Fig. 1

Overview of G. fascicularis and its two tentacle types. a The result of aggressive behavior between Galaxea fascicularis and Pavona sp. The yellow arrow shows the Galaxea, the white arrow shows the dead area between the two corals. Photo taken in Eilat by Tali Mass. b Catch tentacles (CT, green arrow) and extended sweeper tentacles (ST, black arrow) of G. fascicularis. c and i Overall histology of Alcian Blue-stained CT (c) and ST (i). Red boxes show the approximate location of high-magnification micrographs in this figure, blue squares show approximate location of micrographs in Fig. 2. d-f Histological sections of the tip of the CT, stained with H&E (upper section in d and panel e) and with Alcian Blue (lower section in d and panel f). e and f are magnifications of the orange square in (d). Note the tightly-packed large nematocytes in the ectoderm (ec, sp - spirocysts, MpM - Microbasic p Mastigophores) and the abundance of symbiotic algae (zo) in the endoderm (en). g, h Main types of nematocytes of the acrosphere (Microbasic p Mastigophores, MpMs, which have a smooth shaft, unlike the barbed shaft of the MbMs of the sweeper tentacles). i-l Histological sections of the tip of the ST, stained with H&E (upper section in j and panel k) and with Alcian Blue (lower section in j and panel l). k and l are magnifications of the yellow square in (j). Note the mucocytes in the ectoderm between the large, elongated nematocytes, and the lack of symbionts in the endoderm. Mucocytes (mc), mucus vesicles (mv) and nematocysts are stained in blue, the latter perhaps due to the presence of poly-gamma-glutamate (an acidic polyanion) in the capsule matrix of the nematocysts. Hi-Holotrichous isorhiza nematocytes. m, n Main types of nematocytes of the acrosphere of the ST (Microbasic b Mastogophores, MbMs). Scale bars are 100 μm for (c and i), 50 μm for (d and j), 200 μm for (e, f, k and l), 25 μm for (g, m) and 10 μm for (h, n)