Invertebrate Biology という雑誌の頭足類関連論文をWeb上でざっと見ていたときに見つけ，少し気になった論文をピックアップ.
リファレンス-1
Ontogeny of the fused tentacles in three species of ommastrephid squids (Cephalopoda, Ommastrephidae).
Shea, E. K. (2005). Invertebrate Biology, 124(1), 25-38.
https://doi.org/10.1111/j.1744-7410.2005.1241-04.x
要旨-1
The tentacles of ommastrephid squids fuse during embryonic development and remain fused as they grow through hatching, but eventually separate to become two fully functional adult tentacles. The external anatomy of individuals at several post-hatching ontogenetic stages of three species of ommastrephid squids (Ommastrephes bartramii, Sthenoteuthis oualaniensis, and Hyaloteuthis pelagica) was examined using scanning electron microscopy and morphometrics. The fusion of the transverse muscle mass of the tentacles was examined using light microscopy. Five ontogenetic stages of tentacle separation were defined based on landmark features such as the extent of the fusion and the presence of suckers or sucker buds at the distal tip. The total tentacle length and fused tentacle length reached a maximum when the dorsal mantle length (ML) equaled 3–4 mm (H. pelagica) or 4–6 mm (O. bartramii, S. oualaniensis), and then decreased with increasing ML. The average split length (measured from the base of the tentacles to the point of tentacle fusion) increased gradually with increasing ML, and the separate tentacle diameter was roughly half the diameter of the fused portion at all sizes. In all three species, separation of the fused tentacles began earlier in development (2–3-mm ML) and was more advanced at smaller sizes than previously reported. The sizes presented here are conservative because excess epithelium at the location of the split may disguise the actual site of separation. Post-separation tentacles were much shorter than the arms, and the carpal region appeared torn in 2 of the 4 specimens of S. oualaniensis examined. Finally, none of the original distal tip suckers were retained on the post-separation tentacles of S. oualaniensis. These observations are consistent with the hypothesis that the tentacles separate gradually then rupture at the “wrist” (presumptive carpus), and argue against the possibility of prey capture by the fused tentacles.
コメント-1
スルメイカなどを含むアカイカ科の稚仔の持つ触腕の特殊な形態について調べた論文．アカイカ科の稚仔は他のイカとは異なり，左右の触腕が融合した独特の形態を持つことが知られている．この触腕がどのように左右に分かれて成体に見られる機能的な2本の触腕になるのか調べるため，3種のイカについて触腕が分かれる様子をSEM等を用いて観察している．融合していた際に先端に見られる吸盤が分裂が進むにつれて失われ，新たに他の腕と同じような吸盤原基が作られる様子なども見られ，かなりダイナミックに形態の変化が起きているのは非常に興味深い．
リファレンス-2
Histology and ultrastructure of ink gland and melanogenesis in the cuttlefish Sepia pharaonis.
Jiang, M., Xue, R., Chen, Q., Zhan, P., Han, Q., Peng, R., & Jiang, X. (2020). Invertebrate Biology, 139(4), e12306.
https://doi.org/10.1111/ivb.12306
要旨-2
The ink gland in cephalopods is a modified part of the digestive tract that generates melanin used in defense activities against predators. In the present study, we document the histological structure of the ink gland of the cuttlefish Sepia pharaonis following light microscopy (LM) and transmission electron microscopy (TEM) examinations. Based on the observations, we infer the series of cellular events leading to the formation and secretion of melanin. LM observations revealed that the cuttlefish ink gland was cord-like, and could be separated into two distinct regions with different functions, based on markedly different cellular and biochemical characteristics. The region with the function of secreting ink was the epithelial cell region (i.e., mature ink gland cells) and was mainly composed of melanosomes, rough endoplasmic reticulum, Golgi apparatus, and mitochondria. Conversely, the connective tissue region was the site of formation of cells and energy supply. In addition, the most probable sequence of activities from melanogenesis to the release, based on TEM observations, is as follows: (a) formation of a matrix of opaque electronic substances (melanin precursors); (b) melanization in the precursor matrix to form melanin-like particles; (c) further melanin granule development and enlargement, and gradual intensifying of the hue; (d) production of melanin granules (melanosomes) in the precursor matrix; (e) binding of melanosomes to the cytoskeleton and migration to the cell surface; (f) fusion of melanosomes with the cell membrane at the apical pole, and discharge of melanin granules into intercellular spaces or lumen by exocytosis or cell fragmentation. We also investigated how the process of continuous ink release influences ink gland histology and ultrastructure. Inking stimulation induced the release of large amounts of ink by epithelial cells, and was associated with cell disorganization, and severe cellular vacuolization, in addition to severe organelle damage.
コメント-2
トラフコウイカ Sepia pharaonis を用いて墨を生成するink glandの形態を組織学的に調べた論文．構成する細胞の種類などから，ink glandはメラニン合成を行う上皮細胞層と，上皮細胞に分化する前の細胞増殖やエネルギーの供給を行う層に分かれていることが組織切片の観察により示され，またTEMにより詳細なメラニン合成の様子が記載されている．
ink glandは頭足類に独特の新規形質の一つであり，消化管が変化して獲得されたと考えられている．生理学的なメラニン合成のメカニズムなどは良く研究されているが，形態学的な情報は意外と少ないので，記載的とはいえこのような基礎的な研究はとても大事だし，興味深い．消化管からの進化を考える上でも非常に重要な知見かと思われる．
