A key to the Indo-Pacific reef coral genera (Skeletons)


Version 1.0, July 2023.

This key for identification of all genera of Indo-Pacific reef corals (skeletons only) is primarily based on the photos and work done by the late Mr. Hans Ditlev, M.Sc. (1946-2013). He was a coral specialist educated and working at Aarhus University, Denmark. For a longer period, he was also employed as a biologist by the local government (Aarhus Amt, Danmark).

Hans Ditlev published the book “A field-guide to the reef-building corals of the Indo-Pacific” in 1980. This book has been very important as it is the only field guide that really is possible to use in the field. It requires only a simple magnifying glass and two pairs of forceps. Correct identification is fundamentally important for work in ecology, evolution, conservation etc., because the name of an organism is a carrier of information. Hans Ditlev also published scientific articles and described some new coral taxa.

When I got a position at Institute of Biological Sciences, Aarhus University, Denmark, in 1995, I contact Mr. Hans Ditlev. This motivated him to quit the work at the local government and return to Aarhus University where he took up the work with scleractinians with the goal to publish a revised edition of his field guide. He travelled over large areas of the Indo-Pacific and gathered photographs and new knowledge. Important parts of his collections are deposited at Phuket Marine Biological Center, Thailand; and Universiti Malaysia Sabah, Malaysia. One important aspect of his work is that he observed and photographed the living coral in the field. After that, he used hammer and chisel to detach the coral and brought it to the laboratory where he made a preparation of its skeleton. So, he got photos of, and information about the living soft parts as well as the skeleton details of the same coral individual.

His too early death motivated me and my colleague, the late prof. Jørgen Hylleberg, to try to complete his work. It resulted in the books “Indo-Pacific corals identified and illustrated by Hans Ditlev” by Jørgen Hylleberg & Tomas Cedhagen as authors; vol 1: viii+406 pp. 2015, Thailand National Science Museum; and vol 2: xxii+374 pp. 2018, Phuket Marine Biological Center.

All his colour and grayscale photos are also uploaded to iNaturalist (inaturalist.org) (c. 550 species, more than 3.900 photos).

Mr. Hans Ditlev (right person) during field work in Lahad Datu, Sabah, Malaysia, in 1998 (Universiti Malaysia Sabah). Photo: Tomas Cedhagen.
Mr. Hans Ditlev (right person) during field work in Lahad Datu, Sabah, Malaysia, in 1998 (Universiti Malaysia Sabah). Photo: Tomas Cedhagen.

This key

This key can only be used for identification of coral skeletons – not corals with the living soft parts. Presently, I also work on another key intended for identification of the living corals by Lucid AI (Artificial Intelligence).

The nomenclature follows WoRMS (www.marinespecies.org).

The key is mostly dichotomous but sometimes polytomous. For most alternatives (couplet questions) there is a picture (photo or drawing) illustrating the features in question. In some cases, the same genus appears in more than one location in the key. The reason is the large ecophenotypic variation among many species.

When the key has guided you to a genus name, there are two options:

  1. One is a link to the webpage for the genus in the database WoRMS (www.marinespecies.org). Here, it is possible to find a complete list of all existing species within the genus, as well as other information.
  2. The other option is to click on the image icon of the genus. Clicking on it results in a series of photos of species belonging to that genus. In some cases, there is only one photo. For most genera, there are more photos showing entire coral colonies as well as details. I decided to upload every photo of coral skeletons that were available. There is a species name below each photo, but in many cases just a selection of species is presented. Some common or important species of a genus can be absent because they were not yet photographed by Hans Ditlev. Use the arrows on the left or right side of the screen to move through the images within the genus.

In a few cases (Simplastrea, Zoopilus) Hans Ditlev did not have any photos. Illustrations were therefore gathered from the scientific literature, preferable the original description of the taxon.

For identification below the genus level, to species, users are recommended to use the species list in WoRMS (www.marinespecies.org) combined with other sources (handbooks, scientific literature, reliable webpages etc). Identification to species can be very difficult or uncertain because of the enormous ecophenotypic variation, hybridization, growth of chimaeras etc.

If this key for identification to genus level seems to be useful, I intend to expand and amend it.


I thank the late Mr. Hans Ditlev’s wife, Mrs. Else Ditlev, Hornslet, Denmark, for permission to use the photos in the Lucid key(s) as well as upload to iNaturalist.

Aarhus, 2023-07-11

Dr. Tomas Cedhagen (emeritus)
Aarhus University, Department of Biology, Section of Aquatic Biology
Building 1135, Ole Worms allé 1
DK-8000 Aarhus C, Denmark
Email: [email protected]