Thunnus Thynnus the Atlantic Bluefin Tuna is one of the largest species of Tuna that occupies the atlantic ocean. It is a member of the Scombridae Family, subfamily Scombrinae .
The Atlantic Bluefin Tuna, closely related to the Pacific and Southern Tuna, is one of the largest predators in the ocean potentially growing to sizes of 3m in length and almost 700kg. They are extremely aqua-dynamic in shape, like a torpedo, and their well adapted muscles and tendons allow them to reach great speeds, up to 80kmph, and swim for long periods of time . They are well adapted for travelling long distances as well as at high speeds and have been recorded diving down 1,000 metres . Their body colouration is metallic blue with the lower sides and belly silver or white. This makes them less visible from above and below because they are more camouflage with their environment . The dorsal fins are often different with the first being blue or yellow and the second, often taller then the first, is red or brown . Bluefin Tuna were once considered worthless by many fisherman and a nuisance as they often tore wholes in nets but in current day life Atlantic Bluefin are prized, and unfortunately endangered due to over fishing . In Japan BluefinTuna is one of the most sought after fish meats for making sushi and is sold at high prices .
Anatomy and Physiology:
Bluefin, just other fish, have a single circulation system. Their heart only has two chambers (one atrium and one ventricle) which means that blood only passes through the heart once each time it circulates. As blood flows into the heart it collects temporarily in the atrium before the heart muscles contract and force the blood into the ventricle, which in turn collects the blood and then also contracts to push the blood down arteries to the gills. In the gill capillaries Oxygen is diffused into the blood as it passes the sea water. The blood is then transported by vessels to the body capillaries where the oxygen and other chemicals and nutrients diffuse into the body tissue. Once the blood has passed through the body capillaries it is then carried by veins back to the heart and the circuit is continuously repeated . The blood pressure in capillaries is low which limits the rate of blood flow in the entire body however as the fish swims, it vigorously contracts and relaxes its muscles to accelerate the rate of circulation. BluefinTuna also have very high haemoglobin concentrations in blood and a thin blood-water barrier that ensures rapid uptake of oxygen .
For Bluefin Tuna the gills are where gas exchange with the environment occurs. The water passes over the gills and exchanges Oxygen with the Carbon dioxide that is in the fishes blood. The flow of the blood through the capillaries is opposite to the flow of water over them which creates a counter current. This is effective because the countercurrent maintains a partial pressure gradient which allows diffusion Oxygen and Carbon dioxide. The Bluefin does not have a very flexible head, this helps them to swim faster but prevents them from being able to pump water over their gills, like many other fish . Instead they swim at high speeds to force water over the gills to promote respiration .
Bluefin, and most species of Tuna are endothermic animals, are warm-blooded. Metabolic processes within the body convert energy into heat which give the animal its warmth. This however means that a lot of food (energy) must be consumed in order to maintain the warmth. The bluefin most likely developed it's warm-bloodedness due to its migrant patterns and because it dives to great depths (up to 3000ft) and can endure a variety of different temperatures  . Some heat within the fish is created due to the action of swimming and using it's muscles. Much of the heat is lost due to the blood in the gills coming into contact with the cold sea water so to counter-act this the Bluefin has adapted a vascular countercurrent heat exchange system, where blood from arteries and veins pass by each other flowing in opposite directions to maximize the heat diffusion from arterial blood to venous blood .
Many fish, including the BluefinTuna, are oviparous which means that fertilisation and development of the egg occurs outside the body. Tuna create spawning aggregations in which the males and females simultaneously release eggs and sperm into the water, because of this they are called Polygynandrous as many fish are mating at the same time. The spawning aggregations are made mainly in two major sites; the Gulf of Mexico and the Mediterranean . Each location has a different time of the year for spawning, in the Gulf of Mexico it is usually April and June whereas in the Mediterranean it is June and August .
