The sandy sea floor

So-called moveable surfaces (silt, sand and gravel) are the Adriatic’s most common types of sea habitats. Sandy seabeds start from the tidal zone. The most significant fauna which inhabits the surface level are flatfish (wide-eyed flounder, sole, turbot), cartilaginous fish (stingrays and sharks), and numerous invertebrates such as crustaceans, bivalves and sea urchins.

(Raja clavata, Linnaeus, 1758)

Thornback ray

The thornback ray is the most well-known ray species inhabiting the Mediterranean; it has a rhomboidal body and large spines on the back and tail. It is classified as a near threatened (NT) species. It is well known that thornback rays may grow up to 1.10 meters in the Adriatic, they can weigh up to 8 kg and live up to 15 years. Female thornback rays reach sexual maturity at a length of 78.4 cm (7 years old) and males at a length of 67.6 cm (5 years old).

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Aquarium Pula has set up a programme of breeding sensitive cartilaginous fish, represented by Lucky, the first thornback ray born under human care in Croatia. He hatched on 8 January 2020 and was 7 cm in length, with pectoral fins 4 cm long and weighing 7 g. After a year, before being released, he was 43 cm in length and 26 cm in width and weighed 326 g. Lucky was tagged under the designation Aquarium Pula 001 for possible further tracking.

(Scyliorhinus stellaris, Linnaeus, 1758)

Nursehound

The nursehound is a nocturnal shark species that lives at a depth of 100 to 200 m but sometimes ends up in shallow waters. Females lay eggs in a chitin sheath, a capsule (“mermaid’s purse”) from which live young are born after about nine months. A more dense population can be found on the west coast of Istria in the eastern part of the Adriatic and around the islands north of Zadar. In contrast, only individual specimens are found along the west, Italian Adriatic coast. According to the IUCN threatened species category, in Croatia, it is classified as a near threatened (NT) species.

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Insemination occurs within the shark’s body. During mating, the male swims in a spiral around the female and wraps around her body. A transformed pelvic fin or, rather, the copulatory organ (mixipterygium) turns forwards and is inserted into the female cloaca which is inseminated. The female lays the capsules in shallow areas of the sea throughout the year. In Aquarium Pula, along with the small-spotted catshark, we collect 180 egg capsules during the year and share the young with other aquariums, or we return them to the sea.

(Bothus pada, Delaroche, 1809)

Wide-eyed flounder

The wide-eyed flounder is a demersal species of flatfish which lives in sandy or silty seabeds. It usually inhabits the south Adriatic at a depth of up to 400 m. It can grow up to 45 cm long and feeds on invertebrates and small fish.

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Flatfish undergo an unusual physical transformation during their lifetime. Immediately after hatching, young individuals have eyes on both sides of their head, but in just a few weeks, their bodies lean to one side and one eye moves to the opposite side. This way, both eyes become part of the upper, pigmented side of the body, while the other side becomes the bottom side and begins to turn pale. It is interesting that males, unlike females, have a larger gap between their eyes and more pronounced supraorbital eye spines.

(Chelidonichthys lucerna, Linnaeus, 1758)

Tub gurnard

The tub gurnard, a gurnard from the Triglidae family, inhabits the northern Adriatic, along with three other species; grey gurnard (Eutrigla gurnardus), large-scaled gurnard (Lepidotrigla cavillone), and streaked gurnard (Trigloporus lastoviza). It spawns in late autumn and during the winter. Its roe is pelagic, however, juvenile animals love seawater mixed with fresh water, which is why they enter lagoons, estuaries or even fresh water. When it inhabits shallow water, it is darker in colour and is red when living in deeper waters. It has been included in Croatia’s Red Book of sea fishes in the least concern (LC) family category.

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Did you know it was once believed that different colored tub gurnard (dark, brownish, and red) belonged to other species? Today, it is known that the reason for this distinction is their habitat; if they live in shallower waters, they appear dark, whereas if they inhabit deeper areas, they appear red. They are also called butterfly fish because they spread their long and wide pectoral fins like a butterfly's wings. Another name for them is cuckoo fish because they make sounds by croaking or grunting, produced by their swim bladder being touched by vibrating muscles.

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