MIGRATIONS OF THE HORSESHOE CRAB, LIMULUS POLYPHEMUS, 

 IN PLUM ISLAND SOUND, MASSACHUSETTS 



INTRODUCTION 



The predation on soft -clam ( Mya aren - 

 aria) by the horseshoe crab ( Limulus polypher - 

 mus) is of major significance in the soft -clam 

 producing areas of Massachusetts (Turner, 

 Ayers, and Wheeler, 1948; Shuster, 1950; 

 Smith and Chin, 1951). The migratory habits 

 of the horseshoe crab in Plum Island Sound, 

 Mass., were investigated as a part of studies 

 to develop methods for control of this predator. 



Observations on the distribution, abun- 

 dance, and migration of Limulus were made in 

 the sound from 1949 to 1955. Preliminary ob- 

 servations indicated a seasonal pattern of abun- 

 dance within the sound during the summer 

 months. During 1951 a program of marking 

 the crabs by cutting off half of the tail and cer- 

 tain abdominal spines was started. This system 

 gave some returns, but many were not definite 

 enough to be useful because it soon became evi- 

 dent that some individuals had lost a tail or 

 spines before we marked them . 



A program of tagging with Petersen - 

 disk tags was initiated in 1951 and continued 

 through 1954. A total of 1,639 horseshoe crabs 

 was tagged and released from flats within Plum 

 Island Sound during these years. An additional 

 141 horseshoe crabs were released from areas 

 outside Plum Island Sound making a total of 

 nearly 1,800 Limulus tagged in or near Plum Is- 

 land Sound. 



The authors were assisted in the recov- 

 ery of tagged crabs and the collection of untagged 

 crabs by Lionel Sheppard, Clam Commissioner, 

 Charles Bayley, Deputy Clam Commissioner, of 

 Ipswich, Mass., and by Daniel Pierce, Conser- 

 vation Officer, Massachusetts Department of 

 Marine Fisheries. 



METHODS 



Most of the horseshoe crajjs tagged 

 were captured and released in the intertidal 



zone of Plum Island Sound. A few, encountered 

 while searching for previously tagged individuals, 

 were tagged in the Annisquam River near Glou- 

 cester, Mass., and in Hampton Harbor, N.H. 



Subtidal collections were made with a 

 30-inch-wide scallop dredge or beam trawl 

 hauled by a 16 -foot outboard motorboat. The 

 warp was made fast in the boat, but by holding 

 on to it one could feel whether the dredge was 

 scraping the bottom . Although the dredge did 

 not catch large numbers of crabs, it did provide 

 some observations in the subtidal zone. 



Intertidal collections were made while 

 crabs were moving to and from the flats during 

 a flooding tide or sometimes an ebbing tide. We 

 observed that fairly large numbers of horseshoe 

 crabs began their movements onto the flats al- 

 most as soon as did the tide and would stay until 

 just before the flats became exposed at low tide. 

 Crabs that had buried themselves in the flat dur- 

 ing some previous tide, rose out of the soil and 

 moved about as the tide flooded the flat . Several 

 hundred of these crabs could be harvested by 

 three men during one tide. Best results were 

 obtained at the narrow entrance to a channel . 

 By wading in water 2 to 3 feet deep the three of 

 us picked the crabs off the bottom and placed 

 them in the boat which was towed behind or an- 

 chored nearby. Of the techniques used, this one 

 yielded the largest number of crabs . 



During low tide, the presence of Limulus 

 was indicated by depression that liberally pock- 

 marked the flats . These depressions, which 

 were circular, 6 to 8 inches wide and 2 to 3 in- 

 ches deep, were made by crabs in search of food 

 or in preparation for laying eggs . By digging 

 with a clam hoe in or at the edge of fresh pits 

 we often uncovered a crab or a mating pair, but 

 this method was not nearly as productive as col- 

 lections made during a flooding tide . 



Petersen tags were attached to the craba 

 through a hole pierced by an awl at the right 

 rear point of the prosoma or "head" . The 



