June 26, 1890] 



NATURE 



203 



Tank C. — Dendronotus. 



(2) Pounced upon at once by three bullheads {Coitus), 

 which made rapid dabs at it successively, until one secured 

 it and carried it off to a quiet place, where he seized it in 

 his mouth and ejected it nine times in succession, each 

 time taking it half into the mouth and keeping it there 

 for some seconds, then spitting it out and at once pouncing 

 upon it again. Finally, the now somewhat mangled re- 

 mainder of the Dendronotus was taken out and put into tank 

 A, where one of the shannies at once seized and swallowed 

 it. This Dendronotus was large. It was larger than the 

 head of the Coitus, and caused the mouth-cavity to bulge 

 out greatly when it was taken in. The general impression 

 was that the Coitus found the Dendronotus desirable food, 

 but an uncomfortably large mouthful, and was trying to 

 worry it to pieces. 



Tank A.—Eolis. 



(i) Tried, and at once rejected by three shannies in rapid 

 succession, then seized by the large shanny and carried 

 behind some rock-work. Immediately, numerous red 

 cerata were seen scattered through the water in that 

 neighbourhood, showing that the Eolis had been forcibly 

 ejected in pieces. The cerat^ floated about for some 

 time in the water, but were not touched by any of the 

 fish. 



(2) Pounced upon by several fish together ; one secured 

 and at once rejected it, and then, seizing the white body, 

 managed to bite it across, setting free the dorsal portion 

 with all the cerata. It then retired to the back of the 

 tank, while the cerata — after separating, as they very 

 readily do in Eolids — were left floating about in the 

 water, untouched. 



Tank A.— Doris. 



(i) Tried and rejected by two shannies, then seized 

 Ijy the largest shanny and carried off to the back of the 

 tank. 



Tank B.— Doris. 



(2) Several fish darted at the Nudibranch, but a large 

 sole suddenly slipped up vertically between them and 

 bolted it. 



(3) Tried and rejected by six or eight plaice, and finally 

 left on the sand. 



(^4, 5, 6) These three specimens were gently lowered 

 into the tank by a net, so as to reach a shelf of the rock- 

 work without attracting attention. They soon began to 

 expand and move. One plaice swam up and looked or 

 smelled at them, but did not touch them. 



The action of the large sole in bolting Doris No. 2 

 above may possibly be explained as a result of the habit 

 of competing for their food. Three or four other fish 

 were darting at the Nudibranch, and the sole took the 

 only possible course by which it could secure the prey — 

 it made a rapid movement upwards between the snouts 

 of its competitors and swallowed the Doris entire ; there 

 was evidently no time for examination.' 



In the above experiments I have used altogether 53 

 Xudibranchs, offered to twelve different kinds of fish and 

 other predaceous shore animals, and there have been 

 over 130 distinct transactions between the fishes and the 

 Xudibranchs. My general impression is that the order 

 of edibility of the Nudibranchs offered to the fishes is : 

 Dendronotus, Doris, Ancula, Eolis j Eolis being the 

 most distasteful form, Ancula next, Doris less so, and 

 Dendronotus edible, but, from its size, offering difficulties 

 to the rather small fishes which we tried. 



' The last number of the Joum. Mar. Biol. Assoc., containing Mr. Bate- 

 <n's paper on the sense-organs and perceptions of fishes, only reached Liver- 

 pool after this article had been "sent to Nature. In regard to the sole being 

 one of those fishes which hunt for their food and recognize it by the sense of 

 smell alone, 1 would remark that the specimens in the aquarium here cer- 

 tainly seem to perceive their food as the plaice do by sight, the two kinds of 

 fish often darting together at a f >od morsel— and, as I have just shown abave, 

 the sole being sometimes more alert than its competitors. Possibly these 

 soles have changed their habits like the ruckling described (p. 238) by Mr. 

 Bateson. 



