Greater weever

Appearance

''Trachinus draco'' is an elongated and laterally flattened fish with upstanding eyes and a distinct superior mouth that is inclined upwards. The lower jaw is longer than the upper jaw. The head is compact flat and relatively big and the eyes sit almost on top of it.⁠ The upper rim of the eye has two to three small spines, in front of each eye. The five to seven spiny fin rays on its first dorsal fin and the thorns on each of the gill covers have venom glands at their basis.⁠

Behavior

''Trachinus draco'' is a so-called ambush predator that digs itself into fine sand during daytime only with its eyes and the tip of the dorsal fin exposed.⁠ There it lurks until prey emerges which is then attacked by the predator in a swift and sudden manner.
During the night time the greater weever is believed to swim around freely, even pelagically. This thesis is indirectly supported by the finding, that prey in the stomach of ''T. draco'' which were caught during daytime was already mostly digested. Subsequent the feeding activity of ''T. draco'' seems to be higher during night time.⁠⁠⁠ ⁠
''T. draco'' is a carnivore that mainly feeds on Decapods, Bonefish and Opossum shrimps as catches in the eastern-central Adriatic Sea suggest. To a lower extent their diet contains also Isopoda, Amphipoda, Cephalopoda and Shells . In general the former make up to over 90% of the total Index of Relative Importance . The most common prey that could be identified to the rank of species, were decapods namely Liocarcinus depurator and Galathea strigosa and mysids namely Anchialina agilis.
But the compounds of the diet altered significantly with fish size. While the relatively small mysids were the most common prey for ''T. draco'' specimens under a size of 20 cm, their importance in the diet composition decreased with size, as the importance of amphipods did, too. On the other hand, the share of cephalopods and fishes increased with fish size.
The feeding habit of ''T. draco'' in the eastern Adriatic Sea differs very little over the year and decapods have been the most common prey through all seasons but peaked through summer and autumn while teleostei consumption peaked in winter.⁠⁠
In Danish waters on the other hand Merlangius merlangus and to smaller extent clupeoides were the main feeding source in the time of winter.⁠
The frequency of empty stomachs, in the specimens found in the eastern Adriatic Sea was around 15% in general, but differed significantly through the year. In winter the frequency of empty stomachs was highest, with a maximum at around 43.3% and was lowest in summer with 6.8% and in autumn with 7.8%.⁠
These findings were somewhat similar to those in Danish waters. In Kattegat the amount of empty stomachs has been observed in the months of January to April and is described as “very high, close to 100% in some month”.⁠⁠⁠
Those findings may be due to the lower water temperature of around 12 to 13 °C and the therefore lowered metabolism and feeding activity respectively.⁠
Aquarium experiments with stable light, temperature, salinity and sustained food supply on the contrary suggest an endogenous feeding pattern because even under such conditions the fishes refused to eat in the time from January to April and from October to March.⁠The reproduction period of ''T. draco'' seems to lasts from June to October.⁠ This very broad range is limited by the observations of Ole Bagge who states that his findings strongly indicate that the spawning time of ''T. draco'' is indeed restricted to the period between June and August with a peak in July. Bagge also says that there have been no findings of greater weever in spawning condition from September to May.⁠
''T. draco'' is an oviparous spawning fish that lays its eggs into the pelagic zone. ⁠The larvae have a size of about 4.8 mm to 6.8 mm ⁠ and hatch after approximately three months. The larvae and the eggs can both be found in the pelagic zone and the surface plankton. The larvae of ''T. draco'' may have a need for relatively warm water with a relatively low salinity to hatch in greater numbers.⁠⁠
''T. draco'' is probably not territorial, even in the mating season. It has not been observed to carry out any form of parental care.

Reproduction

The reproduction period of ''T. draco'' seems to lasts from June to October.⁠ This very broad range is limited by the observations of Ole Bagge who states that his findings strongly indicate that the spawning time of ''T. draco'' is indeed restricted to the period between June and August with a peak in July. Bagge also says that there have been no findings of greater weever in spawning condition from September to May.⁠
''T. draco'' is an oviparous spawning fish that lays its eggs into the pelagic zone. ⁠The larvae have a size of about 4.8 mm to 6.8 mm ⁠ and hatch after approximately three months. The larvae and the eggs can both be found in the pelagic zone and the surface plankton. The larvae of ''T. draco'' may have a need for relatively warm water with a relatively low salinity to hatch in greater numbers.⁠⁠
''T. draco'' is probably not territorial, even in the mating season. It has not been observed to carry out any form of parental care.

