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Biology

Class:Chondrichthyes Family: Cetorhinidae
Subclass: Elasmobranchii Genus: Cetorhinus
Super-order: Selachiformes Species: Maximus
Order : Lamniformes

 

 

 

 

Cetorhinus maximus (Gunnerus) is known by as wide a variety of common names as the waters in which it is found. In many cases these names describe aspects of its typical behaviour or biology e.g. ‘sailfish or sunfish’.

The basking shark was first documented in the mid to late 1700’s by some naturalists of note including J.E. Gunnerus the bishop of Trondheim and Thomas Pennant the author of British Zoology, 1769, who is also credited as giving the shark its current common english language name (Fairfax 1998). It was not until later years that the traditional oral names of Gaelic origin were recorded in Ireland and Scotland. Some of these names like their European cousins also offer local insights on the shark’s characteristics and fishermen’s perceptions. ‘Liop an dá lapa’ – unwieldy beast with two fins, ‘Liabhan Mór’ - great leviathan, ‘Liabán gréine’ – great fish of the sun (McNally, 1976). One very informative localised name from northern Spain is that of ‘Peregrino’ meaning pilgrim. This name has strong significance for the timing of basking shark sightings as pilgrims for Santiago de Compostella usually arrive into northern Spain during the early summer months and are gone again by Autumn (Compagno, 1984).

Most of the sharks key life history characteristics such as growth rate, natural mortality and fecundity have been studied in limited detail but are at present assumed rather than measured (Canadian Science Advisory Secretariat, CSAS, 2008). The biological knowledge currently available is mainly due to the efforts of early naturalists and in more recent years the zoologists, L.H. Matthews and H.W. Parker. In 1947 these two zoologists conducted a number of post mortems at Soay on the invitation of Gavin Maxwell, an infamous entrepreneur of the day and owner of the then Soay based basking shark fishery (Fairfax 1998).

Similar in many aspects to other large sharks Cetorhinus maximus has a relatively narrow girth and pointed snout. It average size is 5-7 meters although specimens have been recorded at 11 meters. The liver for which it was prized commercially is given to make up approximately 25% of its body weight and acts as a ‘hydrostatic float’ to keep the shark at near neutral buoyancy. Like other sharks it uses its large heterocercal tail (caudal fin) to propel its self, while its paired pectoral and smaller pelvic fins control its accent and decent. It has two dorsal fins the first of which gives the shark its characteristic profile. This Dorsal fin has been recorded at over a meter in height and is sometimes known to droop over when above the water surface. The male and females are distinguished by the presence of two claspers on the male. These claspers are misnamed and although used in mating activity they are not used to ‘clasp’ the female, instead they are used to deliver spermatophores into the female in a similar way to a mammalian penis (Fairfax 1998, Shark Trust, 2008).

The shark is a filter feeder and uses expanding gill rakers located at the side of its mouth to catch large quantities of copepods. This feeding strategy is described as Ram filter feeding due to the sharks action of swimming head first with open mouth through waters in which its prey is found (Shark Trust 2008). There has been a positive correlation found between the presence of basking sharks and the density of the copepod calanus finmarchius (Sims & Quayle, 1998).

The basking shark is of the cartilaginous family containing no bones and has a number of interesting biological features including a small ‘suspended’ brain. It has exceptional olfactory senses, is highly sensitive to motion and also possesses a large number of tubules known as the Ampullae of Lorenzini (an electronic field sensor) (Fairfax, 1998). Many aspects of its physical character are not fully understood and are usually associated with the belief that it spends much of its life at extreme depths (UK Government CITES proposal, 2002).

Courtship behaviour and reproduction cycles have not been studied and current knowledge is based on limited eyewitness accounts. There is only one peer reviewed eyewitness account of copulation from the east coast of North America. However what is believed to be courtship behaviour has been reported many times with differing theories on the manor of the sharks practices. Nose to tail following, parallel swimming and breaching are all theorised as being linked to courtship. This behaviour has been recorded in most of the basking shark ‘Hotspots’ around Britain and Ireland and is closely associated with thermal fronts (SIMS et al, 2000). There is also only one recorded eyewitness account of birth by two Norwegian fishermen, Hans Goksoyr and Jonas Sordal. They were towing a harpooned basking shark when ‘ ….it threw a live young shark roughly as large as a common habrand. The young shark was alive and immediately started swimming at the surface. Then another young shark and so on to five young sharks and a sixth was dead’ (Kunzlik, 1988).

UK led studies into Basking shark genetic patterns and gene regions offered limited results due to the small amount of samples taken. It does at this stage of investigation report to allow for differentiation between two mtDNA gene regions; The Northern temperate waters and southern temperate waters (Noble et al, 2006). The major contribution to shark conservation that genetic studies have achieved is to facilitate enforcement of CITIES regulations by the identification of basking shark parts at low concentrations in processed products. As a result the presence of basking shark fins has been confirmed in the Hong Kong and Japan markets and a positive relationship has been discovered between numerous fin trader categories and valuable parts of basking sharks (Magnusson et al, 2007).

More in this category: « Species Distribution »