Figure 1.
Development of Pseudosiderastrea tayamai.
Pseudosiderastrea tayamai development is similar to that of Acropora spp. in that it passes through a stage consisting of a cellular bilayer lacking a central space. (A) Eggs (e) and debris enmeshed in the mucus net shortly after release. (B) 2-cell stage. (C) 4-cell stage. (D) A section of a 4-cell stage, with offset blastomeres. (E) Section of the 16 cell stage. (F) Compact spherical embryo. (G) Flattened embryo. (H) Intact embryos resembling a tightly cupped hand. (I) Section of an embryo similar to those in H, which if unwound would resemble a prawn chip. (J) A bowl-shaped embryo; (K) Enlargement of Figure 4J. Lipid bodies (white arrowheads) are gradually coalescing to form larger masses of lipid as they move centrally (black arrows). (L) The embryo forms a flattened sphere. (M) Later the embryo becomes more rounded. (N) Section of a spheroidal embryo. The lipids are moving centrally, out of the future ectoderm at the periphery and into the central future endoderm. (O) Section of a pear-shaped planula. Mesoglea formation is apparent between ectoderm and endoderm (arrows) and invagination has started (asterisk). Most of the lipids are in relatively large droplets but lipids are still present in the ectoderm. (P) In this elongate planula the pharynx (p) has formed, leading inward from the oral pore. Diverse cell types (e.g. nematocysts, granular cells) are now apparent in the ectoderm (ec), all lipids have moved into the endoderm (en), and the mesoglea (m) is clearly apparent separating endoderm from ectoderm. Traces of the mucus net are apparent surrounding many of the embryos (black arrowheads), even after histological processing.
Figure 2.
Development of Galaxea fascicularis.
(A) Eggs (pinkish orange, below the line in the lower left corner) and pseudo-eggs (lighter in color, above the line, upper right corner). (B) True eggs are featureless and appear more dense and (C) a section reveals that they are filled with small lipid bodies. In contrast, pseudo-eggs appear vacuolated (D) and the lipid is localized to much larger bodies (E). (F) 2-cell stage. (G) 4-cell stage. (H) 8-16 cell stage. (I) Morula stage. (J) Bowl stage (concave side up). (K) Bowl stage (embryo shown in J-viewed from above). (L) Glancing section of an embryo comparable to J and K. (M) Enlargement of the embryo shown in L, with two sizes of lipid-containing bodies. It appears that the large arise by fusion of the small. (N) Spheroidal embryo with closing blastopore showing center filled with lipid. (O) Mesoglea is gradually forming at this stage. (P) Whole mount of stage similar to O. Red pigment localized at the animal pole as the result of a marking experiment persists in G, H, J, K and P. The asterisk in L and N marks the blastopore.
Figure 3.
Development of Montipora hispida.
(A) Egg-sperm bundle; Symbiodinium (darker spots within the eggs) are distributed throughout the eggs. (B) Section of egg-sperm bundle, showing sperm (s) tightly packed in the middle of the bundle, surrounded by eggs (e). (C) Polar bodies (pb) on the surface of the egg. (D) Enlargement of Figure 3C; (E) Section of egg. Symbiodinium are marked with arrowheads in this and succeeding panels. (F) The first cleavage results in a heart-shaped embryo. (G) Four-cell stage; the nucleus occupies a substantial part of each cell. (H) Embryos at the 4-16 cell stage, showing unevenly dispersed Symbiodinium in the arrowed embryos. (I) The 32-64 cell stage; again with Symbiodinium unevenly dispersed near the surface of the embryo. (J) Section of an embryo comparable to I. (K) Section of slightly older embryo. (L) Section of prawn chip stage. (M) Bowl-shaped stage, viewed from above. (N) Section of stage comparable to M, the embryo has become thicker. (O) Spherical stage, the asterisk in this and the following panels indicates the blastopore. (P) Section of an embryo similar to that shown in O. At this stage the mesoglea is gradually forming and Symbiodinium are moving into the endoderm. (Q) Later spherical stage; the mesoglea has clearly formed, with a row of ectodermal nuclei just above it (arrow). Few Symbiodinium are now seen in the ectoderm. (R) Pear-shaped planula stage. (S) Section of embryo comparable to that shown in R. There is a line of nuclei located in the columnar ectodermal cells, just above the mesoglea (arrows). (T) Section of elongated planula; the ectodermal cells are variously differentiated and the pharynx has grown inward.
