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Please use this identifier to cite or link to this item:
http://hdl.handle.net/2031/5525
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| Title: | The conservation of horseshoe crabs in Hong Kong |
| Other Titles: | Xianggang ma ti xie de bao yu yan jiu
香港馬蹄蟹的保育研究 |
| Authors: | Li, Hiu Yan (李曉恩) |
| Department: | Department of Biology and Chemistry |
| Degree: | Master of Philosophy |
| Issue Date: | 2008 |
| Publisher: | City University of Hong Kong |
| Subjects: | Limulus polyphemus -- Conservation -- China -- Hong Kong. |
| Description: | CityU Call Number: QL447.7 .L53 2008
xxvi, 250 leaves : ill. (some col.) 30 cm.
Thesis (M.Phil.)--City University of Hong Kong, 2008.
Includes bibliographical references (leaves 227-249) |
| Type: | thesis |
| Abstract: | Horseshoe crabs in Hong Kong are facing a rapid population decline and local
extinction risk. This study focused on some aspects of the conservation of horseshoe
crabs in Hong Kong, including 1) most up-to-date information of the present status and
distribution of horseshoe crabs; 2) morphological measurements and genetic analysis of
two horseshoe crab species Tachypleus tridentatus and Carcinoscorpius rotundicauda,
and geographical variations in population genetics of T. tridentatus; 3) the degree of
human exploitation of horseshoe crabs; and 4) trials on artificial insemination and
breeding of horseshoe crabs and investigations on the optimal environmental conditions
for enhancing the survival and hatching success of horseshoe crab eggs.
Juvenile horseshoe crabs were sampled on the soft shores in Hong Kong using a
quantitative random quadrat sampling along shore transects at 17 survey sites in Hong
Kong. A total of 15 juvenile Tachypleus tridentatus were found at Tsim Bei Tsui, Pak
Nai, two locations at Ha Pak Nai in northwestern New Territories, and San Tau and Yi
O on Lantau Island. In comparison with a similar territory-wide study in 2002 by
Morton and Lee (2003), a significant decline in the mean population density was recorded, with the decrease of 96, 90 and 80% at Pak Nai and two locations in Ha Pak
Nai, respectively. No Carcinoscorpius rotundicauda was found in the transect study at
the 17 sites. Additional walk-through observations on the shores revealed the presence
of T. tridentatus at other sites including Tsim Bei Tsui in the northwestern New
Territories, and Shui Hau Wan, Tai Ho Wan, Sham Wat, Yi O and Tung Chung on
Lantau Island. The walk-through study also revealed the presence of C. rotundicauda at
Tsim Bei Tsui, Sheung Pak Nai, Pak Nai, two locations at Ha Pak Nai in northwestern
New Territories, Luk Keng and Lai Chi Wo in the northeastern New Territories, and Tai
Ho Wan, Yi O and Tung Chung on Lantau Island.
Further intensive distribution surveys were conducted at four key horseshoe crab
nursery grounds, namely Pak Nai, Ha Pak Nai, Shui Hau Wan and San Tau, at monthly
intervals from March to August 2005, using both the transect study and walk-through
observations. All the sites supported T. tridentatus populations with a maximum
abundance of 85 individuals at Shui Hau Wan in April. Few (1-2 individuals) C.
rotundicauda were found at Pak Nai and Ha Pak Nai, while a higher abundance (total
64 individuals) was recorded at San Tau on Lantau Island in the 6-month survey.
However, its overall population was much lower as compared with T. tridentatus.
Among the sites, San Tau and Shui Hau Wan were the key nursery grounds for C.
rotundicauda and T. tridentatus, respectively. The active period for horseshoe crabs
lasted from May to July with more individuals being found as compared with the
inactive period in March, April and August. A wide range of age groups of horseshoe
crabs were recorded, with a maximum prosomal width ranging from 10.1 to 96.1 mm
and 10.9 to 43.3 mm for T. tridentatus and C. rotundicauda, respectively. A significant
spatial variation in the abundance of T. tridentatus on the shore was obtained with larger
individuals on the lower shores. As individuals of T. tridentatus <12.0 mm (maximum prosomal width) were recorded in the present survey, these sites were certainly
important nursery grounds for juveniles and newly hatched individuals.
