762 JOVRNAL, BOMBAY NATURAL Hist. SOCIETY, Vol. XXV. 



Besides the interest which is attached to Noctiluca as the cause of the 

 red colouration of the sea, there is another property of this I'lagellate which 

 brings it even more prominentlj^ before one's notice. As its name suggests, 

 Noctiluca is perhaps the commonest organism to which the diffused phospho- 

 rescence of the sea at night is due. So many marine animals and plants are 

 luminous that it is impossible to say off-hand to which one of them the phos- 

 phorescence on any particular night is mainly due; but, whatever other light- 

 emitting creatures are present, Noctiluca usually accompanies them, and it 

 is frequently the predominating form, both in tropical and home seas. 



A common method of studying the minute marine floating organisms 

 (Plankton) is to run seawater from a bathroom tap through a very fine- 

 meshed silk net. By this means the solid contents of a considerable 

 quantity of water can be concentrated and examined. The solid particles, 

 animals and plants, are strained oft', and collect in a receptacle at the 

 bottom of the net. Unfortunately Noctiluca is a very fragile creature, 

 and is so broken up, if subjected to this process, tha^ nothing remains of 

 it, but its empty and shrivelled up cell wall ; in which condition it ceases 

 to be phosphorescent and easily escapes notice. It is easy, of course, to 

 procure undamaged specimens of Noctiluca by lowering a bucket over the 

 side of the ship at night, but it is difficult to isolate such a small creature, 

 for microscopic examination irj such a large quantity of water. 



On a recent voyage through the Persian Gulf, I collected a bucketful of 

 * red water " about midday. With the exception of a very few minute 

 Entomostvaca, the only organism present was Noctiluca, which was in large 

 numbers. It remained alive and active up to 11 p.m. Whenever the bucket 

 was jarred, as by a sudden kick, the upper layer of the water became 

 intensely phosphorescent immediately, and remained so for a second or 

 two. Any concussion seems to cause these Flagellates to become luminous ; 

 one sees this very well in the momentary glow which appears in the waves 

 caused by the ship's progress through the water. 



Although Noctiluca seems to be the usual cause of red water in the 

 Persian Gulf, a red colouration of the sea is not always due to it. 

 Dr. G. S. West mentions several other Peridinians (Dinoflagellates) 

 which cause a distinct colouration of the sea, by their presence in very 

 large numbers. He says (Cambridge Botanical Handbooks, Algpe, Vol. I.) 

 " In the Indian Ocean, Ceratiuni volans sometimes gives the water a 

 brownish purple colour ; in the Japanese Seas, Gonyaula.T polygramma is 

 frequently the cause of a brown colouration of the water ; and in the 

 vicinity of Bombay, Peridijiium sanyuineum has been known to colour the 

 sea red. Gonyaulax polyedra causes a red colouration of large areas of the 

 sea off the coast of California during the summer months ; it also exhibits 

 a luminosity at night," 



Another Alga, one of the Myxophycete, Trichodesmiuni evythraum, 

 frequently colours the Red Sea. One of my brother officers tells me that 

 he also collected some " red water" in the Persian Gulf about a year 

 ago; the organism that he found was "rod-shaped "; possibly this was 

 Trichodesmium, which, however, I have not observed myself. 



In spite of much good work that has been done for its determination 

 in all these cases, the exact nature of the red pigment, and its disposition 

 in the cell, remain uncertain. The species of Noctiluca found in European 

 Seas is N. miliaris. That which is so common in the Persian Gulf appears 

 to be indistinguishable morphologically from this form, but I do not know 

 whether it is actually specifically identical. 



H. J. WALTON, C.M.Z.S., Lieut. -Col., i.m.s. 

 Bombay, February 1915. 



