Isometra vivipara and Phrixometra nutrix are two brooding feather stars (order Comatulida) that are widely distributed along the southern tip of South America to the Antarctic. We examined 210 specimens of I. vivipara and 38 specimens of P. nutrix collected during five cruises in the southwestern Atlantic between 91 and 642 m depth. Specimens were analyzed and deposited in the National Invertebrate collection, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”. Isometra vivipara reaches an arm length of 49 mm long and shows two stages of brooding: in the female genital pinnules and on the cirri of the mother. In this paper, we propose that spermatozoa reach the ovary via the ambulacral groove. By contrast, P. nutrix is smaller with a maximum recorded size of 26.8 mm, and brooding occurs only inside the female genital pinnules. Here, we record, for the first time for P. nutrix, spermatozoa shape and a cleaved egg attached externally to a genital pinnule of a female. The distributions of both species and their abundances in some areas could be related to their doliolaria larval stage. In addition, we extend the bathymetric range of P. nutrix to 512 m in Marine Protected Area Namuncurá/Burdwood Bank and distribution to the Mar del Plata Submarine Canyon area. We also propose mode of reproduction and developmental patterns as attributes for identification and possible explanations for their differences in distribution.

Isometra vivipara和Phrixometra nutrix是两个育雏羽星（Comatulida 目），它们广泛分布于南美洲南端至南极。我们检查了在大西洋西南部 91 至 642 m 深度的五次航行中收集的210 个I. vivipara标本和 38 个P. nutrix标本。标本被分析并存放在国家无脊椎动物收藏馆，Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”。胎生等角线虫 达到 49 毫米长的臂长，并显示出两个育雏阶段：在雌性生殖器小叶和母亲的卷毛上。在本文中，我们提出精子通过脐带沟到达卵巢。相比之下，P. nutrix较小，记录的最大尺寸为 26.8 毫米，育雏仅发生在雌性生殖器叶尖内。在这里，我们第一次记录了P. nutrix 的精子形状和一个从外部附着在女性生殖器小叶上的卵。这两种物种的分布及其在某些地区的丰度可能与其幼虫阶段有关。此外，我们扩展了P. nutrix的测深范围512 m 在海洋保护区 Namuncurá/Burdwood Bank 和分布到 Mar del Plata Submarine Canyon 区域。我们还提出繁殖模式和发育模式作为识别和可能解释它们分布差异的属性。