| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Marine Biological Laboratory, Woods Hole, Massachusetts
The bay scallop Argopecten irradians (Lamarck 1819) is harvested commercially and recreationally throughout its range along the east and Gulf coasts of North America. At its peak, the Massachusetts harvest exceeded 1200 metric tons with a monetary value of $11 million (1). On Martha’s Vineyard, bay scallop harvests are an important economic commodity and can represent, depending upon the size of the catch and price per pound, between 4% and 10% of the island’s annual economy (2).
Argopecten irradians irradians, the northern bay scallop, and the southern subspecies (A. i. concentricus, A. i. amplicostatus) exhibit extreme natural variability in harvestable stocks from year to year; but, in general, natural populations have declined over the past quarter century (1). Harvest data from Cape Cod, Martha’s Vineyard, and Massachusetts as a whole, compiled from 1965 to 1997, reveal that the maximum harvests (in bushels) occurred in the 1980s, but that scallop harvests have dropped precipitously since 1985 (3).
The variability and population declines are attributed to predation and habitat loss, to harmful algal blooms, and to the species’ short life (1,4,5). The consequent economic pressures have led to increased emphasis on scallop aquaculture, development of field grow-out techniques, transplantations, and seeding programs using hatchery- or field-collected seed (1,6). However, the notion that these efforts contribute significantly to population stabilizations is poorly supported by hard evidence (3,7).
One of the difficulties in ascertaining the success of seeding or stock enhancement programs is that seeded animals cannot be distinguished from the natural population. The colored tags used for shrimp and fin fish are not useful for scallops. Allozyme differences were insufficient to discriminate between native and transplanted animals (8). In contrast, DNA-based molecular markers are excellent at distinguishing between subpopulations (9) and also have the great advantage of being neutral, while not generating artifacts due to predator preferences or survival. In this paper, we report progress in the development of scallops with molecular tags—an aid in the evaluation of stock enhancement programs.
We chose to develop RAPD (Random Amplification of Polymorphic DNA) genetic markers that can be detected by the polymerase chain reaction (PCR) (10). RAPD-PCR has the great advantage that DNA sequence information is not required for the development of useful markers.
Representative adult bay scallops were initially collected from Nantucket Island and, more recently, from Martha’s Vineyard. Mantle tissue was sampled after the valves opened spontaneously. This procedure does not kill the animal, which can then be kept alive for mating experiments. After the tissue was thoroughly rinsed to remove extraneous biological material, DNA was purified from it. DNA purification procedures, RAPD-PCR protocols, and electrophoresis conditions have been described previously (11). Fifteen primers were screened for amplifiability and reproducibility, and the relative frequency of bands was determined. Results for two primers are shown in Table 1. In both cases, the larger size bands were present at higher relative frequencies when compared with those of the smaller size bands. Figure 1 shows a representative gel image for the same two primers. Panel A shows an example of a RAPD marker that occurs at 100% frequency in the test population. Panel B shows a RAPD marker that appears at low frequency in the same population.
|
|
To circumvent this problem, we performed bulk matings with adults carrying distinctive markers. Crosses were done with four selected groups of 10 to 25 individuals. These have now gone through two generations, and are being screened. Animals testing positive for the high frequency marker (arrow, Fig. 1) will be conditioned and spawned. From the results obtained thus far, bulk mating of animals carrying selected markers appears to be the best approach to genetically tagging Argopecten.
Field trials will be carried out by transplanting animals carrying the selected marker into test areas where the marker is absent or present at a very low level, and then determining the relative frequency of the tag in sampled animals. To the best of our knowledge, this is the first attempt to tag a cultured mollusc species with molecular markers for the evaluation of seeding programs. If this proof-of-principle experiment is successful, it can be extended to commercial aquacultured species such as Placopecten magellanicus, Mya arenaria, and Mercenaria mercenaria.
This work was supported in part by a grant from the MIT/WHOI Sea Grant Program to A.M.K. and H.M.C. (Project: R/A-34). H.M.C. and A.M.K. are indebted to Dr. Dale Leavitt of SEMAC and Rick Karney of the Martha’s Vineyard Shellfish Group for assistance and advice. We thank the Marine Resources Center, Marine Biological Laboratory, for providing facilities for maintenance of the scallops.
Footnotes
1 Brown University, Providence, RI. 
Literature Cited
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
