Aposymbiotic culture of the sepiolid squid Euprymna scolopes: Role of the symbiotic bacterium Vibrio fischeri in host animal growth, development, andlight organ morphogenesis

The sepiolid squid Euprymna scolopes forms a bioluminescent mutualism with the luminous bacterium Vibrio fischeri, harboring V. fischeri cells in a co mplex ventral light; organ and using the bacterial light in predator avoida nce. To characterize the contribution of V. fischeri to the growth and deve lopment of E. scolopes and to define the long-term effects of bacterial col onization on light organ morphogenesis, we developed a mariculture system f or the culture of E. scolopes from hatching to adulthood, employing artific ial seawater, lighting that mimicked that of the natural environment, and p rovision of prey sized to match the developmental stage of E. scolopes. Ani mals colonized by V. fischeri and animals cultured in the absence of V. fis cheri (aposymbiotic) grew and survived equally well, developed similarly, a nd reached sexual maturity at a similar age. Development of the light organ accessory tissues (lens, reflectors, and ink sac) was similar in colonized and aposymbiotic animals with no obvious morphometric or histological diff erences. Colonization by V. fischeri influenced regression of the ciliated epithelial appendages (CEAs), the long-term growth of the light; organ epit helial tubules, and the appearance of the cells composing the ciliated duct s, which exhibit characteristics of secretory tissue. In certain cases, apo symbiotic animals retained the CEAs in a partially regressed state and rema ined competent to initiate symbiosis with V. fischeri into adulthood. In ot her cases, the CEAs regressed fully in aposymbiotic animals, and these anim als were not colonizable. The results demonstrate that V. fischeri is not r equired for normal growth and development of the animal or for development of the accessory light organ tissues and that morphogenesis of only those t issues coming in contact with the bacteria (CEAs, ciliated ducts, and light organ epithelium) is altered by bacterial colonization of the light organ. Therefore, V. fischeri apparently makes no major metabolic contribution to E. scolopes beyond light production, and post-embryonic development of the light organ is essentially symbiont independent. (C) 2000 Wiley-Liss, Inc.