Reference: Bio/. Bull. 204: 256-269. (June 2003) 2003 Marine Biological Laboratory Larval Behavioral, Morphological Changes, and Nematocyte Dynamics During Settlement of Actinulae of Tubularia mesembryanthemum, Allman 1871 (Hydrozoa: Tubulariidae) KEIJI YAMASHITA 1 . SATORU KAWAII 2 . MITSUYO NAKAI, AND NOBUHIRO FUSETANI 3 Fmetani Biofouling Project, ERATO, JST (formerly Research & Development Corporation of Japan), Isogo-ku, Yokohama 235-0017, Japan Abstract. The marine colonial hydroid Tubularia mesem-bryanthemum produces a morphologically unique disper-sive stage, the actinula larva. Detailed observations were made on the behaviors and nematocyte dynamics of actinula larvae during attachment and morphogenesis by employing microscopic and time lapse video techniques. These obser-vations produced four primary results. ( 1 ) Actinula larvae demonstrated two forms of attachment: temporary attach-ment by atrichoiis isorhi~a (AI) nematocysts discharged from the aboral tentacle (AT) tips and permanent settle-ment by cement secretion from the columnar gland cells of the basal protrusion. (2) During larval settlement, numerous AIs were discharged from the AT tips with sinuous move-ment and rubbing of the tentacles onto the substrata, leading to "nematocyte-printing" around the settlement site. (3) Simultaneous with the discharge of the AIs, migration of stenoteles, desmonemes. and microbasic mastigophores oc-curred, resulting in a dramatic change of nematocyte com-position in the ATs after larval settlement. This was in parallel with changes in larval behavior and the tentacle function. (4) Nematocyte-printing behavior during settle-Received 20 June 2002; accepted 30 January 2003. 1 To whom correspondence should be addressed. Present address: Bio-fouling Group Himeji EcoTech Co.. Ltd.. 841-49 Koh, Shirahama-cho, Himeji-shi, Hyi.Lii. 672-8023, Japan. E-mail:
[email protected] 2 Present address. Faculty of Biotechnology. College of Science and Engineering, Tokyo Denki University, Ishizaka, Hatoyama, Hiki-gun, Saitama 350-0311. Jap,,.. 3 Present address: Laboratory' of Aquatic Natural Products Chemistry. Graduate School of Agricultural and Life Sciences. The University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku. Tokyo 113-8657. Japan. Abbreviations: AI, atrichous isorhiza: AT, aboral tentacle; D. desmo-some; MM. microbasic mastigophore; S, stenotelc. ment could be recognized as metamorphic behavior respon-sible for irreversible changes in AT function, from attach-ment to feeding and defense. Introduction Many marine sessile invertebrates produce planktonic or benthic larvae as a dispersive phase. These larvae develop to competent stages, attach to suitable substrata, and metamor-phose into juveniles in response to certain environmental (physical, biological, and chemical) cues (Crisp, 1974, 1984; Chia and Bickell. 1978; Pawlik, 1992). Many marine colonial hydroids (Cnidaria) produce free-swimming plan-ula larvae for dispersal. Attachment and metamorphosis of some hydroid planulae are induced by certain bacteria, various pharmacologically active compounds, or neuro-transmitter peptides (Miiller, 1985; Leitz and Miiller, 1987; Berking, 1988; Leitz and Klingman, 1990; Leitz el ai, 1994). The biochemical and physiological mechanisms in-volved in attachment and metamorphosis of Hydractinia have been described (Chia and Bickell. 1978; Berking, 1991: Leitz, 1993. 1997). Marine hydrozoans in the genus Tubularia are widely distributed in shallow waters throughout the world (Pe-tersen, 1990). Their relatively large polyps have been ex-cellent subjects for biological studies in areas such as re-generation (Barth, 1940: Tardent and Eymann, 1958), early development of the gonophores (Brauer, 1891; Berrill, 1952; Nagao, 1965), growth in culture (Mackie, 1966), field ecology (Hughes, 1983; Ostman et al., 1995), physiology (Josephson and Mackie, 1965; Neufeld et al., 1978; Michel and Case, 1986), and taxonomy (Tardent, 1980; Petersen, 1990). 256
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