specimens or specific records—but there were several specimens of Hygrosoma spp. in Museum Victoria mis-identified as P. rigidum , so maybe this is what is being referred to.

In the end, the unresolved investigations into this species may have been best summed by Mortensen’s (1935: p 148) comment on P. zelandiae “ Altogether too much ado about a young unidentifiable specimen”.