Abstract
A comprehensive understanding of coral reproduction and development is needed because corals are threatened in many ways by human activity. Major threats include the loss of their photosynthetic symbionts (Symbiodinium) caused by rising temperatures (bleaching), reduced ability to calcify caused by ocean acidification, increased storm severity associated with global climate change and an increase in predators caused by runoff from human agricultural activity. In spite of these threats, detailed descriptions of embryonic development are not available for many coral species. The current consensus is that there are two major groups of stony corals, the "complex" and the "robust". In this paper we describe the embryonic development of four "complex" species, Pseudosiderastrea tayamai, Galaxea fascicularis, Montipora hispida, and Pavona Decussata, and seven "robust" species, Oulastrea crispata, Platygyra contorta, Favites abdita, Echinophyllia aspera, Goniastrea favulus, Dipsastraea speciosa (previously Favia speciosa), and Phymastrea valenciennesi (previously Montastrea valenciennesi). Data from both histologically sectioned embryos and whole mounts are presented. One apparent difference between these two major groups is that before gastrulation the cells of the complex corals thus far described (mainly Acropora species) spread and flatten to produce the so-called prawn chip, which lacks a blastocoel. Our present broad survey of robust and complex corals reveals that prawn chip formation is not a synapomorphy of complex corals, as Pavona Decussata does not form a prawn chip and has a well-developed blastocoel. Although prawn chip formation cannot be used to separate the two clades, none of the robust corals which we surveyed has such a stage. Many robust coral embryos pass through two periods of invagination, separated by a return to a spherical shape. However, only the second of these periods is associated with endoderm formation. We have therefore termed the first invagination a pseudo-blastopore.
Highlights
Corals are highly variable in their reproductive patterns
Embryos of Pseudosiderastrea tayamai, Galaxea fascicularis, Montipora hispida and M. digitata were collected in Okinawa Prefecture; embryos of Platygyra contorta, Favites abdita, Echinophyllia aspera, Goniastrea favulus, Pavona Decussata and Phymastrea valenciennesi in Kochi Prefecture; embryos of Dipsastraea speciosa and Oulastrea crispata in Wakayama Prefecture
After spawning Symbiodinium were not found immediately adjacent to the site of polar body emergence (Figure 3C) and were sometimes unevenly dispersed (Figure 3H), there was no consistent pattern of spatial restriction except that essentially all had moved to the endoderm before settlement
Summary
There are species in which colonies may have a single sex throughout life, while other species may be sequential hermaphrodites, with sex based on size, or simultaneous hermaphrodites They may be synchronous spawners, releasing eggs and sperm or egg-sperm bundles, or brooders in which early development takes place within the colony and planulae are released. Based on molecular sequence data, scleractinian corals can be divided into two large clades, the robust and the complex, that appear to have diverged more than 200MYA [1,2,3,4,5]. Before the widespread availability of sequence data, coral taxonomy was based on morphological characters (e.g. 6,7) but few of the families created on this basis are presently regarded as monophyletic based on recent molecular studies [8].
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