116
• The recent predominance of O. davisae in
Tokyo Bay may partly be explained by the
increase in the abundance of moon-jelly due
to the heavy increase of artifi cial constructions
on the coast offering the substrata of the asexually growing polyps of the moon-jelly (Ishii
and Katsukoshi 2010 ), directly followed by
the ephyra and medusae.
• In addition, the increase in the occurrence of
oxygen depletion in the lower layer of inner
Tokyo Bay (Ando et al. 2005 ) may cause the
general decrease of species diversity of planktons except for the moon-jelly that is less vulnerable in such oxygen-depleted environment
(Ishii et al. 2004 ).
• Increase in ephyras of moon-jelly due possibly to the above reasons, which voraciously
eat zooplanktons except for O. davisae , may
eventually lead to enhance the predominance
of O. davisae .
• Moreover, not spawning but carrying the
eggs, being different from many other copepods, reproduction of O. davisae may not
seriously be affected by the development of
oxygen depletion in the bottom layer (Uye
1994 ).
It has been pointed out that the predominance
of O. davisae , with a very small size (even
adults >0.5 mm) and thus grazing smaller phytoplankton such as dinofl agellates but not diatoms, leads to leftovers and excess growth of
diatoms that sink to the aphotic bottom (7). The
increase of organic materials in the aphotic
bottom deteriorates the oxygen depletion due to
excess decomposition. This possible process of
enhancement of oxygen depletion may also
cause the increase of moon-jelly and predominance of O. davisae again. These processes may
lead to some loss of species diversity in Tokyo
Bay. To avoid this spiral, reduction of nutrient
input and mitigation of artifi cial construction of
the coastline are possible measures. Resilience
of the Tokyo Bay ecosystem may not well function without reviving the natural coastline or
without reducing the nutrient loading. To examine if the above speculation is valid, we are
working on more quantitative studies on the
prey-predator relationship between zooplanktons including O. davisae and the moon-jelly
A. aurita (not only the ephyra stage but also the
polyp and medusa stages).
References
Anakubo T, Murano M (1991) Seasonal variation of zooplankton in Tokyo Bay. J Tokyo Univ Fish
78(2):145–165
Ando H, Kashiwagi N, Ninomiya K, Ogura H, Kawai T
(2005) Changes in the state of water pollution in
Tokyo Bay since 1980 – trend analysis of water
quality using monitoring data obtained by Local
Governments. In: Annual report of the Tokyo
Metropolitan Research Institute for Environmental
Protection, Tokyo Metropolitan Research Institute
for Environmental Protection, Tokyo, pp 141–150
(in Japanese)
Ferrari FD, Orsi J (1984) Oithona davisae , new species, and Limnoithona sinensis (Burckhardt, 1912)
(Copepoda: Oithonidae) from the Sacramento-San
Joaquin Estuary, California. J Crustac Biol
14(1):106–126
Ishii H, Katsukoshi K (2010) Seasonal and vertical distribution of Aurelia aurita polyps on a pylon in the
innermost part of Tokyo Bay. J Oceanogr
66(3):329–336
Ishii H, Kojima S, Tanaka Y (2004) Survivorship and
production of Aurelia aurita ephyrae in the innermost part of Tokyo Bay, Japan. Plankton Biol Ecol
51(1):26–35
Itoh H, Aoki N (2010) Temporal and spatial distribution of
planktonic copepods in Tokyo Bay: seasonal occurrence
in the innermost part of the bay, in the early 1990s. Bull
Plankton Soc Jpn 57(2):94–104 (in Japanese)
Itoh H et al (2011) Vertical distribution of planktonic
copepods in Tokyo Bay in summer. Plankton Benthos
Res 6(2):129–134
Nishida S (1985) Taxonomy and distribution of the family
Oithonidae (Copepoda, Cyclopoida) in the Pacifi c and
Indian Oceans. Bull Ocean Res Inst Univ Tokyo
20:1–167
Omori M, Ishii H, Fujinaga A (1995) Life history strategy
of Aurelia aurita (Cnidaria, Scyphomedusae) and its
impact on the zooplankton community of Tokyo Bay.
ICES J Mar Sci J du Conseil 52(3–4):597–603
Tsuda A, Nemoto T (1988) Feeding of copepods on natural suspended particles in Tokyo Bay. J Oceanogr
44(5):217–227
Uye S (1994) Replacement of large copepods by small
ones with eutrophication of embayments: cause and
consequence. Hydrobiologia 292/293:513–519
Y. Tanaka and T. Akiba
• The recent predominance of O. davisae in
Tokyo Bay may partly be explained by the
increase in the abundance of moon-jelly due
to the heavy increase of artifi cial constructions
on the coast offering the substrata of the asexually growing polyps of the moon-jelly (Ishii
and Katsukoshi 2010 ), directly followed by
the ephyra and medusae.
