Dear Sir:

During the summer of 1879 a very striking case of protective resemblance came under my observation, which I have not seen recorded and which, it seemed to me, might possibly interest you.¹ I trust, therefore, you will pardon the liberty I take in addressing you for the sake of giving a brief description of it.

The creature is a naked mollusk of the genus Scyllaea and apparently the S. Edwardsii described by Verrill a few years ago (though he did not observe the protective resemblance, the specimens having been sent him in a bottle from the sea-shore).² It lives upon common species of Sargassum which was found in considerable quantity floating in the harbor at Beaufort, N.C.³ The fronds of the sea-weed are lanceolate with irregularly serrate or ragged edges; they are of a clear olive color usually more or less blotched with reddish or greenish tints. The fronds are attached to a cylindrical stem which is usually of a decided reddish tint with darker blotches. Scattered over the stem and fronds were numbers of calcareous bodies, remains of the shells of Spirorbis etc., while growing on the fronds were many little tufted Hydroids—mostly Sertularia.⁴

Now, every one of these things was mimicked by the Scyllaea. The body was about two inches long and as thick as a pencil. The resemblance in color to the sea-weed was simply perfect and extended even to the presence of dead-white little tubercles along the sides which produced the same effect as the calcareous bodies on the Sargassum. Behind, the body terminated in a vertically compressed portion the upper edge of which was serrated in the same peculiar ragged fashion characteristic of the Sargassum fronds.

From the creature’s back extended two pairs (one behind the other) of mimic fronds which, except for being considerably shorter, were in all respects similar to the Sargassum fronds. They were of the same color, the same irregularly lanceolate form, and with the same ragged edges, and, what is still more remarkable, were covered with delicate tufted processes which in their ensemble, tho’ not individually, bore the closest resemblance to the little Hydroids living on the Sargassum fronds. These processes were translucent and very contractile; they apparently serve as branchiæ. They also extend to the dorsal surface of the animal and a few are found on the sides of the frond-like posterior extremity of the body.

Lastly, the small ringed head tentacles are borne on a pair of larg⁠⟨⁠e⁠⟩⁠ flattened processes from the upper sid⁠⟨⁠e⁠⟩⁠ of the head. These processes exp⁠⟨⁠and⁠⟩⁠ at their tips into rou⁠⟨⁠  ⁠⟩⁠ flat⁠⟨⁠  ⁠⟩⁠ lobes with irregularly serrated edges. While these processes do not resemble, except in color, the perfect fronds they do resemble very closely some of the old or imperfectly developed fronds, especially those towards the basal part of the stem.

The creature, unlike most of the small naked mollusks I have seen, is very sluggish and usually lies perfectly motionless with its body along the stem and its mimic-fronds extended among the real fronds of the sea-weed. Under these circumstances its resemblance to a part of the plant is so wonderfully close that the creature almost defies detection. It was by a mere accident that I discovered two of these creatures on a small piece of Sargassum about as large as my two hands which had for some time been floating in a glass jar in our laboratory and on which I had already found the convoluted egg-strings of the animal and remarked upon their presen⁠⟨⁠ce.⁠⟩⁠ I then called the attention of Dr. Brooks,⁵ a practised observer, to the Sargassum, and it was only after close scrutiny that the presence of the Mollusks flashed upon him.

The perfection of the mimickry seems clearly correlated with the very exposed habitat of the animal; for drifting about at the surface it must be in full view both from air and water, and any resemblance to the sea-weed would be of the greatest service. I observed that the color was changeable, for after the animals had been kept some days in the aquarium they became more definitely mottled. This suggests the idea that they may be able to adapt themselves to the color of the particular bit of Sargassum on which they find themselves—and the Sargassum varies a good deal in color. On the other hand the change may have been due to the abnormal conditions existing in the aquarium. Each specimen had two or three small but very vivid violet spots on the body which did not, apparen⁠⟨⁠tly⁠⟩⁠ correspond to anything on the Sargas⁠⟨⁠sum⁠⟩⁠ The position of these spots was not the same in the two specimens—and I cannot form a conjecture as to their meaning.

In spite of numerous efforts I never succeeded in finding other individuals, and therefore cannot state how far the mimicry is a constant feature of the species. The Sargassum is extremely variable in color and form and does not invariably bear the hydroids and Spirorbes, though both are very common. It seems, therefore, quite possible that the Scyllæa inhabits only special varieties of the plant. Verrill’s specimens were said to be from “eel-grass” (I suppose Zostera) to which our Scyllæa certainly does not bear any remarkable resemblance in form.⁶ Still, they may have been taken from Sargassum entangled among the Zostera or may easily have been washed off by waves or even have crawled upon the Zostera as the Sargassum lay entangled in a bed of it.

I am in hopes of being able to find other specimens next summer, and would be very glad to know whether it has ever been recorded.

I am, Sir, with the greatest respect | Yours very truly | Edmund B. Wilson | (Assistant in Biology, J.H.U.).