Figure 4.
Development of Pavona Decussata.
(A) Spawned egg. (B) 2-cell stage. (C) Section of 2-cell stage; the two nuclei are offset. (D) 4-cell stage. (E) Section of 4-cell stage. Successive divisions create additional blastomeres (F-G) 16-cell stage. (H) Section of 16-cell stage showing the start of the blastocoel (bl in this and subsequent figures). (I) 32-cell stage. (J) The cells surrounding the blastocoel are starting to become columnar. (K) The embryo starts to flatten and the first sign of the pseudo-blastopore appears (plus sign in this and subsequent panels). (L) The surface of the embryo becomes smoother due to continuing cell division. (M) Bowl-shaped embryo. (N-P) The embryo becomes pear-shaped, as the ectodermal cells become more elongate. (Q) Spheroidal blastula. (R) Section of the same, showing the blastocoel starting to fill as material moves inward. (S) Embryo just before appearance of the blastopore/oral pore. (T) Invagination of the blastopore/oral pore has begun (asterisk in this and subsequent panels). (U) Section of invaginating planula. (V) More elongate whole mount planula. (W) Invagination has proceeded, mesoglea has formed (arrows), and ectoderm (ec) and endoderm (en) are clearly apparent. (X) Transverse section of planula similar to W.
Figure 5.
Development of Oulastrea crispata.
(A) Spawned egg. (B) 2-cell stage. (C) At the 4-cell stage the blastomeres are offset. (D) 8-cell stage. (E) 16-cell stage. (F) Another embryo at the 16 cell stage. (G) This embryo has started to flatten. The blastocoel is apparent by the lightening at the center of the embryo. (H) By this stage the ectodermal cells are becoming columnar rather than circular in outline. The central blastocoel is apparent. (I-L) As cell division continues the flattened spheroid gradually resumes a spherical shape. (M) Pear-shaped planula. (N) Invagination (inv) has started at the oral end of the planula. (O) There is a ledge-like constriction at the oral end of the planula and the aboral end has thickened in preparation for settlement. (P) The constriction at the oral end of the planula is now less sharp and has moved aborally, the aboral end has become more rounded and the oral pore is now clearly apparent. In M-P the blastopore/oral pore is marked with an asterisk.
Figure 6.
Development of Platygyra contorta.
(A) Spawned egg. (B) 4-cell stage-the blastomeres are offset. (C) Section of slightly older embryo. (D) The 16-cell stage is much less regularly arranged than in the other species discussed here. (E) The embryo has started flattening and is now highly irregular in shape. (F) At a slightly older stage a blastocoel (bl) becomes apparent. The pseudo-blastopore is marked by a "+" in this and subsequent panels. (G) This stage corresponds to the prawn chip stage of Acropora, but it has a cushion shape due to the enclosed blastocoel. (H) Cell division has continued, leading to a smoother surface as the pseudo-blastopore deepens. Loosely consolidated lipid fills the area that will eventually be occupied by endoderm. (I-K) Embryos of this species are often highly variable in shape. At this stage lipid and cells are starting to move inward to fill the blastocoel (bl). (L, M) At gastrulation two separate pores appear initially (asterisks in L) and then expand and grow together until ultimately a single blastopore is formed (asterisk in M). (N) Eventually a slightly elongate planula is formed with the oral pore at one end (asterisk). (O) Transverse section of planula showing endoderm (en), mesoglea (arrows) and ectoderm (ec). (P) Elongate planula showing pharynx extending inward from the oral pore (asterisk).
Figure 7.
Development of Favites abdita.
