A new species of Anabaena (Cyanophyta, Nostocaceae) from a Minnesota lake. III

Christmas Lake, in which Anabaena [usca was found, is about 30 km west of Minneapolis, Minne­ sota. The lake is roughly 1·2 k m in diameter, has a maximum depth of 30 m, a thermocline at about 6 m, and, during the period of sampling in 1974, May-November, secchi disk readings of 3'5 to 5·5 m. The lake water was thus always clear. At no time was there a bloom of any alga. Samples were obtained by long tows of the plankton net. The shore terrace of Christmas Lake is generally narrow, and the slope to deep water is steep. A belt of aquatic plants extends all around the lake. It contains principally Potomogeton spp., Vallisneria americana, Myriophyllum exalbescens, Chara vulgaris and especially Ceratophyllum demersum. The lake bed in deep water is clay. The bed is soft and friable in the area of plant growth, and sandy in the shallow water shoreward of the plant growth. Anabaena [usca was first seen in the plankton 20 August 1974. It developed immature akinetes by 8 September. It had many akinetes in the period 1-12 October, and at the end of this period the alga showed signs of disintegration. A few remnants of the alga, without akinetes, were seen in the samples of 23 October and in the last sample taken on 8 November 1974. Figure 1 is a colony of A. [usca spread under a cover glass. Figures 3 and 4 are trichomes in open water, without a cover glass. The trichome of Fig. 3 is helical, but the helix changes from left hand to right hand near the centre of the trichome. Figure 4 shows the more usual colonial arrangement, i.e. many circular parts, irregularly arranged, with wide spacing. The circular parts have diameters of 50-75 /-Lm. Figure 2 is a trichome, containing two immature akinetes and a heterocyst, in an ink solution, to show the mucilage. The narrow mucilage shown is typical. The cells of A. [usca are lemon shaped, with a very narrow connection between cells (Figs 5 and 6). The cells are a brown colour at all stages of growth. The cells of A. [usca have gas vacuoles. They also have several filaments in each cell which run the length of the cell and which appear to be gas vacuoles. The cells of A. [usca are 5'5-6'5/-Lm wide and (6'0)-7'0-8'5-(10,0) /-Lm long. The heterocysts of A.f usca are spherical, or nearly so (Figs 5, 6 and 7). They have a wide hyaline wall. The heterocysts have a ho mogeneous light green colour when mature. They are granulated when immature. The heterocysts are spaced 100-450 /-Lm apart along the trichome. The heterocyst dimensions are (6'0)-6'5-8'0-(8'5) /-Lm wide and (6'5)-7'0-8'5(9'0) /-Lm long. The akinetes of A. [usca form one, two or rarely three, cells from a heterocyst, and usually on both sides of the heterocyst (Figs 1, 5, 6 and 7). The shape of the akinetes is that of a short cylinder early in growth ( Figs 5 and 6), but in later growth the akinetes are sausage shaped ( Figs 1 and 7). The akinetes form at nearly all heterocysts. They have gas vacuoles when immature, includiQg, as in the cells, several filaments which run the length of the akinete and which appear to be gas vacuoles. The gas vacuoles disappear as the akinete ripens, and surface granules then appear. The akinetes are blue-green in colour when imnlature, yellowish-green when ripe. Two akinetes in series on one side or on both sides of the heterocyst occur occasionally. The akinetes do not at any time form central masses in the colony, as the akinetes of Anabaena lemmer­ mannii do. The akinete dimensions are (7'5)-8,010·0-(12·0) /-Lm wide and (16)-26-42-(52) /-Lm long. The following literature has been examined and no Anabaena sp. similar to A. [usca was found in