The diet of the Bluefin is quite diverse and they are able to modify their behaviour to suit the type of prey they are trying to catch. They often aggregate in groups of fish that are of a similar size and together they chase down other small fish at high speeds, however they can also filter feed in order to catch small organism and have been known to eat kelp occasionally. Small fish such as mackerel, herring and anchovies make up part of their diet as well as eels, squids and come crustaceans . Bluefin larvae and juveniles feed on small organisms such as rotifers, shrimp and other larvae until they are big enough to eat small fish .
Tuna are a migratory species and some Bluefin have been recorded to have circled the entire Atlantic ocean, travelling about 16,000km and living as old as 40 years. Their reasons for migration are to find locations for feeding and breeding. To aid them in navigation the BluefinTuna has a pineal 'window' or 'eye' that allows them to steer by the sun on their long migration. They also rely on scent trails, and temperature gradients  .
72 parasites are known to live on the Atlantic Bluefin Tuna however only 9 of these are host-specific. Some of the parasites occupy the brachial cavities such as Euryphorus bracypterus, whereas other like Brachiella thynni are found on the fins, Pennella filosa that are in the gills and Caligus bonito on the body of the fish  .
The commercial demand for tuna has led to overfishing due to the food industry, mainly in Japan, and fishing for sport – especially in the USA. Fisherman usually use a hook and line to catch Bluefin however, they have been reported to not take bait in some areas so some fisherman use kites to make their bait look and move more realistically. Some scientists believe they don't readily bite unless in mixed schools with other fish because it may stimulate them to feed whilst others speculate that they may be beginning to identify bait on hooks  .
In order to counter-act the overfishing of BluefinTuna, much of the tuna on the market is farm-raised. Initially, wild juveniles were caught and reared on farms but in 2002 laboratory research had developed technology to artificially hatch and raise Bluefin larvae to adults, collecting their eggs and artificially hatching the next generation to create a sustainable method . This means that the fish are completely farm-raised and wild populations won't be negatively affected, giving them a chance to
In fisheries the Bluefin's fertilised eggs are contained in a tank where they float to the waters surface. They are only 1mm in diameter and take around 48 hours to hatch. The Bluefin larva is only 3mm in length and are fed plankton. They are referred to as “Fry” at around 20 days and are move from their tank to nets in the ocean where they grow for 3 years until they are adult size. After 3 years the fish are harvested and shipped as meat .
In Japan sushi is an extremely popular dish which comprises of raw fish, it can be served in different ways such as sashimi; a variety of raw fish and/or meat thinly sliced. Japan has utilised the ocean that surrounds them consuming about 80% of all Atlantic and Pacific BluefinTuna.
Because tuna is a read meat the Japanese initially use to bury the tuna for four days in order to mellow their taste of blood. This is how it got it's name 'Shibi” - 'four days' .
The main threats of BluefinTuna are predation from whales, sharks, other predatory fish and overfishing from humans.   In 1966 the International Commission for the Conservation of Atlantic Tuna (ICCAT)to address the conservation issues with highly migratory fish, specifically the Bluefin. Over 20 countries now have active roles in the organisation attempting to organise international research and management regulations. The major problem for the ICCAT is that the Bluefin is considered “Data deficient” by the International Union for Conservation of Nature (IUCN) however they have labelled it “Critically Endangered”  . The IUCN is a global environmental organisation comprised of governmental, non-governmental organisations and individual members .
Currently fishing is being monitored and new fishery/aquaculture methods are helping to bring wild numbers back up
Thunnus thynnus (Linnaeus, 1758)
- Linnaeus, C. 1758: Systema Naturae per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis, Tomus I. Editio decima, reformata. Holmiæ: impensis direct. Laurentii Salvii. i–ii, 1–824 pp doi: 10.5962/bhl.title.542: page 297.
- Integrated Taxonomic Information System
- Swedish Museum of Natural History Ichthyology name database
- IUCN: Thunnus thynnus (Linnaeus, 1758) (Endangered)