Abstract
A new species of the genus Microcosmus was described in this study based on specimens collected from the coast of Xilian Town, Xuwen County, Zhanjiang, Guangdong Province, China. The morphological and molecular characteristics of this new species, Microcosmus sp. z YZ-2024 (YZ-2024), distinguish it from other sea squirts. Firstly, YZ-2024 can be preliminarily distinguished by the following morphological features: (1) smooth surface of the tunica, without protuberances; (2) except for the attachment site, the tunica is orange-red; (3) absence of neural ganglia, and the tunica does not contract abruptly when disturbed; (4) the heart is oval-shaped. Most importantly, the mitochondrial genome characteristics of YZ-2024 effectively and accurately distinguish it from other congeneric species, with a mitochondrial genome size of 14,520 bp (GenBank No. PP067884) and the proportions of bases A, T, G, and C comprising 26.83%, 47.16%, 16.91%, and 9.10%, respectively. Except for the gene COX1, the mitochondrial genome arrangement of YZ-2024 is totally different from that of 22 other species in the class Ascidiacea. Evolutionary analysis has indicated that YZ-2024 clusters with its congeneric species Microcosmus sulcatus. Interestingly, YZ-2024 belongs to Pyuridae, but it clustered with 12 species of Styelidae into a clade. Based on this, it can be suggested that YZ-2024 is a new species of sea squirt in the northern Beibu Gulf of the South China Sea. Moreover, this study is the first report of molecular identification of sea squirt species in the northern Beibu Gulf of the South China Sea.