(A) Spawned egg. (B) This heart-shaped embryo is just beginning its first division. (C) Section of a heart- shaped embryo. The nucleus (n) appears to be just starting to divide. (D) A 2-cell embryo. (E) 4-cell stage (F) 16-cell stage. (G) The embryo has flattened and a blastocoel (bl in this and succeeding panels) has formed. (H-I) The embryo has now become cushion-shaped with a depression, the pseudo-blastopore (plus sign in this and subsequent panels) appearing in one side. (J-K) Embryos vary in shape as the pseudo-blastopore deepens. (L) The embryo swells, becoming more spherical, at the same time maintaining the pseudo-blastopore. (M) Section of a nearly spherical embryo. (N) Spherical embryo with the remains of the pseudo-blastopore. (O) A new invagination, the blastopore (asterisk in this and succeeding panels) starts in a different location from the pseudo-blastopore. (P) Section of an embryo comparable to O, showing that the two pores are quite distinct (labels as in O). (Q, R) Invagination has proceeded: the asterisk marks the blastopore. (S) The blastopore has now become smaller. (T) The blastocoel has now disappeared and cells at the margins of the invaginating tissue have taken on an elongate bottle shape. (U) The mesoglea is now clearly apparent, separating endoderm from ectoderm. (V) Higher magnification of U, showing highly elongated cells at the margins of the invaginating tissue. (W) Two germ layers, ectoderm (ec) and endoderm (en), are apparent surrounding the space that will eventually form the gastrovascular cavity. (X) Cellular differentiation is apparent in the ectoderm and lipid-filled endodermal cells have invaded the central cavity. (Y) Higher magnification of the oral pore region showing sharply invaginated margins of the pharynx (arrowheads). (Z) Section of the elongating planula showing the central cavity filled with lipid-containing endodermal cells (en). (AA) The pharynx (arrow) has elongated; (BB) Primary polyp immediately after settlement. The ectoderm is translucent while the endoderm is opaque white. The mouth (m) is central.
Figure 8.
Development of Echinophyllia aspera.
(A) A spawned egg. (B) A possible polar body pinching off. (C) The beginning of the cleavage furrow that will result in a 2-cell embryo; (D) Section of the stage shown in C. (E) Section of a 2-cell embryo. (F) At the 4-cell stage the cells are firmly attached to each other. (G) Section of a 4-cell embryo, with nuclei moving to the center of each cell. (H) 16-cell stage (I) Section of 16-cell stage with apparent blastocoel. (J) 16-32 cell stage (K) The embryo has started to flatten. (L) Cushion-shaped embryo (the plus sign marks the pseudo-blastopore in this and subsequent panels). (M) The return to a spherical state starts with swelling from the edge. (N) The pseudo-blastopore is still apparent. (O) A spheroidal embryo. (P) Section of an embryo comparable to O with material moving toward the blastocoel. (Q) Invagination of the blastopore/oral pore (asterisk) has begun. (R) Invagination has continued and the blastocoel has disappeared. (S) The cavity formed by invagination is being filled by the breakdown and migration of cells from the inner layer, and mesoglea formation has started (arrows). (T) Elongate planula.
Figure 9.
Development of Goniastrea favulus.
(A) Spawned egg. (B) There is no lipid in the immediate vicinity of the nucleus. (C) 2-cell stage. (D) 4-cell stage. (E) Section of the 4-cell stage. (F) At the 16-cell stage the cells are organized into a sphere; (G) A section of the embryo at this stage reveals that the sphere is hollow, and that the cells surround a central blastocoel (bl, in this and succeeding panels). (H) 32-cell stage. (I) A spherical blastula. (J) Section of an embryo comparable to that shown in I. (K) The pseudo-blastopore is now apparent. (L) The embryo is spherical and material is starting to move into the blastocoel at one point (arrowheads). (M) Cell division has continued, material is moving into the blastocoel (arrowheads) and mesoglea is beginning to form (arrows). (N) Invagination of the blastopore (asterisk in this and subsequent panels) has started and there is material moving into the blastocoel from the sides. (O) The blastopore/oral pore has now formed a significant depression as the embryo resumes its formerly spherical shape (P). (Q-R) The oral pore shrinks in diameter as the embryo begins to elongate. (S) Planula. (T) Section of a planula showing the central area now filled with lipid and delimited by a well formed mesoglea (arrows).
Figure 10.
