Crinoids (Echinodermata) have long been considered as a typical example of suspension feeders. Crinoids are also regarded as a "non-selective" suspension feeder in general. In order to test whether crinoids select their foods, and also whether they positively control surrounding water current actively, the following three experiments were conducted.

The existence or absence of food selection of M. rotundus is tested. Standardized gut contents were examined after supplying model food. The model food contained zooplanktons, grass beads (medium- and small-sized), and phytoplanktons in descending order. Among the grass beads, crinoids tend to contain larger particles. But comparing among all kinds of particles, crinoids took much greater amount of phytoplanktons than other particles. In descending order, phytoplanktons, zooplanktons and medium beads are almost in the same volume, followed by small beads, respectively. This result strongly demonstrated that crinoids select food materials at the particle level. This is the first quantitative analysis of the crinoid food selection, and the first documentation about existence of active food selection of crinoid.

By the previous submersible observations, crinoids are thought to change their postures responding to the changes of current direction and velocity. On the other hand, a preliminary experiment by the author indicated that crinoids also respond to approaching particles in a current. Based on the food selection and this result, the crinoid behaviors were examined in aquarium condition. Zooplanktons, phytoplanktons and quartz powder were applied to crinoids, firstly as not mixed (singly), then as mixed status. Crinoids showed a common posture change after the supply of all kind particles. If the suspended particles contained organic matters, crinoids swung the arms and swelled pinnules. On the other hand, when the particles were quartz powder only, crinoids did not show any movements, and their arms completely remained in a same posture. This result indicates that 1) crinoids can distinguish whether the suspended particles contained organic matters or not, depending on the chemical properties, and 2) the behaviors are regarded as adaptive or effective in the sense of feeding efficiency.

The possibility that crinoids could distinguish if the suspended particles contained organic matters, is examined by food extraction. The chemical properties of zooplanktons and phytoplanktons were extracted and packed into agarose gel. The gels were ground up and applied to crinoids. Crinoids showed peculiar behaviors as they showed when organic matters were supplied, but in the case of agarose gel only, they showed almost no movement. Thus it is concluded that crinoids can recognize the chemical properties of suspended particle. This is also the first discovery that the crinoids behave differently according to the different qualities of diets.

The means of crinoid behavior during their feeding were examined in the following two steps. At first, the water current around the crinoid individuals were visualized by carmine dye. Before feeding, currents were uniform; passing through the crown, but sometimes they were slightly disturbed by their arms. During feeding, currents were much more disturbed than those before feeding, and sometimes circular currents along curled arms occurred directly inside of crown. After feeding, the currents were passing away the crown. Such disturbed/passing seem to be produced by the current modification by the crinoid. Then the current changes were measured quantitatively. The current velocity and directions were recorded by a velocity meter around a crinoid individual among following three phases, before feeding, among feeding, and after feeding. The changes of current directions and the velocity are arrhythmic through all the phases. But during the feeding phase, the changes of current direction and velocity seemed to be maintained as minimum, indicating that the current conditions were kept in a certain range. Thus it is possible that crinoids maintain current conditions by their posture and by their behavior, during their feeding.

Living crinoids consist of about 600 species, by contrast fossil crinoids are more than 9,000 species. Crinoids were most diversified in the Paleozoic. General features of Paleozoic type crinoids are 1) dominance of ligamentaly arm articulations, 2) sessile lifestyle, and 3) absence of pinnules. On the other hand, Recent crinoids differ from Paleozoic type in the following characteristics; 1) arms are flexibly moving by well developed muscular articulations, 2) free living lifestyle (especially in comatulids and isocrinids), and 3) possession of pinnules. Arm flexibility and existence of pinnules have enabled Recent crinoids to change and adjust the density of filtration fan to the ambient current. Former study showed that crinoid linages which have variable fan density are long ranging. This study revealed that the arm flexibility serves not only for changing the fan density, but also for current stabilization by the moving. By the submersible observation, crinoids tend to stand on the environment where relatively strong currents prevail. Free living lifestyle allows crinoids to replace the micro habitat. Moreover, pinnules are used for the crawling locomotion. Free living lifestyle is an advantageous character also in the sense of escape from predators.

It is strongly suggested that 1) fossil crinoids (at least post-Paleozoic forms) could recognize, distinguish and select the suspended particles, 2) posture changes and arm behaviors among feeding are predominant characters in Recent crinoids. The other echinoderms are reported to use chemical sensory organs during their feeding. For example, an Atlantic starfish can recognize its favorable clam and distinguish the prey bivalves. Thus it is reasonable that echinoderms' chemical sensing is used in feeding not only in living but also in fossil echinoderms. Furthermore, particle sorting should be more important in the Paleozoic type crinoids than in case of Recent type crinoids to get a nutritious food, because the Paleozoic type crinoids could not feed efficiently as the Post-Paleozoic forms by adjusting their fan to catch desirable foods in optimum amount. In the Paleozoic, many crinoids formed highly dense communities so called "crinoid meadows" or "crinoid gardens" in shallow water. It is possible that food selection by crinoids and their low metabolic rate are a key factor for such highly dense communities.

