Scientists from the University of Tel Aviv and the National University of Ireland discovered that polyps of the coral Lobophyllia corymbosa are able to recognize foreign tissue, a key feature of an immunological response. This species also transfers nutrients to injured polyps within the same colony, but not to genetically distinct ones. These results indicate that even ancient, primitive animals such as corals have developed an immune system.

For their research, the scientists used the budding species Lobophyllia corymbosa. This species looks somewhat similar to members of the genus Caulastrea, having separate polyps in close proximity when the colony is mature.

Left: Lobophyllia corymbosa. Notice that some polyps have not disconnected yet, which is shown by the white arrow. When the colony reaches full maturity, all polyps are completely separate. There is no common tissue, called coenosarc or coenenchyme, connecting the polyps (Bricker et al, Journal of Experimental Biology, 2006).

The researchers excised 52 polyps from 14 different Lobophyllia colonies at Eilat, the Red Sea, Israel. The polyps were then either kept intact or cut longitudinally (cut through the axis as seen from the top), and grafted together with copper wire. They created three different graft types; autografts (two polyp halves from the original one), isografts (two halves from different polyps, but from the same colony, meaning they are genetically identical) and allografts (two halves from genetically different polyps).

The results of all grafting experiments were evaluated after 6 weeks. They documented all aspects of interaction; tissue fusion, skeleton fusion, no response, cytotoxic rejection and tissue overgrowth. They found that all intact polyps which had been connected close together showed no response, and simply fell apart when the copper wire was removed. However, the polyps which were cut first and then grafted together showed interesting results.

All auto- and isografts had fused completely by tissue and skeleton within 6 weeks! Histological sections (tissue which has been imbedded into paraffin, cut, and then observed by microscope) of fused partners confirmed the continuity of their tissues across the original contact area. By contrast, none of the allografts had fused. Instead, they either remained in a non-responsive state, healing the wounds caused by the sectioning, or cytotoxically (by means of toxic substances released by the polyps) rejected their allogeneic partners using an unknown mechanism.

This is completely in line with the results from Stylophora pistillata (see CORALZOO news), which showed that this species can distinguish between foreign and self tissue as well. Both species recognize allografts and isografts, but reject most allografts, much like a donor kidney is repelled by a host’s immune system after transplantation.  Fungia species are also known to recognize genetically identical polyps (Jokiel and Bigger, 1994). Contact between intact polyps usually resulted in no visible response (independent of the genetic identity of the counterparts), while cut polyps fused only with cut clonemates, rejecting allografts. This shows that corals really have evolved an immune system.

The fact that fusion of polyps only occurs after tissue injury (such as by cutting polyps in half, and them placing them closely together), is due to the fact that regeneration is activated. Corals are known to have stem cells, and after tissue injury, the regeneration process is activated, after which genetically identical coral tissue is fused.

Polyp energy transfer 

Next, the scientists determined whether these solitary polyps are able to transfer energy (photosynthates such as carbohydrates) between polyps. It is known that this occurs in many species, such as Montastrea and Porites sp. They set up a creative experiment, by which polyps were removed, incubated with radioactively labeled carbon (¹⁴C) with 20 hours of light per day. This ensured that the polyps incorporated the ¹⁴C into their tissue by high photosynthesis rates.

After this, they placed the polyps back into either the original colony (isogeneic corallum) or a genetically different one (allogeneic corallum). Some polyps were wrapped into plastic as well. One neighbouring polyp of either an isogeneic or allogeneic colony was injured as well. The experiment is shown in the diagram below:

 
Schematic diagram of the ¹⁴C-labeling and sampling locations in Lobophyllia corymbosa clones. 48 hours after reattachment of the ‘hot’ polyp (radioactive polyp) to the experimental clones, they sampled four fragments (marked as black circles) from each clone, using a round stainless-steel corer that enabled collection of similar-sized fragments (1 cm² each). One fragment was taken from the ‘hot’ polyp, one from the injured polyp adjacent to the hot polyp (injury size, 2 cm²), one from an intact polyp adjacent to the hot polyp, and one from an intact remote polyp. ¹⁴C activity in the tissues of each fragment was determined by liquid scintillation counting (Bricker et al, Journal of Experimental Biology, 2006).

They found that polyps only transfer radioactive carbon to isogeneic injured polyps, and not to either isogeneic healthy polyps, allogeneic healthy ones or allogenetic injured ones. The scintillator device detected no increase in radioactivity of samples from anything else than injured, isogeneic polyps. This is very interesting, and in line with the results above. Not only are coral polyps able to detect genetically identical clonemates, they also choose to help their injured neighbouring polyps when they are damaged, by translocating nutrients to areas of high demand. Nutrients such as carbohydrates, produced by photosynthesis, could stimulate recovery of injured polyps. This shows that a coral colony really is a collaboration of organisms, which can support one another. It is not surprising that coral polyps only help genetically identical ones, as they would have no evolutionary benefit by helping others.

The next question the scientists had, was how these nutrients are transported between polyps, as this Lobophyllia species shows no connective tissue between them. In other words, Lobophyllia clonemates displayed a typical colonial behavior in regard to energetic collaboration. The polyps were also able to choose the direction of metabolite (such as carbohydrates) transfer, without an existing tissue bridge. How is all of this possible?

The scientists hypothesized that this transfer might occur by coral mucus, which is moved to a specific direction by the polyp’s tentacles. Coral mucus is now known to be very important in nutrient cycling on coral reefs. Another theory is that the nutrients are relocated by cells, released from the donor polyp. It is also possible that both mucus and cells are transferred to injured polyps. All of this might take place during night-time, when polyp tissues are closely together. Indeed, when polyps were wrapped with plastic, no energy transfer occurred, supporting their theories.

Right, top picture: Lobophyllia corymbosa polyps expand at night, causing touching polyp tissues, temporarily. This might explain the results of the energy-transfer experiment. Energy might be transferred through mucus, cells or both. This can easily be achieved when the tissue of both donor and receiving polyps is in close proximity Right, picture below: Polyps which were wrapped in plastic showed no energy transfer to any other polyps (Bricker et al, Journal of Experimental Biology, 2006).

Concluding remarks

All these results show that Lobophyllia corymbosa polyps are highly integrated; they recycle nutrients to polyps with high demand, such as injured ones. The question is, why do some coral species choose a connective lifestyle (such as Acroporids, Montiporids, Stylophorids, etc), while others remain solitary (such as the genera Trachyphyllia, Lobophyllia and Caulastrea).

The scientists theorized that both modes of existence have their advantages. True colonial growth is ideal for transferring energy and nutrients such as carbohydrates and cells. L. corymbosa polyps, which are solitary, were not able to transfer energy over longer distances, in contrast to true colonial polyps which are connected by tissue. A solitary lifestyle, in contrast, may offer increased protection against harmful pathogens such a Vibrio bacteria (see the Coral Science archive). This is because the pathogen has more difficulty jumping from one polyp to another, while an infected Montipora or Pachyseris colony dies off quickly (unless it is fragmented, which aquarists often do when they see rapid tissue necrosis or bleaching of colonies). A second advantage of being a solitary polyp, according to the scientists, could be that individual polyps are fragmented more easily. This allows a coral to disperse itself more efficiently on the reef.

After all, biology is all about survival and reproduction…

References:

Itzchak Brickner, Uri Oren, Uri Frank and Yossi Loya, Energy integration between the solitary polyps of the clonal coral Lobophyllia corymbosa, Journal of Experimental Biology, 2006, 209, pp 1690-1695