"OO 



NATURE 



[February 27, 19 13 



The Occurrence of the Portuguese Man-of-War 

 (Physalia), and of a Giant Spider-Crab, " Homola 

 (Paromola) cuvieri," in the English Channel. 



The occurrence of the Portuguese man-of-war 

 (Physalia) on the English coast is so unusual as to be 

 well worth recording. During March and the early 

 part of April, 1912, numbers of the Atlantic form of 

 Physalia were cast up on our shores at various points 

 between Cardigan Bay and Seaford in Sussex. It 

 was also reported by M. CauUery ' as occurring along 

 with Velella on the French side of the Straits of 

 Dover in the early part of April, 1912. Specimens 

 were also sent to this laboratory on February 10 this 

 year from Looe, on the south coast of Cornwall. 

 There can be little doubt that the presence of Physalia 

 on the south coast of England in March and April, 

 19 12, was due to the almost continuous high southerly 

 to south-westerly winds indicated in the south-eastern 

 part of the North Atlantic in the meteorological 

 reports for the early part of that year. 



Phvsalia is believed to occur normally only in the 

 warmer currents of the Atlantic Ocean,- but in the 

 earlv months of the year large specimens are not 

 infrequently blown into the Mediterranean, and after 

 storms thousands have been found about the same 

 time of the year on the beaches of the Canary Islands. 

 It is therefore not improbable that the Physalia 

 stranded on the English coasts had been driven by 

 the wind from the eastern portion of the subtropical 

 North Atlantic. 



It is interesting to note that at the end of March 

 northerly winds set in in the eastern part of the 

 English Channel. This circumstance probably ex- 

 plains why Physalia and Velella were driven on to 

 the French side of the Straits of Dover. 



It may also be of interest to mention that a specimen 

 of tlie very large spider-crab Homola (Paromola) 

 cuvieri has been taken for the first time on record in 

 the Enclish Channel. The specimen is a very fine 

 male. When the large clawed legs are held out at 

 right angles to the body, the span is nearly 4 ft. 

 (117-3 cm.), while the length of the carapace is rather 

 more than 6| in. (i6-6 cm.). We are informed by 

 Messrs. M. Dunn and Sons, who presented the crab 

 to the Marine Biological Laboratory at Plymouth, 

 that it was taken by fishermen on December 16 last, 

 in a trammel-net three-quarters of a mile E.S.E. of 

 Pen-a-Maen Point (north-east of Dodman Point), on 

 the Cornish coast. At this spot the depth of water 

 is about 15 fathoms, whereas in the Mediterranean 

 this crab is said to inhabit the deeper waters, and 

 has been taken there in about 215 fathoms. Off the 

 north-west coast of Africa " it has indeed been re- 

 torded^ from a depth of 350 fathoms. 



There is an abundant growth of marine animals 

 fviz. Anomia, Pomatoreros, Serpula, Sabella, Botryl- 

 lus, Antennularla, Ascidiella, Plumularia) on the back 

 and legs of the crab, a fact which seems to indicate 

 that the animal has been living at least some months 

 in relatively shallow water. This specimen of Homola 

 is still living in the tanks at the Marine Biological 

 Laboratory, and is feeding well. 



The occurrence of Homola in the English Channel 

 is one more instance of the close relationship of the 

 fauna of this region with that of the Mediterranean 

 and neighbouring parts of the Atlantic. Homola, 

 however, has been recorded rarely from the west 

 coasts of Ireland and Scotland,* hence there is the 



1 M. Caullery, BjiU. de la Soc. Zool. de FrajtcCy tome xx,\vii., 1917, pp. 

 iSo-2. 



n, Ud. 11., 



lifiquesdu 



2 C. Chun, " Ergeh. der Plankton-ExpeH." Die Siphonopho 

 K.b, i8q7, p. Sq. 



A. Milne Fthv.-irdsand E. L.-L. Bouvler, " ExpMitions Sci. 



Ft du T.lli! 

 "Ouide 10 Cnisla 

 ish Museum (N.iti 



Crvist. T)ecap.. 

 sa. ^c. Exhibited in the Department of Zoology, 

 .•1I History). 



possibility that this northern distribution may havc- 

 been effected partially by means of the current" which 

 flows from the Mediterranean, and is believed to 

 spread along the western shores of Europe. There 

 is, moreover, reason to believe that the more typical 

 planktonic forms of life might be carried considerable 

 distances in such a wind-drift as that in which 

 Physalia must have been. It is not suggested, how- 

 ever, that the larvae of Homola, for example, would 

 be driven so far as Physalia and Velella in similar 

 circumstances, for it is well known that these two 

 Siphonophores, inasmuch as they are semi-aerial, 

 comprise a separate category of plankton with regard 

 to their adaptation for distribution. 



The recent distribution of Physalia mav therefore 

 be said to offer us a picture by means of which we 

 can more readily understand, for example, the close 

 relationship between the fauna of the English Channel 

 and that of the Mediterranean and neighbouring 

 Atlantic region. J. H. Orton. 



The Laboratorv, Plvmouth. 



Actual Conditions affecting Icebergs. 



In the interesting discussion by Dr. Aitken on the 

 relation between laboratory experiments and actual 

 conditions, as affecting icebergs (Nature, January 9), 

 there is one element of primary importance which 

 appears to be too much overlooked. To put it in the 

 most general terms, icebergs are almost always 

 moving in the drift of an ocean current ; and the 

 point to which I wish to direct attention is the 

 relative movement of the water with respect to the 

 iceberg. 



It is the nature of constant currents to have a 

 greater speed at the surface and to decrease in 

 velocity with the depth. This is a usual character- 

 istic, as foimd in my investigations in the Tidal and 

 Current Survey, in several such currents. For 

 example, when the surface speed is one knot or more 

 the velocity may fall to about half this at 30 fathoms, 

 and it may be only distinctly appreciable at 90 

 fathoms. An iceberg in such a current will, of course, 

 move with the average velocity corresponding to its 

 draught, and, as a consequence, the normal condition 

 is that an iceberg has a superficial current running 

 past it. It is also probable that this current will 

 usually be much greater than such movements of the 

 water around it as are set up by convection, from 

 difference of temperature. 



It is also to be noted that this relative movement 

 is independent of wind disturbance and tidal effect, 

 which often accentuate it. In the work of this Sur- 

 vey, I have had ample opportunity to observe these 

 effects while at anchor in the open amongst icebergs- 

 for davs at a time. In Belle Isle strait, they ground 

 in a depth of 30 or even 50 fathoms, which shows 

 the draught they may have; and the strong tidal 

 streams of 2 to 2J knots running past them create 

 a wake behind them, like a vessel under sail. This 

 may be considered an unusual condition, but it should 

 not be overlooked in discussing practical safeguards. 

 for an iceberg aground on the 30-fathom bank in 

 the middle of Belle Isle strait is as much of a menace 

 to navigation as any. 



Although there may thus be many modifications of 

 general conditions, it will be on the safe side to 

 assume in this discussion that there is always a 

 superficial current of appreciable amount flowing past 

 an iceberg, even in the open, while it is carried along 

 in anv berg-bearing current. 



W. Bf.ll Dawson. 



(Superintendent of Tidal and Current Surveys, of 

 Canada.) 



Ottawa, Canada, January 25. 



2261, VOL. go] 