Moreover, Paleozoic type crinoids and blastoids went extinct at the P/T boundary. They both had no "muscular arm" and they were both sessile benthos. According to this study, namely the absence of arm mobility or absence of optimal adjustment in feeding, are also considered as a possible cause of their decline. Through the experiment, the current velocity was kept as constant. The current velocities around crinoid individual were kept in a certain range (statistically not significance), and in this condition crinoids were moving. This moving and posture changing were made possible by the muscular articulations. Thus it is concluded that acquisition of muscular articulation has allowed the Recent type crinoids to adapt the wide-range of environmental changes, resulting in the long-term survivor of their linage.

本論文は、生きている化石として有名な有柄ウミユリの1種,トリノアシの生体を用い,この種が能動的に懸濁物を捉えて採餌していることを実験的に明らかにした先駆的,独創的な研究である.

有柄ウミユリ類はオルドビス紀前期に出現し,その後基本的な体制を変えることなく5億年以上も生き続けてきた動物である.また現生の棘皮動物門の中では最も原始的な系統的位置を占める動物と考えられている.有柄ウミユリ類は一般に深海に生息するため,その生体を用いた実験的研究はほとんど行われて来なかった.従来,有柄ウミユリ類は海底の流れの中で,腕を伸ばして懸濁物を非選択的に捉えていると考えられてきた.北沢君は日本近海に生息する有柄ウミユリのうち,割合浅海に生息する1種,トリノアシを用い,それを実験水槽で飼育しながら観察することにより,その餌のサイズによる選択,餌の種類による選択,餌や化学物質による採餌行動の違い,さらに冠部で積極的に水流をコントロールすることによる採餌効率の向上の有無について検討を行った.

本論文は5章から構成されている.第1章では,有柄ウミユリの基本的体制と採餌方法,そして本論文の目的について述べられている.第2章では,トリノアシを実験水槽で飼育しながら,異なるサイズのガラスビーズと,異なる種類の餌を与えた場合,どのようなものを選択的に採取しているのかを,消化管内容物を検討することによって定量的に明らかにした.この結果,トリノアシは大(直径350-500μm),中(直径117-250μm)小(直径37-63μm)のビーズ玉のうち,中ビーズを選択的に多く取り込むこと(大ビーズは急速に沈降し捉えることができない),そして中ビーズと餌のカイアシ類,植物性プランクトンとを比べると,植物性プランクトンを選択的に多く採餌していることが明らかになった.このことは,トリノアシが餌のサイズと種類の両方を関知して選択していることを示している.この有柄ウミユリ類による餌の選択性の存在は初めて確認されたことである.第3章ではトリノアシが採餌時にとる特有の行動,すなわち腕とその付属機関である羽枝を振る運動が,どのような餌と,どのような餌から抽出した化学物質に反応するのかを調べたものである.与えるモデル餌料としてはカイアシ類,植物プランクトン粉末,石英粒子,またカイアシ類と植物プランクトンからの抽出物質それぞれについて与えた後の反応を観察した.その結果,トリノアシは無機物質(石英)には物質の接触時以外にはほとんど反応を示さないこと,有機物が含まれているときには特有の採餌行動を示すこと,餌からの抽出物質には反応すること,などが明らかになった.これらの事実から,トリノアシは化学受容器を有し,餌からの化学的物質を関知して採餌行動を行っていることが示された.第4章では,トリノアシの採餌行動時に,冠部の周囲にどのような水流が生じ,効率の良い採餌に役立っているのかを検証した.冠部の上流側,下流側にそれぞれ8点の測点を設け,採餌前,採餌中,採餌後の各点の水流の強さと向きを水流計で計測した.その結果,トリノアシは採餌中のみ上流側に回転する水流を起こし,また同じく採餌中のみ周囲の水流を一定の変化幅に抑える傾向が見いだされた.つまりトリノアシは積極的に周囲の水流をコントロールし,効率の良い採餌を行っていることが示された.第5章では,このような積極的な採餌行動を起こすメカニズムとして,トリノアシを含む現代型ウミユリが,古生代型ウミユリには存在しなかった腕の筋肉を用いて最適な採餌行動を行える能力を備えたこと,そしてそれがこのグループの成功に寄与している可能性を議論している.

従来行えなかった有柄ウミユリ類の生体を用いることにより,北沢君は実験的研究を行い,初めてその能動的,選択的な採餌行動を明らかにすることができた.これは棘皮動物に限らず,一般的に多くの海棲懸濁物食者が非選択的であると従来考えられていることに対し,検討する必要があることを示している.また有柄ウミユリ類が化学受容器を用いて化学物質を関知し,反応することも初めて示された.これらの事実は現代型ウミユリが大量絶滅を乗り越え,生きている化石として生き続けていること関係している可能性が高い.

なお,本論文は,大路樹生との共同研究であるが,北沢君が主体となって実験を行い,考察を行ったもので,北沢君の寄与が十分であると判断する.

これらの点から,審査委員会では全員が論文の独創性・萌芽性と今後の進化古生物学の新しい発展に寄与した点を高く評価し,本論文を博士(理学)の学位に受けるに値すると判断した.