Cave Spider Crabs, scientifically known as Herbstia condyliata, are fascinating marine crabs found along the eastern coast of the Atlantic Ocean, western Europe’s coastal waters, and the entire Mediterranean Sea, including the Maltese Islands. Known for their distinctive appearance and unique behaviors, these crabs are a common sight for divers exploring Malta’s underwater caves and rocky crevices.
Identifying a Cave Spider Crab is quite straightforward due to its distinctive features. The shell, or carapace, is triangular with a rounded posterior edge, pear-shaped, and longer than wide, measuring up to 47mm by 38mm in males. The carapace is convex and adorned with small tuber-like stems and a median hump at the rear with three small spines. The rostrum has short horns ending in two divergent triangular points. Adult males have strong pincers with tuber-like stems on the upper edge, while juveniles and females have weaker pincers. The legs are long, roughly twice the width of the shell, and ringed with alternating light and dark areas. The general color is mottled reddish-brown, with large specimens displaying orange to pink pincers.
Herbstia condyliata possesses several unique characteristics. Its triangular, convex carapace is adorned with small tuber-like stems and a median hump with three small spines. The strong pincers of adult males, the long legs, and the red eyes are notable features. The crab’s color can vary based on its environment, often appearing grey-dirty in sediment-rich waters due to deposits on the carapace or microscopic epibionts. Juveniles are less colorful and quite diaphanous, often camouflaging themselves with elements from their surroundings.
Cave Spider Crabs inhabit caves, coralligenous habitats, and rocky areas inside cracks and crevices. They are often found in dark caves or on hard walls and substrates at night. These crabs can be found from the coastline to depths of about 50 meters, sometimes reaching up to 80 meters. Solitary by nature, they rely on their camouflage to avoid predators such as octopuses, starfish, and various fish species.
An intriguing aspect of the Cave Spider Crab is its camouflage ability, especially in juveniles. Young crabs often decorate themselves with various elements from their environment, such as Alcyonium coralloides polyps and sponge fragments, creating a Christmas tree-like appearance. This behavior helps them blend into their surroundings and avoid predators. As they grow, they gradually lose this decorative behavior.
Diving in Malta offers the exciting opportunity to observe Cave Spider Crabs in their natural habitat. Some excellent diving sites to spot these crabs include:
Billingshurst Cave – Located near Reqqa Point on Gozo’s north coast, this site is known for its abundance of Cave Spider Crabs.
Blue Grotto – Situated on Malta’s southern coast, this popular dive site features underwater caves and crevices where these crabs can often be found.
Hondoq Bay – A coastal area on Gozo’s southeast coast, offering clear waters and rocky habitats ideal for spotting these fascinating crabs.
Diving in these locations, especially during night dives, provides an excellent chance to observe the unique behaviors and characteristics of Cave Spider Crabs.
