LUMINESCENCE IN MARINE ORGANISMS — NICOL 449 



MODES OF LUMINESCENCE 



Three different ways of luminescing are employed by animals. 

 By far the commonest is intracellular luminescence in which the ani- 

 mal j)roduces light within special cells or photocytes. These are ag- 

 gregated together at special positions, and often form part of complex 

 light organs provided with reflectors, lenses, black pigmented sheaths 

 and even with shutters in some instances. Complex light organs are 

 foimd in deep-sea shrimps, squid, and fishes (pis. 1 and 2). Another 

 form of luminescence consists of discharging a cloud of luminous 

 matter into the sea water and we find this method in use among several 

 quite distinct animals; namely, polychaete or bristleworms, squids, 

 several bivalve mollusks (the group containing mussels, oysters, etc.), 

 crustaceans (ostracods, mysids, and shrimps, allies of the water-flea 

 and the lobster), and deep-sea fish (pi. 1, fig. 1, and pi. 3, fig. 2). The 

 third method of luminescence involves a symbiotic relationship with 

 luminous bacteria. The animal harbors these within special glandlike 

 organs and supports them; in return, the bacteria produce light for 

 their host. Luminous bacteria occur normally in certain squids and 

 fishes. 



A few animals, like bacteria, shine continuously, but in most ani- 

 mals light emission is discontinuous and is under reflex or voluntary 

 control. Jellyfish, sea pens, and comb- jellies (or ctenophores ; pi. 3, fig. 

 1), to cite a few examples, flash only when stimulated. The normal 

 stimulus is touch or a mechanical shock, giving rise to a battery of 

 impulses in the nervous network which characteristically forms the 

 nervous system of these animals. The impulses pass to all parts of 

 the animal and the light cells are excited and made to flash. Fol- 

 lowing excitation of a sea pen, waves of light can be seen running 

 over the surface of the animal, these waves corresponding to the 

 spread of excitation in the underlying nervous network. 



For experimental and quantitative studies, the light einitted by 

 animals can be recorded with the aid of photoelectric apparatus. This 

 usually consists of highly sensitive photomultiplier tubes, amplifiers 

 and cathode ray oscilloscopes or pen recorders. Indeed, the lumines- 

 cent response can be treated like the classical nerve-muscle prepara- 

 tion, with the added advantage that certain features can be moni- 

 tored or watched by the experimenter. For example, successive flashes 

 can be distinguished, and the sj)read of luminescence can be followed. 



Animals that shine or flash spontaneously include luminous crusta- 

 ceans, fireflies, squid, and fish (pi. 1 and pi. 2, fig. 1). In the fireflies, 

 which are, of course, beetles (Coleoptera), the onset and duration of 

 flashing are controlled by nerve impulses proceeding from the ventral 

 nerve cord. It had long been supposed that peripheral control of 

 flashing in fireflies was exercised by regulating the admission of 



