Great Scallop: Pecten maximus (L.)

Phylum: Mollusca        Class: Pelecypoda (Bivalvia)       Order: Ostreoida       Family: Pectinidae

 

Species description

The great scallop, Pecten maximus is a bivalve mollusc (it has a shell consisting of two valves). The lower right valve is convex and off-white in colour whilst the upper left valve is flat and reddish-brown. It is has many common names including the great scallop, the king scallop, the giant scallop, escallop and Coquille St. Jacques.

 

BIOLOGY and ECOLOGY

Biological factor   Size Most <150mm shell width [1] but up to 210mm shell width [3] Lifespan 20 years [1 & 2] Size at reproductive maturity Minimum 60mm [2] Age at reproductive maturity Full maturity 3 - 5 years [2] Fecundity 15 - 21 million oocytes per emission for a 3 years old [11] Larval phase 11 - 30 days [2] Adult mobility Limited swimming mobility (benthic)

Fig 1. The great scallop Pecten  maximus © Sussex IFCA

 

 Reproductive behaviour

The great scallop is a hermaphrodite, fertilization is external and either sperm or eggs may be exuded first [3]. Temporal gonad studies indicate there maybe a bi-modal spawning pattern with partial spawning in the spring (April or May) and then further spawning in the autumn (late August) [3], however some studies have hypothesized that a part of the decrease in gonad contents in the autumn may be attributed to re-absorption. Following fertilisation a planktotrophic veliger larva develops, this stage is free living and subject to water transport. Within 30 days the veliger will settle from the water column and attach by the form of byssal thread to a suitable substrate. Young scallops generally remain attached until they are between 4 - 13 mm in length [5]. Then detaching and settling on the seabed.

 

Migratory behaviour

The great scallop is capable of ‘swimming’ and ‘jumping’ however research indicates that these movements are small and localised [10] and that this behaviour is generally exhibited as an escape reaction. Swimming is achieved by rapidly ‘clapping’ the valves and expelling the water on either side of the dorsal hinge so that the scallop moves with the curved edge of the shell foremost [6]. Jumping is achieved through the gradual relaxation of the adductor muscle followed by the rapid opening and closing of valves, which jumps the scallop hinge forward [6]. It is generally believed that adult scallops do not migrate and they will only move if disturbed, instead they rely on larval dispersal for distribution and consequently are affected by local hydrographic regimes. Estimates of how far the larvae may travel range from 10 - 40km [2]. It is therefore possible that recruitment to one scallop bed may actually be supported by another scallop bed some distance away; it is also possible that a scallop bed may be self-sustaining, depending on the hydrographics (there is scientific evidence for both scenarios) [2 & 7].

 

Habitat

The great scallop is found offshore to a depth of 100m [8], normally on a seabed of fine sand and gravel where it lies recessed in slight hollows. Recessing is achieved through a series of powerful adductions (valve closures) where water is ejected from the mantle cavity and lifts the shell at an angle to the seabed so that subsequent water jets blow a hollow into the sediment [7]. The great scallop has an aggregated distribution within their geographic range which is most likely due to the hydrography as they are dispersed as larvae and the availability of suitable settlement substrate. Major fishing grounds are generally widely separated, so much so, that respective environmental conditions produce marked differences in population parameters [7].

 

Predators and prey

The great scallop feeds by filter feeding; pumping water through a filter in the gill chamber to remove particulate organic matter and phytoplankton [2]. Predators of the great scallop include a range of starfish, commonly Asterias rubens, large crabs and cephlapods. Scallops posses primitive ‘eyes’ and on detection of a change in light caused by a shadow or a movement a scallop may swim, re-orientate itself or close its shell in response [9]. Research also suggests they may ‘smell/taste’ danger. Experimental contact with different starfish species elicited distinct, energy adaptive types of response; full swimming response was initiated only by extracts of Asterias rubens and Astropecten irregularis which prey on molluscs, while limited jumping or valve-closing responses were induced by non-predatory starfish [6].

 

THE SUSSEX FISHERY

Fishing methods

The scallop fishery in the Sussex IFCA district is solely commercial using a specific dredge type; the ‘spring-loaded Newhaven dredge’, this is the only type of dredge permitted within the district. It was originally designed in Newhaven but is now used throughout the British Isles. Typically the inshore trawlers within the district alternate between beam trawling, otter trawling and scallop dredging depending on the time of year and economics. The duration of a scallop fishing trip will typically be between one day and one week depending on the size and storage capacity of the individual vessel. Under chilled conditions scallops can stay alive for a week. At present there are no established SCUBA diving activities that target scallops commercially within the Sussex IFCA district.

Fig 2. Scallop dredges hanging over the gantry of a fishing vessel and the catch on deck © Sussex IFCA

 

Fishing activity

The scallop fishery within the Sussex IFCA district is predominantly east of Brighton. No scallop dredging occurs within 3 nautical  miles of the shoreline nor during their spawning period which makes it a winter and spring fishery, this is because  of local Sussex IFCA byelaws. The best price for scallops is obtained when the gonad/roe is in good condition; typically this is better in the winter fishing months. Fishing activity for scallopsoccurs equally during daylight and night time.

[1] Hayward, P., Nelson-Smith, T., and Shields, C. 1996. Collins Pocket Guide: Sea Shore of Britain and Europe. Harper Collins Publishers, London. 

[2] Marine Life Information Network for Britain and Ireland. 1998. MarLIN. Marine Biological Association, Plymouth, England. Available from: http://www.marlin.ac.uk/speciesfullreview.php?speciesID=4056 (accessed 09/04/09)  

[3] Mason, J. 1983. Scallop and queen fisheries in the British Isles. Fishing News Books Ltd, Farnham, Surrey, England. 

[5] Fish, J.D., and Fish, S. 1996. A student's guide to the seashore: 2^nd edition. Cambridge University Press, Cambridge, England. 

[6] Thomas, G.E., and Gruffydd, L. D. 1971. The types of escape reactions elicited in the scallop Pecten maximus by selected sea-star species.Marine Biology, 10, 87-93. 

[7] Brand, A.R. 1991. Scallop ecology: Distributions and behaviour. In: Shumway S. E., ed. Scallops: biology, ecology and aquaculture. Elsevier, Amsterdam, 517-584. 

[8] Hayward, P. J., and Ryland, J. S. 1998. Handbook of the Marine Fauna of North-West Europe. Oxford University Press, New York. 

[9] Wilkens, L.A. 1991. Neurobiology and behaviour of the scallop. In: Shumway S. E., ed. Scallops: biology, ecology and aquaculture. Elsevier, Amsterdam, 429-469. 

[10] Hartnoll, R. G. 1967. An investigation of the movement of the scallop, Pecten maximus. Helgolander wiss. Meeresunters.15, 523-533.  

[11] Le Pennec, M., Paugam, A., and Le Pennec, G. 2003. The pelagic life of the pectinid Pecten maximus - a review.ICES Journal of Marine Science, 60, 211–223.