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Vol. 48 No. 3, 2009
A Growth Check Deposited at Estuarine Arrival in Otoliths of Juvenile Flathead Mullet (Mugil cephalus L.)
Chih-Chieh Hsu1, Chih-Wei Chang2, Yoshiyuki Iizuka3, and Wann-Nian Tzeng1,*
1Institute of Fisheries Science, College of Life Science, National Taiwan University, Taipei 106, Taiwan
2National Museum of Marine Biology and Aquarium, Pingtung 944, Taiwan
3Institute of Earth Science, Academic Sinica, Nankang, Taipei 115, Taiwan
Chih-Chieh Hsu, Chih-Wei Chang, Yoshiyuki Iizuka, and Wann-Nian Tzeng (2009) An
accessory primordium (AP) and secondary growth zone (SGZ) with unique
microstructure and microchemistry were observed in otoliths of juvenile
flathead mullet (Mugil cephalus
L.) collected in estuaries. The microstructure, microchemistry,
and crystalline structure of the mullet otoliths collected from 4
estuaries of western Taiwan from 1996 to 2004 were examined by optical
and scanning electron microscopy, Raman spectroscopy, and an electron
probe microanalyzer to evaluate the age at formation of the AP and
whether the SGZ was deposited during a habitat transition from offshore
to the estuary. The age was determined by the daily growth
increments in mullet otoliths. Mean ages (32.9 ± 5.5 d) and
lengths (28.3 ± 2.8 mm) of the fish at estuarine arrival did not
significantly differ among years or estuaries (p
= 0.82 and 0.31, respectively). The mean age when APs were
deposited in juvenile mullet otoliths was 27.1 ± 2.5 d, which was
consistent with the age of fish without an AP in the otolith (26.9 ±
3.0 d). The consistency in ages indicated that the AP was
deposited very soon after estuarine arrival. The Raman shift and
elemental composition indicated that the SGZ was a normal aragonite
crystal without polymorphic vaterite or calcite inclusions, indicating
that the formation of the SGZ was not due to a changing crystalline
structure of CaCO3. Sr/Ca concentration ratios significantly decreased from approximately 8.0 × 10-3 in the primordium of the otolith to < 4 × 10-3
at the AP when the juvenile mullet migrated from highly saline offshore
to fresh water in the estuary, during which time fish behavior changed
from pelagic to benthic habits. This suggests that the AP can be
used as a biological tracer to determine the age of the fish at
recruitment and refine the reconstruction of the early life-stage
environmental history of flathead mullet.
Key words: Mugil cephalus, Otolith, Microstructure, Microchemistry.
*Correspondence: Tel: 886-2-33662887. Fax: 886-2-23639570. E-mail:wnt@ntu.edu.tw
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