Food

''Trachinus draco'' is a so-called ambush predator that digs itself into fine sand during daytime only with its eyes and the tip of the dorsal fin exposed.⁠ There it lurks until prey emerges which is then attacked by the predator in a swift and sudden manner.
During the night time the greater weever is believed to swim around freely, even pelagically. This thesis is indirectly supported by the finding, that prey in the stomach of ''T. draco'' which were caught during daytime was already mostly digested. Subsequent the feeding activity of ''T. draco'' seems to be higher during night time.⁠⁠⁠ ⁠
''T. draco'' is a carnivore that mainly feeds on Decapods, Bonefish and Opossum shrimps as catches in the eastern-central Adriatic Sea suggest. To a lower extent their diet contains also Isopoda, Amphipoda, Cephalopoda and Shells . In general the former make up to over 90% of the total Index of Relative Importance . The most common prey that could be identified to the rank of species, were decapods namely Liocarcinus depurator and Galathea strigosa and mysids namely Anchialina agilis.
But the compounds of the diet altered significantly with fish size. While the relatively small mysids were the most common prey for ''T. draco'' specimens under a size of 20 cm, their importance in the diet composition decreased with size, as the importance of amphipods did, too. On the other hand, the share of cephalopods and fishes increased with fish size.
The feeding habit of ''T. draco'' in the eastern Adriatic Sea differs very little over the year and decapods have been the most common prey through all seasons but peaked through summer and autumn while teleostei consumption peaked in winter.⁠⁠
In Danish waters on the other hand Merlangius merlangus and to smaller extent clupeoides were the main feeding source in the time of winter.⁠
The frequency of empty stomachs, in the specimens found in the eastern Adriatic Sea was around 15% in general, but differed significantly through the year. In winter the frequency of empty stomachs was highest, with a maximum at around 43.3% and was lowest in summer with 6.8% and in autumn with 7.8%.⁠
These findings were somewhat similar to those in Danish waters. In Kattegat the amount of empty stomachs has been observed in the months of January to April and is described as “very high, close to 100% in some month”.⁠⁠⁠
Those findings may be due to the lower water temperature of around 12 to 13 °C and the therefore lowered metabolism and feeding activity respectively.⁠
Aquarium experiments with stable light, temperature, salinity and sustained food supply on the contrary suggest an endogenous feeding pattern because even under such conditions the fishes refused to eat in the time from January to April and from October to March.⁠

Defense

This section may be too technical for most readers to understand. Please to , without removing the technical details. '''' The venomous apparatus of ''T. draco'' consists of one spine on each operculum and five to eight dorsal spines. The spines on the operculum point towards the cauda, or tail, and are slightly bent downwards. The opercular spines arise from the upper edge of the operculum and are connected to the operculum with one third of their complete length. The other two thirds of their length lies free along the operculum. The total length of the opercular spines is approximately 27 mm. The spine itself is “covered by an integumentary sheath”.⁠⁠ If this sheath ruptures because there is force applied to the spine, it allows the venom to leak from the venom gland cells and to run through a deep groove along the spine and into the wound caused by the sting.
Each of the dorsal spines is covered in an individual integumentary sheet. The spines are connected through a fine interspinous membrane. The spines have different length and are by that arranged in a curve like manner. The spines observed by Russel & Emery had minimum lengths of 5 mm and maximum lengths of 29 mm and were quite heterogeneous in their appearance on a microscopic level. The dorsal spines are all bent very slightly towards the tail of the fish. The mechanism of envenomation is quite similar to that of the opercular spines.⁠The glands producing ''T. draco''’s venom are located in the derma of the fish. The venom glands are cased in connective tissue coated with a basal lamina which has a length of 0.1 μm. The venom glands are built up of polygonal cells with a long axis length of 40-50 μm.
These cells show a relatively heterogeneous cytoplasm with noticeable large vacuoles and heterogeneous granular .
As in the tissue of the lesser weever supporting cells can be found in ''T. draco'' which develop from epidermal cells. These supporting cells form pockets inside the venom glands which are filled with differentiated glandular cells. The supporting cells most probably play a role in the cohesiveness of the venom gland and in the regeneration of the glandular cells.