• In addition, the increase in the occurrence of
oxygen depletion in the lower layer of inner
Tokyo Bay (Ando et al. 2005 ) may cause the
general decrease of species diversity of planktons except for the moon-jelly that is less vulnerable in such oxygen-depleted environment
(Ishii et al. 2004 ).
• Increase in ephyras of moon-jelly due possibly to the above reasons, which voraciously
eat zooplanktons except for O. davisae , may
eventually lead to enhance the predominance
of O. davisae .
• Moreover, not spawning but carrying the
eggs, being different from many other copepods, reproduction of O. davisae may not
seriously be affected by the development of
oxygen depletion in the bottom layer (Uye
1994 ).
It has been pointed out that the predominance
of O. davisae , with a very small size (even
adults >0.5 mm) and thus grazing smaller phytoplankton such as dinofl agellates but not diatoms, leads to leftovers and excess growth of
diatoms that sink to the aphotic bottom (7). The
increase of organic materials in the aphotic
bottom deteriorates the oxygen depletion due to
excess decomposition. This possible process of
enhancement of oxygen depletion may also
cause the increase of moon-jelly and predominance of O. davisae again. These processes may
lead to some loss of species diversity in Tokyo
Bay. To avoid this spiral, reduction of nutrient
input and mitigation of artifi cial construction of
the coastline are possible measures. Resilience
of the Tokyo Bay ecosystem may not well function without reviving the natural coastline or
without reducing the nutrient loading. To examine if the above speculation is valid, we are
working on more quantitative studies on the
prey-predator relationship between zooplanktons including O. davisae and the moon-jelly
A. aurita (not only the ephyra stage but also the
polyp and medusa stages).
References
Anakubo T, Murano M (1991) Seasonal variation of zooplankton in Tokyo Bay. J Tokyo Univ Fish
78(2):145–165
Ando H, Kashiwagi N, Ninomiya K, Ogura H, Kawai T
(2005) Changes in the state of water pollution in
Tokyo Bay since 1980 – trend analysis of water
quality using monitoring data obtained by Local
Governments. In: Annual report of the Tokyo
Metropolitan Research Institute for Environmental
Protection, Tokyo Metropolitan Research Institute
for Environmental Protection, Tokyo, pp 141–150
(in Japanese)
Ferrari FD, Orsi J (1984) Oithona davisae , new species, and Limnoithona sinensis (Burckhardt, 1912)
(Copepoda: Oithonidae) from the Sacramento-San
Joaquin Estuary, California. J Crustac Biol
14(1):106–126
Ishii H, Katsukoshi K (2010) Seasonal and vertical distribution of Aurelia aurita polyps on a pylon in the
innermost part of Tokyo Bay. J Oceanogr
66(3):329–336
Ishii H, Kojima S, Tanaka Y (2004) Survivorship and
production of Aurelia aurita ephyrae in the innermost part of Tokyo Bay, Japan. Plankton Biol Ecol
51(1):26–35
Itoh H, Aoki N (2010) Temporal and spatial distribution of
planktonic copepods in Tokyo Bay: seasonal occurrence
in the innermost part of the bay, in the early 1990s. Bull
Plankton Soc Jpn 57(2):94–104 (in Japanese)
Itoh H et al (2011) Vertical distribution of planktonic
copepods in Tokyo Bay in summer. Plankton Benthos
Res 6(2):129–134
Nishida S (1985) Taxonomy and distribution of the family
Oithonidae (Copepoda, Cyclopoida) in the Pacifi c and
Indian Oceans. Bull Ocean Res Inst Univ Tokyo
20:1–167
Omori M, Ishii H, Fujinaga A (1995) Life history strategy
of Aurelia aurita (Cnidaria, Scyphomedusae) and its
impact on the zooplankton community of Tokyo Bay.
ICES J Mar Sci J du Conseil 52(3–4):597–603
Tsuda A, Nemoto T (1988) Feeding of copepods on natural suspended particles in Tokyo Bay. J Oceanogr
44(5):217–227
Uye S (1994) Replacement of large copepods by small
ones with eutrophication of embayments: cause and
consequence. Hydrobiologia 292/293:513–519
Y. Tanaka and T. Akiba