Croatian Undersea World IV (Definitive)

Yellow gorgonian (Eunicella cavolini)

Yellow gorgonian (Eunicella cavolini), is a coral that belongs to the Gorgoniidae family, widely spread in the Mediterranean Sea. The multi-branches colony, up to 50 cm in height, consists of thousands of interconnected tiny polyps approximately 2 mm in length. The branches are short and about 3 mm thick. The colony grows in a single plane, always perpendicular to the direction of the sea current. This is the best position to filter sea water, from which polyps collect plankton and organic matter using their tentacles. The colonies inhabit vertical segments of the seabed, with their depth ranging from 15 to 120 metres.

Yellow gorgonian is unisexual and reaches maturity at about 15 years of age. Polyps in male colonies liberate sperm into the water and the eggs are fertilised inside female polyps. The fertilised eggs are then released into the sea and they drift in the current for one to four weeks. When they settle on suitable surface at the end of their journey, they develop into a new colony with the growth rate of only 1 to 3 cm per year. It is believed that yellow gorgonian has a lifespan of more than 25 years. In the Mediterranean Sea, colonies inhabited by more than 180 individuals per square metre were recorded. Their density is smaller in Croatian part of the Adriatic Sea, reaching some 20 individuals per square metre.

Today, the yellow gorgonian is a protected species in Croatia, nevertheless susceptible to detrimental human influence. It is often destroyed by fishing gear, especially nets. The underwater temperature anomalies as consequence of the global climate change represent the biggest threat to its survival.

Sea orange (Suberites domuncula)

Sea orange (Suberites domuncula) is almost always found in symbiosis with a hermit crab. Their cohabitation begins in the early life phase of the hermit crab. A young crab inhabits an empty marine gastropod shell, to which the sponge larva then attaches. The sponge grows and it overgrows the gastropod shell over time. Since the crab still grows, the shell becomes too small for him, so he abandons it, inhabiting the inside of the sponge. Although abandoned, the gastropod shell still remains within the sponge. Thus the sponge becomes the home to the crab, always matching its dimensions through its growth. In addition, the sponge produces toxins so there are no animals that feed on it, protecting in that way the hermit crab as well. The symbiosis is also beneficial for the sponge in several ways. The crab transports it to various places, providing it with better conditions to obtain food from sea water; in case of danger it flees together with the sponge.

The sponge usually reaches 5 cm in diameter. Its colouring is mostly light orange, but it can also be blue, white, brown, yellow or it can feature a combination of colours. As a response to stress, it can shrink to less than 50% of its volume. It can often be found in northern Adriatic, on sandy and muddy seabeds, where it was first described as a species in 1792.

Smooth sea star (Hacelia attenuata)

Smooth sea star (Hacelia attenuata) is a species we know little about. It is a thermophilic organism that prefers warm seas. Due to that, it is widespread in the Adriatic Sea only in its central and southern part. It can often be found in the vicinity of the outer islands – Vis, Lastovo and Mljet. It can be found in sea starting from the surface to the depth of 150 metres, at rocky soil, often in coralligenous communities, also penetrating the undersea caves. It is unknown what does it feed on, but it is likely a predator like the majority of sea stars, preying on benthic invertebrates, such as molluscs and gastropods.

It spans up to 30 cm in diameter. It is predominantly bright red, with occasional occurrence of yellow specimens. The name of the species, attenuata, is related to the shape of its arms, which end in narrow tips. When in danger, it sheds some of its arms that can regenerate and create a new sea star. When the regrowing of the new limbs begins, the sea star looks like the drawing of a comet, thus giving it the so-called comet form.

Ante Žuljević, senior research scientist
Institute of oceanography and fisheries