Watson (1969) used relative abdomen width to determine maturity of female 

 Chionoecetes opilio . Relative abdomen width was used as the criterion of matur- 

 ity of female arrow crabs in the present study. In male brachyuran crabs, 

 external morphological changes, particularly affecting the chelae, may coincide 

 with maturity (Hartnoll, 1969). Onset of sexual dimorphism, as shown by allo- 

 metric growth of the chelae, was used as the criterion of maturity of male arrow 

 crabs . 



Maturity in both sexes of arrow crabs appears to occur when the crabs measure 

 about 8.5 mm to 9.5 mm in carapace length. The smallest adult female crabs 

 were 8.5 mm. In females, presence of eggs on the pleopods provides functional 

 proof of maturity; one individual of 8.5 mm with eggs was found. Among 35 

 females of 9.0 mm or larger, all but two (one 9.0 mm and one 10.5 mm) were mature 

 and 27 were ovigerous. It is more difficult to provide functional proof of 

 maturity in male crabs. Males as small as 11.0 mm were observed grasping or 

 mating with females, indicating that those individuals were mature. Such obser- 

 vations were few, however, and do not provide a complete indication of the mini- 

 mum size at maturity. The fact that male crabs larger than 8.0 mm showed strong 

 morphometric differences from females indicates that maturity of the males prob- 

 ably occurs soon after they exceed 8.0 mm. 



Mature female arrow crabs were categorized according to presence or absence of 

 eggs on the pleopods and state of development of the eggs. Among 35 mature 

 females, 28 were ovigerous and 7 were not. Among the ovigerous crabs, 17 had 

 early, uneyed eggs; 9 had late, eyed eggs; and 2 had eggs that were hatching. 

 The wide range of reproductive stages observed among the mature females in early 

 August indicates that this species probably has an extended (or possibly year- 

 round) reproductive season. 



Arrow crabs were observed displaying various aspects of mating behavior. Most 

 mating activity seemed to occur during the late afternoon and early evening. 

 Seven pairs of mating crabs were observed on various days between 1630 hr and 

 1930 hr. One sighting of a pair of grasping crabs at 0200 hr was the only 

 instance of mating behavior observed outside the late afternoon-early evening 

 period. 



The mating behavior was basically of two types--grasping and copulation. During 

 both, the male is the active participant and the female is passive. When grasp- 

 ing, the male loosely holds the female, usually by a walking leg, in one of his 

 chelae; both crabs are in normal upright posture, side by side, and facing the 

 same direction. Sometimes a pair moves while grasping, the male providing all 

 or most of the locomotion while dragging or herding the female along. During 

 attempted or actual copulation, the male crab assumes a ventral side-up position, 

 brings the female, still ventral side-down, directly over him, and grasps her 

 tightly with his chelae and posterior three pairs of walking legs. The ventral 

 surfaces of the two crabs are then pressed closely together, the abdomen of the 

 female is extended posteriorly, and the weight of both crabs is supported above 

 the substrate by the female's walking legs. Crabs remain in the copulatory 

 position for 3 to 9 minutes. The actual mechanism of sperm transfer was not 

 observed. 



Three pairs of mating crabs were collected and placed in water-filled containers 

 in the wet room of the TEKTITE habitat. Copulatory behavior was not inhibited 



VI-216 