Both morphological measurements and genetic analysis were used for the
differentiation of the two commonly occurring horseshoe crab species in Hong Kong.
For morphological measurements, T. tridentatus and C. rotundicauda were significantly
different in various ratios of body parts. Only 5 out of the 9 parameters were proven
important in differentiating the two species; they are the ratios of prosomal length to
maximum prosomal width, maximum prosomal width to distance between two
compound eyes, carapace length to telson length, first opisthosomal spine length to
maximum prosomal width, and sixth opisthosomal spine length to maximum prosomal
width. For phylogenetic comparisons, the inter-specific variations of both 18S and 28S
rDNA sequence were very small, and was <1% and 1.57%, respectively. Thus, these
two rDNA regions may not be powerful enough for the differentiation of T. tridentatus
and C. rotundicauda. The 18S and 28S rDNA of “abnormal” juvenile T. tridentatus
individuals with only 1 immovable spine on the dorsal surface of the opisthosoma above
the insertion of a post-anal spine, and “normal” individuals with 3 immovable spines
were also sequenced. Both juvenile groups were genetically closer to C. rotundicauda
but morphologically closer to adult T. tridentatus. There was no significant spatial
variation in the genetic pattern of juvenile T. tridentatus at five nursery grounds, Pak
Nai, Ha Pak Nai, San Tau, Shui Hau Wan and Yi O, based on 18-28S intergenic spacer
sequence (ITS) rDNA, suggesting that these horseshoe crab populations may come from
a common population.
The degree of human exploitation of horseshoe crabs in Hong Kong was estimated
through interviewing 34 seafood restaurants, 150 fish sellers and fish handlers. A total of 1,023 horseshoe crabs were caught in 2004-05, with 72% from mainland Chinese
waters. Of these, 33% were released back to the sea after being caught and only 690
individuals were retained on board. The majority (62%) of them were sold and used for
the Chinese traditional set-free rituals, while the remaining were kept and sold in fish
stalls and seafood restaurants. An average sale of 17 horseshoe crabs per month was
estimated throughout the 13-month study, with 45% of them being obtained from Hong
Kong waters. The sale of horseshoe crabs was high by comparing with other popular
commercial marine species, in view of the low population density, long maturity period
and low breeding rate of horseshoe crabs. Hence, human exploitation is possibly one of
the major impacts affecting the long-term survival of this animal in Hong Kong.
In view of the low natural breeding and hatching success, and low juvenile
survival, artificial insemination and breeding may be reasonable options to enhance the
horseshoe population in the natural environment. Trials of artificial breeding were
conducted in the summer of 2004. Several thousand trilobite larvae hatched with some
of them further molted into juveniles of second to fifth instars. Mortality rate was high
after hatching and varied over time. After almost two years of laboratory rearing, only
some 20 individuals of the first batch of trilobites survived to become juveniles. To
elucidate the optimum conditions for the survival and hatching of eggs, a two factorial
experiment was conducted with combinations of four salinities (15‰, 20‰, 25‰ and
30‰) and three temperatures (20°C, 25°C and 32°C) being studied. Horseshoe crab
eggs survived at a wide range of temperatures (20–32°C) and salinities (20–30‰).
However, no hatching was observed at 15‰. Although the survivorship at low
temperatures and low salinities were relatively high, the developmental and hatching
rates were reduced. The highest hatching rate was obtained at 32°C and 30‰ which
should be recommended for future artificial breeding practices. Trials on non-invasive artificial breeding methods using electrical stimulation for the collection of unfertilized
eggs and sperms were also performed. Several successful trials were undertaken,
however, the number of eggs obtained was small. Further studies are required to
ascertain the applicability and repeatability of this method.
The present study provides comprehensive baseline information on the horseshoe
crabs in Hong Kong that may form a basis for the implementation of conservation
measures in the future. |
| Online Catalog Link: | http://lib.cityu.edu.hk/record=b2340766 |
| URI: | http://hdl.handle.net/2031/5525 |
| Appears in Collections: | BCH - Master of Philosophy
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