December 15, 1923] 



NA TURE 



861 



Letters to the Editor. 



{^The Editor does not hold hi tn self responsible for 

 opinions expressed by his correspondents. Neither 

 ca7i he undertake to return, ?ior to correspond -with 

 the writers of rejected manuscripts intended for 

 this or any other part of Nature. No notice is 

 taken of aitonymous communications ^^ 



Some New Commensals in the Plymouth District. 



While collecting by digging on the shore at 

 Millbay, Salcombe, in April this year, I found three 

 consecutive pairs of Synapta and a polynoid worm 

 living together, and at the same time three distinct 

 associations of the brittle star, Ophiocnida brachiata, 

 and a small mollusc. These finds led to five later 



Fig. I.— The brittle star, Ophiwnuia brachiata, with thp polynoid, Harmothoi 

 lunulata, clasping and curving over the disc, opposite the arrow head 

 and alongside the projecting, ami which is broken ofl short ; and the 

 mollasc (M), Mt/tUacula biJmtala : three animals living associated 

 together buried in muddy sand at Salcombe. X ca. ?. (From a photograph 

 by Mr. R. S. Clark.) 



monthly visits during the best spring tides, and 

 resulted in the observations given below. 



The polynoid, HarmothoS lunulata, was found in 

 muddy sand at Salcombe on adjacent beds with six 

 different animals belonging to two different phyla 

 and four different classes, but the more interesting 

 point is that the size of the polynoids forms on the 

 whole an increasing series approximately as follows : 

 Harmothoe lunulata, a few to lo mm., with Ophiocnida 

 brachiata, and curved round the disc or over the 

 mouth (Fig. i) ; H. lunulata (var. synapta St. Joseph), 

 15 to 20 mm., with Synapta inhmrens and S. digitata, 

 and also occasionally with Phascolosoma pellucidum. 

 On the same beds occur larger specimens of a polynoid 

 somewhat different in general appearance from the 

 small Harmothoi lunulata just mentioned, but they 

 appear to be merely later-growth stages of the same 

 species, and agree generally with the var. nigra, 

 Ala;j6s. Specimens of this form about 20 to 30 mm. 



NO. 2824, VOL. I 12] 



were taken with Phascolosoma vulgare, and of 35 to 

 50 mm. with Amphitrite Edwardsi. Further work 

 will be required to put this last observation on a 

 secure basis, but the animals can only be obtained 

 by special search in small numbers at considerable 

 intervals of time. There is little doubt, however, 

 that this species of Harmothoe at Salcombe starts 

 life commensally with Ophiocnida, and changes its 

 mate as it grows bigger and requires more accommoda- 

 tion in the burrow provided by the messmate, until 

 it finally consorts with polychaetes up to 30 cm. long, 

 such as Amphitrite Edwardsi. 



Harmothoe lunulata was also taken at the mouth 

 of the River Yealm with Synapta inhcerens, and will 

 no doubt later be found in the same association in 

 Plymouth Sound. 



During the same expeditions the almost constant 

 association of the mollusc Montacuta hidentata ^ with 

 Ophiocnida brachiata was confirmed. The mollusc in 

 this case is frequently found in numbers just below 

 or above the disc, and occasionally under an arm 

 in company with Harmothoe lunulata young. The 

 same mollusc was taken also but less frequently \yith 

 the Ge^hYrQaxi, Phascolosoma pellucidum, and occasion- 

 ally with Nereis. With this Gephyrean, however, 

 was found fairly constantly the messmate Lepton 

 ClarkicB,^ which was fairly frequently present in 

 groups of 4 to 7, and sometimes attached to the skin 

 of the blood-worm. 



On both shores at Salcombe another Harmothoe 

 sp. B, not yet identified, was taken in tubes with one, 

 and not in tubes with other species of Nereis. The 

 same species was taken by careful work also in Rum 

 Bay, Plymouth Sound, alongside or under the 

 tentacles oi Amphitrite gracilis, Poly cirrus aurantiacus, 

 and another species of Polycirrus, and at the same 

 time Harmothoe marphysce was rediscovered with 

 Marphysa sanguinea in Plymouth Sound after a long 

 lapse of years. The same Harmothoe sp. B was also 

 taken with Nereis in beds at the River Yealm. It is an 

 interesting fact that Sir Ray Lankester took a similar 

 polynoid under the tentacles and in the tubes of 

 Terebella {Polymnia) nebulosa at Heme so long ago 

 as 1865. 



The frequency with which the associates mentioned 

 above occurred apart from each other was noted 

 during the collecting work, and found to be low except 

 in the case of Phascolosoma pellucidum, which occurs 

 in thousands in a few square yards of ground. 



In none of these cases of association or com- 

 mensalism can a reason for it be asserted with any 

 certainty. The frequent occurrence of polynoids, 

 however, at the bases of the tentacles of polychaete 

 commensals, as Polycirrus, or in or near the grooves 

 of other polychaetes, as Amphitrite, Nereis, Chaeto- 

 pterus, or the grooves of Ophiocnida, suggests the 

 pilfering or scavenging of food-material. In the 

 cases of Montacuta and Lepton it is clear that food- 

 material is abundant in the burrows they inhabit, as 

 their shells are often covered with Polyzoan polyps, 

 and in addition various Foraminifera are not un- 

 common in the mouths or in the region of the burrows. 

 Indeed, the variety of associates of some com- 

 mensals suggests, on the other hand, that an inhabited 

 burrow may be simply and mainly a harbour of 

 refuge, which is used so frequently that the in- 

 habitants learn to know and tolerate each other, 

 while at the same time not necessarily depending 

 directly in any particular way on each other for food. 



i. H. Orton. 

 ^. , , 



November 8. 



' 1 .1111 Mil]. II imlcLitLj to Mr. R. Winokworth for the determination of 

 these spcclo-s. 



