KITTREDGE ET AL: CHEMICAL SIGNALS IN THE SEA 



or a single cell is difficult when the stimulant 

 must be presented in the gas phase. Each 

 species of stimulant molecule must partition 

 between the gas phase and an aqueous film. 

 The active concentration at the receptor 

 membrane is unknown. A study of the physi- 

 ology of pheromone reception by aquatic organ- 

 isms would avoid this limitation. 



A survey of the literature reveals that, as in 

 the field of entomology, there exists a broad 

 basis of behavioral observations suggesting the 

 role of chemical communication in the aquatic 

 environment. These studies suggest that marine 

 invertebrates are primarily dependent on chemo- 

 reception for information from their environ- 

 ment. The input is composed of a broad spectrum 

 of chemical messages ranging from species 

 specific pheromones eliciting stereospecific 

 responses, e.g., mating behavior, epidemic 

 spawning, aggregation, or alarm behavior, 

 through those kairomones triggering metamor- 

 phosis or migration to the cues indicating the 

 proximity of predators or prey. 



The closest parallel to insect pheromone 

 communication observed in marine organisms 

 are the sex pheromones of marine Crustacea. 

 The first experimental demonstration of 

 "chemical recognition" by marine Crustacea is 

 the description of the behavior of male copepods 

 (Labidocem aestiva) by Parker (1902). In a 

 series of elegantly simple experiments he 

 demonstrated that "they [the females] probably 

 give rise to some substance that serves as a 

 scent for the males; in other words, the males 

 are probably positively chemotropic toward the 

 females." Moreover Parker noted that "they [the 

 males] seldom pass near the tube without some 

 characteristic reaction. Usually they made one 

 or two quick circles as they swam by, or even 

 a somersault-like motion; these were observed 

 fifteen times when the females were in the tube, 

 never when they were not." Lillelund and Lasker 

 (1971) observed similar swimming behavior in 

 male Labidocera joUae. Although L. joUae 

 females swim in a seemingly random pattern 

 with only occasionally looped excursions, the 

 males frequently vary their random course of a 

 few seconds duration by swimming in circles, 

 covering a small area intensively. Of greater 

 interest was the observation that rather than 

 circles, the path of the males often resembled a 

 curtate cycloid. The males occasionally pro- 

 gressed for several centimeters in this curtate 



cycloid path (Figure 3). These observations, 

 although obtained during feeding studies, 

 suggest an important aspect of the physiology 

 of pheromone response in small Crustacea — the 

 mechanism of sensing a chemical gradient. Crisp 

 and Meadows (1962) have stated that, because 

 of the small distance between the chemosensory 

 organs of barnacle cyprid larvae, these larvae 

 cannot detect a chemical gradient and thus 



1 cm 

 I 1 



Figure 3. — Swimming behavior of a male copepod 

 Lahidoceru jollae. A' and B' mark the termini of the 

 tracings. The upper trace shows both an occasional circu- 

 lar swimming course, progression in a curtate cycloid 

 course and "doubling back." The lower trace is an 

 extreme example of the "doubling back" behavior. 



