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cold-water coral research in Sweden  
 
 

Lophelia pertusa (Linneaus, 1758)

Caryophylliidae, Scleractinia, Hexacorallia, Anthozoa, CNIDARIA

Lophelia pertusa is a stony coral (Scleractinia) belonging to the group Hexacorallia. As the name implies this group has six tentacles and septa (body segments), or in multiples of six (compare with Octocorallia that has eight). In contrast to many other stony corals L. pertusa is 'pseudo colonial', i.e. the individual polyps is not interconnected by living tissue. Only the young polyps that recently has emerged on the rim of an adult polyp is still attached to it by living tissues. On the image to the right you can see both an isolated polyp with no attachments to others and an adult polyp with a young bud attached. Lophelia pertusa is capable of building reefs stretching over several kilometers and several meters high.

Reproduction in L. pertusa is both sexual, and by asexual budding (cloning). A single colony is either male or female (gonochoric = separate sexes) and the reproductive cycle begins during spring when the surplus of the phytoplankton spring blooms falls down as 'marine snow' to the deep seafloor and gives energy for investment in gamete development. The cycle is completed almost a year later, and results in synchronized mass spawning during January and February (at least in the NE Atlantic). Eggs and sperm are released to the water and eggs are fertilized externally. The early embryo is called a 'blastula' consisting of a uni-cellular layer resembling a soap bubble. The blastula then goes through a phase of 'gastrulation', i.e. a part of the uni-cellular layer forms an invagination, thus creating the precursor of the gastric cavity and a 'blastopore', the primary mouth opening (stomodeum). The embryo has now become a free-swimming planula larva. After three weeks they get competent for settling and start probing the bottom. Finding a suitable substrate they attach, secrete a skeletal plate and begin metamorphosis into a polyp. The body divides into six septa and the tentacles grow out as extensions of the septa. From this single polyp new individuals will be cloned, and a large colony will grow out. Growth is slow, 4-25 mm/yr, or roughly one polyp length. A large reef thus takes centuries to develop. How dense the colony will become depends on the amount of food available, i.e. a rich food supply can support a denser colony. A large reef consist of many colonies of different sexes. However, in a small reef with just a few colonies, both sexes are not always present and reproduction can be mainly clonal, as is the case at Saekken. The adult polyp is a predator, catching copepods, but also feed on particulate matter.

 
Gross Anatomy

A reef consist of many polyps residing in their individual calice (skeletal cup). A polyp has a simple 'bauplan' (body arrangement) in the shape of a funnel with a ring of tentacles at the upper rim. The tentacles is used for catching prey (copepods) and particles and bring them to the mouth opening (stomodeum). The gastric cavity is an simple sac and there is no true organs. Instead all the bodily functions are mediated by specialized cells. There are muscle fibres controling body movements, and the retraction into the calice for protection - they can't run, but they can hide.

 
Microscopic Anatomy

The body walls of a polyp consist of only two uni-cellular layers, the epidermis lining the external body surface, and the gastrodermis lining the gastric cavity. Between these layers is a basically acellular mesoglea with connective tissue, although it can contain some interstitial cells. The gastroderm is packed with specialized cells, e.g. glandular cells excreting digestive enzymes to the gastric cavity, and spirocytes. The spirocyte is a type of cnidocyte, a unique attribute of all cnidarians. A cnidocyte is a cell that produce an organelle, the cnidocyst. The organelle is usually a coiled thread charged with barbed tips that hook onto a prey or attacker. On the cell surface there is a small trigger, called a cnidocil, that fires off the cnidocyst upon contact. The most well-known type of cnidocyte is the nematocytes of stinging jelly-fish with ampullae charged with poison. The spirocytes contain spirocysts, a barb-less coil with smaller fibres that spread out when discharged. They create a very sticky web that the polyp use to immobilize prey until the enzymes has done their job.

Another specialized cell type in scleractinians are the calicoblastic cells that excrete the skeleton. The skeleton is excreted both from inside and outside of the calice, so the calice is completely covered by a layer of living tissue. Calcium and carbonate ions are readily available in the surrounding water, and carbon dioxide from the corals own metabolism can be converted enzymatically to carbonate ions as well. In fact, much of the skeleton is excreted from metabolically derived carbon dioxide. The ions diffuse passively through channels in a first step, and are then actively transported to a closed chamber between the calicoblastic cell and the skeleton where they precipitate as aragonite crystals onto an organic matrix of large molecules. Hydrogen ions are actively pumped out of these chambers, thus creating an alkaline environment driving the precipitation.

 
Histology

NB! The histological training report I had made available here was full of errors! I had misinterpreted several structures. I will replace it as the present project I'm working on is finished and the manuscript is published. (pdf removed May 13, 2013)

 

More information about Lophelia you also find here:

http://www.lophelia.org/

 

 
   
 

A) A living polyp with extended tentacles. B) A piece of coral after chemical fixation, the tentacles are completely retracted into the calice. C) A polyp with the skeleton removedt (dissolved in acid). D-E) An adult polyp with a smaller bud (clone) still attached by living tissue.

 

  
 
 
 

An illustration showing the anatomy of a polyp inside the skeletal cup (calice). Skeletal areas in between calices is called theca. The surface inside the calice consist of lamella (septa) excreted by the sheets of live tissue (the mesenteries) that separates the body compartments. The columella is a porous area that eventually gets filled up by aragonite as the polyp continues to extend the calice. In the centre of the circlet of tentacles is the mouth opening (stomodeum). The mouth has a muscular ring controling the aperture. Underneath the mouth is the gastric cavity.

 

  
 
 
 

This is a histological preparation showing the mouth opening (oa = oral aperture) with a plug of mucus (dark lilac staining) filled with discharged spirocysts (pink coils). The red arrow marks out the group of spirocysts in the enlarged lower image. NB, the gastric cavity (gc) is erroneously marked above the oral aperture.

Clarification: cnidocytes (nemato-, spiro- and ptycho-) are celltypes that produce organelles (-cysts). An organelle is a "small organ", a specialized structure within a single cell. An organ consist of an aggregation of specialized cells, e.g. livercells, nervcells etc.

Abbreviations: gc = gastric cavity; sm = smooth muscles.