Evolution of the Caribbean subfamily Mussinae (Anthozoa: Scleractinia: Faviidae): transitions between solitary and colonial forms

Understanding evolutionary transitions in scleractinian corals is fundamental to predicting responses of coral reefs to climate change. We examine transitions between solitary and colonial corals in the fossil record, focusing on the Caribbean solitary reef coral <i>Scolymia</i> and members of the subfamily Mussinae. Fossil specimens are selected from a large well-documented collection of Neogene Caribbean corals, and a total of 23 species are distinguished using 15 morphological features. Ten of the 23 species are extant Caribbean species, seven are previously described extinct Neogene species, and six other extinct species are formally described as new. The 7 + 6 extinct species are added to a morphological data set consisting of 30 extant molecularly characterized species plus seven additional extinct (five Eocene, two Neogene) species. In addition to the Caribbean subfamily Mussinae, the extant species include the Indo-Pacific families Merulinidae and Lobophylliidae, and the Caribbean subfamily Faviinae. Phylogenetic analysis was performed on the data using maximum parsimony, and the results reveal four clades, which correspond with previously reported molecular clades. Solitary corals group most closely with Caribbean Mussinae and Indo-Pacific Lobophylliidae, whereas colonial corals are present in all four clades. Within Caribbean Mussinae, members of the colonial genera <i>Mycetophyllia</i> and <i>Isophyllia</i> form distinct subclades, as do the extinct solitary genera <i>Antillia</i> and <i>Antillophyllia</i>. The relationships within <i>Scolymia</i> are less well defined but its members appear more closely related to extinct solitary genera dating back to the Eocene. These results indicate that evolutionary transitions between solitary and colonial corals have been rare within the Mussinae. Except <i>Antillophyllia</i>, most Mussinae genera are restricted to the Caribbean. During the late Miocene, <i>Mycetophyllia</i> diversified and three other modern Mussinae genera (<i>Mussa</i>, <i>Scolymia</i>, <i>Isophyllia</i>) originated in association with increased Caribbean productivity. Mussinae that were more likely to survive Plio–Pleistocene extinction may have taken refuge in deep forereef habitats.

理解石珊瑚（scleractinian corals）的演化过渡，对于预测珊瑚礁对气候变化的响应至关重要。本研究聚焦化石记录中独居与群居珊瑚的演化过渡，以加勒比海独居礁珊瑚星珊瑚属（Scolymia）以及肉芝珊瑚亚科（Mussinae）的类群为研究对象。化石标本采自一套规模庞大、记录完备的新近纪加勒比海珊瑚馆藏，基于15项形态特征共鉴定出23个物种。23个物种中，10个为现生加勒比海物种，7个为此前已发表的绝灭新近纪物种，剩余6个绝灭物种则为本次研究正式命名的新种。上述7个已发表绝灭物种与6个新命名绝灭物种被加入至一套形态数据集当中，该数据集原本包含30个经分子鉴定的现生物种，以及额外7个绝灭物种（其中5个为始新世物种、2个为新近纪物种）。现生研究类群除包含加勒比海肉芝珊瑚亚科的物种外，还涵盖了印度-太平洋的裸肋珊瑚科（Merulinidae）、叶状珊瑚科（Lobophylliidae），以及加勒比海的蜂巢珊瑚亚科（Faviinae）的类群。本研究采用最大简约法（maximum parsimony）对该数据集开展系统发育分析，结果揭示了4个演化支，这些演化支与此前报道的分子演化支相吻合。独居珊瑚与加勒比海肉芝珊瑚亚科以及印度-太平洋叶状珊瑚科的类群聚类关系最为紧密，而群居珊瑚则分布于全部4个演化支中。在加勒比海肉芝珊瑚亚科内，群居属类群褶叶珊瑚属（Mycetophyllia）与等叶珊瑚属（Isophyllia）各自形成独立的亚演化支，绝灭独居属类群安替列珊瑚属（Antillia）与安替列叶珊瑚属（Antillophyllia）亦是如此。星珊瑚属（Scolymia）内部的亲缘关系尚未得到明确解析，但其类群似乎与可追溯至始新世的绝灭独居属类群亲缘关系更近。上述结果表明，独居与群居珊瑚之间的演化过渡事件在肉芝珊瑚亚科内十分罕见。除安替列叶珊瑚属（Antillophyllia）外，绝大多数肉芝珊瑚亚科的属类群仅分布于加勒比海海域。中新世晚期，褶叶珊瑚属（Mycetophyllia）发生辐射演化，另外3个现生肉芝珊瑚亚科属类群——肉芝属（Mussa）、星珊瑚属（Scolymia）与等叶珊瑚属（Isophyllia）——伴随着加勒比海海域生产力提升而起源。能够在上新世-更新世绝灭事件中幸存的肉芝珊瑚亚科类群，可能曾在深水礁前生境中寻求庇护。