NO. 1078, VOL. 42] 



The Nudibranchs were all healthy and good-sized speci- 

 mens, and the fish were probably the right kind, being nearly 

 all shore fishes found in the immediate neighbourhood of 

 where the Nudibranchs live. Still, the conditions were, 

 of course, to a certain extent artificial, and that must be 

 taken into account in drawing conclusions. Dropping 

 the Nudibranchs into the tank from above is unnatural, 

 and may give rise to a misleading result, especially where 

 the fish are accustomed to have their food thrown in from 

 above, and only receive edible food. Then, again, at least 

 some of the fish— those that have been some time in 

 captivity— have been educated to compete with one 

 another for the food. When anything is thrown in — a 

 bit of white shell will do— there is at once a rush made 

 upon the falling object, and no time is allowed for inspec- 

 tion or consideration. I would account for the seizing of 

 Eolis by the shannies (very active, voracious, and ap- 

 parently impulsive fishes), even when the prey is evi- 

 dently distasteful and has brilliant warning colours, as a 

 result of this acquired habit of competition, and of 

 pouncing upon anything thrown into the tank. Still, 

 there is a marked difference between the manner in 

 which they take a cockle and say an Ancula. The cockle 

 is taken right in and swallowed at once, while the dis- 

 tasteful Nudibranch, even if seized, is usually only partly 

 taken into the mouth ; in some cases, it is seen to be 

 held by the very front of the jaws, and is then ejected 

 with force. 



Ancula has been a particularly interesting case. Start- 

 with the general opinion that Ancula is a perfectly de- 

 fenceless soft-bodied animal, I have been astonished to 

 find that it is sometimes present on the rocks at Hilbre 

 Island in great abundance, in very prominent and ex- 

 posed situations, and that its colouring was not pro- 

 tective, but rendered it conspicuous. The experiments at 

 the aquarium next showed me that this Nudibranch is 

 distasteful to fishes and other shore animals, but for a 

 time I did not understand why. Lately, however, Mr. 

 Clubb and I have found ^ that, besides the abundant 

 mucous glands scattered over the integument, Ancula 

 possesses special large glands occupying the apices of 

 the cerata, and opening to the exterior. These glands 

 are placed just where an offensive organ would be most 

 useful, and where the stinging cells are found in Eolis, 

 and it seems probable enough that it is the presence of 

 their secretion on the most outstanding parts of the body 

 which renders the animal objectionable to fishes. 



The protective colouring of Doris bilamellata may be 

 accounted for in one or both of two ways : (a) it may serve to 

 protect the animal from certain other shore animals which 

 have not yet been experimented with, and to which the 

 spicules and mucus of the Doris are not objectionable ; 

 and {b) it may save the animal from being tried by fishes, 

 &c., not sufficiently aware of its (to them) distasteful 

 nature.^ It is obvious that, if an animal is not thoroughly 

 objectionable, and has not yet become conspicuous with 

 warning colours, it will be better for it to be protectively 

 coloured. So we need not be surprised to find that some 

 inconspicuous protectively coloured animals have certain 

 offensive organs, and are distasteful to certain of their 

 enemies. Eolis is a most distasteful form, and has con- 

 spicuous colours of a warning nature. Ancula is also 

 distasteful, and is conspicuously coloured. Doris is less 

 distasteful, and is still protectively coloured ; while Den- 

 dronotus, which is, I believe, edible, is very effectually 

 concealed, amongst the red seaweeds it lives on, by its 

 large branched cerata and red-brown colours. 



W. A. Herdman. 



' See " Third Report on the Nudibranchiata of Liverpool Bay " (Trans. 

 Biol. Soc. Liverpool, vol. iv.). 



* What seems to be a very similar case has been pointed out by Mr. 

 Garstang (Joum. Mar. Biol. Assoc, October 1889, p. 191), viz. the two 

 British species of //^rwf^a, which are both protectively coloured, and hive 

 no stinging cells, and yet seem to possess the power of emitting, when irri- 

 tated, an offensive fluid. I would expect to find that they were distasteful 

 to at least lomc fishes. 