Development of Dipsastraea (Favia) speciosa.
(A) Spawned egg, showing polar body (pb). (B) 2-cell stage, the blastomeres are almost fully separated. (C-D) 4-cell stage. The cells are tightly adherent. (E) 16-cell stage. (F) Section of 16-cell stage with blastocoel (bl in this and subsequent panels). Nuclei (n) occupy a substantial portion of the volume of each cell. (G) 32-cell stage. (H) The embryo has now started to flatten. (I-K) Further cell division and flattening have resulted in an embryo comparable to the prawn chip stage of Acropora, but for the presence of a significant blastocoel (bl). (L) The pseudo-blastopore has appeared (plus sign). (M) The embryo has now become spherical with a smooth surface except for the invagination associated with the pseudo-blastopore. (N) Section of spherical embryo with the pseudo-blastopore; within the periphery of the embryo is a layer of lipid globules. The blastocoel is starting to fill. (O, P) The pseudo-blastopore has now closed. (Q) This section reveals a flattening where invagination of the blastopore is about to begin (asterisk). (R) Invagination of the blastopore has now started. (S-T) The blastopore has first become circular and then started to shrink in diameter. (U) The embryo has now begun to elongate in the oral-aboral axis. (V) This section shows the blastopore closing and two germ layers being formed. (W) A pear shaped planula with aboral end (ab in this and the next figure) becoming thicker. (X) Barrel-shaped planula.
Figure 11.
Development of Phymastrea valenciennesi.
(A) Spawned egg. (B) 2-cell stage. (C) The first and second cleavage furrows both start from the animal pole. (D) 4-cell stage. (E) The 16-cell stage is roughly spherical. (F) A section of the 16-cell stage reveals that it is a solid mass of cells. (G) By the 32-cell stage a blastocoel (bl) has developed. (H) A pseudo-blastopore (plus sign) has developed in the side of the formerly spherical embryo. (I) Section of an embryo comparable to that shown in H. (J-K) Older embryos, looking down on the deepening pseudo-blastopore. (L) Section of an embryo comparable to that shown in K. (M) The pseudo-blastopore has disappeared and the embryo has expanded to form a spheroid, as shown in N. (O) Formation of the blastopore (asterisk in this and later panels) has begun. (P) The blastopore/oral pore begins to close. (Q) Invagination is proceeding, the blastocoel has disappeared, mesoglea is forming (arrow), and distinct endoderm (en) and ectoderm (ec) are becoming apparent. (R-S) The planula gradually elongates and the translucent ectoderm is clearly differentiated from the opaque cream-colored central endoderm. (T) A longitudinal section of a mature planula reveals a well differentiated ectoderm (ec) separated from a still lipid-filled endoderm (en) by a well-developed mesoglea (black arrows). The pharynx (p) and well-differentiated mesenteries (m) are also apparent.
Figure 12.
Diagrammatic representation of the two extreme forms of coral development.
(A) Early in their development Acropora spp. embryos pass through a prawn chip stage consisting of an extended cellular bilayer lacking a blastocoel [14, 15, 19]. Through changes in cell shape this extended sheet of cells shrinks in diameter, thickens and the sides bend inward, forming a bowl-shaped embryo. The ultimate result of these movements is that the cells lining this concavity are overgrown by the outer cells, resulting in an outer sphere of ectoderm surrounding an inner mass consisting of lipid granules, cellular fragments and cells. This outer sphere is complete, with no trace of a pore to be seen. We regard this process as gastrulation and the initial pore as the blastopore. Whether or not this interpretation is accepted this process is markedly different from that shown in (B) which is that seen in robust corals such as Goniastrea favulus. Rather than a spatially extended prawn chip lacking a blastocoel, these corals pass through a cushion stage, which is flattened but always retains a blastocoel. This cushion then rounds up and develops a depression in its side which we have termed a "pseudo-blastopore", but no material enters the blastocoel. The pseudo-blastopore then disappears, the embryo rounds up again, and then a second pore, the true blastopore, appears, this time associated with the passage of material into the blastocoel. In such corals the blastopore remains open and transitions seamlessly into oral pore/mouth.