Sea spiders (Pycnogonida) are a fascinating and understudied group of organisms. They are the closest relative of all arachnids (spiders, scorpions, mites, etc.) without being an arachnid. Sea spiders and arachnids form a group known as the Class Chelicerata. There are 1,300 species of sea spiders distributed from tropical to polar waters, from intertidal pools to the deep sea. Most of them are in the size range of millimeters to a few centimeters, but some can reach up to 70 cm. In the tropics it is more common to find small species, while on the deep sea or temperate/polar waters they tend to be larger. Their morphology is unique. Many of their internal organs extend into their legs, because of their reduced cephalothorax and abdomen.

Due to their challenging sampling, small size, and cryptic morphology, they remain poorly known. Despite the large taxonomic diversity across many groups of organisms present on the Great Barrier Reef, only 33 named species of sea spiders are known from there. Several of those records come from Lizard Island. However, during the last decade there has not been any systematic survey for the group on the island. These last ten years have witnessed four strong coral bleaching events and several cyclones. As the strength and frequency of these events is increasing due to climate change, it is urgent to characterize this neglected portion of the biodiversity in order to understand comprehensively how the Great Barrier Reef is affected. Moreover, this basic information is essential to understand the ecology and evolutionary history of these organisms.

Darko Cotoras collecting coral rubble to inspect for sea spiders. Photo: Todd Oakely.

In 2019, sponsored by a Lizard Island Reef Research Foundation postdoctoral fellowship, I started a project which aims to understand the arachnid diversity present on Lizard Island. Such research has never been done before on the island. During the field work, I had the opportunity to mentor John McCormack (at that time student at the University of Virginia) on a project to characterize the diversity and distribution of beetles across the island. This work is already published (McCormack and Cotoras, 2021) and, as a result, 229 pictures corresponding to 78 species were uploaded to the Lizard Island Field Guide and a summary poster created.

After 4 years and a pandemic, I have returned to Lizard Island to continue exploring its understudied biodiversity, this time focusing on the sea spiders. In preparation, I consulted with the expert taxonomist Claudia Arango (Griffith University and Queensland Museum) and reviewed museum collections at the Museum für Naturkunde (curator Jason Dunlop) in Berlin and the Bishop Museum (collection manager Holly Bolick) in Honolulu.

The field sampling took place during August 2023 and it was done in collaboration with professor Todd Oakley from the University of California, Santa Barbara. Todd is an expert in ostracods and our sampling strategies complemented very well. Every day we would visit a different coral reef. While Todd deployed his traps, I would collect different types of substrates. They included coral rubble, corals fragments, algae and sediment. These materials were carefully inspected under the stereoscope during the afternoon and into the evening. Perhaps, one of the most striking first impressions was to realize that the coral rubble was a very rich substrate to find the sea spiders. Sorting through coral rubble reminded me of sorting through leaf litter in the terrestrial environment.

Left. Dorsal view of a sea spider. The total body size is approximately 1 mm. This picture was taken on a microscope at Lizard Island Research Station. Right. Coral rubble corresponds to one of the habitats where sea spiders can be found. Photo: Darko D. Cotoras.

After the field work, I focused on species identification. First, I was hosted by curator Michael Rix at the Queensland Museum where I familiarised myself with their collection and started with the classification of my material. Here, I also worked directly with the expert taxonomist Claudia Arango, which was enormously valuable.

Then, at the Australian Museum hosted by collection manager Claire Rowe, I was able to work on their collection and refine even more the species identifications. The museum’s collection is a very important reference material essential to any biodiversity study. During this visit, I also established new collaborations with the arachnology collection manager Matt Shaw in order to further progress the study of the arachnids from Lizard Island.

Darko Cotoras at the Marine Invertebrate collection of the Australian Museum. The sea spider in the jar is a temperate/polar species. Photo: Claire Rowe.

The next step is to synthesize all the gathered information in order to update and expand our knowledge on this cryptic group of inhabitants from the reef. In the context of climate change and a potential sixth mass extinction, biodiversity discovery is an urgent and pressing issue. Species are becoming extinct faster than we can even give them a name, threatening the sustainable function of the planetary ecosystem as we know it. The current situation is analogous to randomly removing pieces of a car while it’s going, with the caveat that we do not even understand how this car works…

By Darko D. Cotoras Viedma, PhD

Senckenberg Research Institute and Natural History Museum

California Academy of Sciences

2019 Lizard Island Reef Research Foundation Post Doctoral Fellow