Menghong Hu1, Qiongzhen Li2, SG Cheung1 & Paul Shin1

1City University of Hong Kong, Hong Kong, China 2Guangxi Institute of Fisheries, Guangxi, China

Distribution, abundance and population structure of

horseshoe crabs along three intertidal zones of Beibu

Gulf, Southern China

Outline

1 Background

2 Materials and Methods

3 Results and Discussion

4 Conclusions

1. Background

Taxonomy

Limulus polyphemus

Taxonomy

Tachypleus tridentatus

Tachypleus gigas

Carcinoscorpius rotundicauda

Asian species:

Taxonomy

Tachypleus tridentatus

Tachypleus gigas

Carcinoscorpius rotundicauda

The populations of three

Asian horseshoe crab

species have decreased

considerably in recent

years owing to

habitat destruction,

pollution,

overharvest for food &

medicinal use.

Life-history characteristics and habitat preferences

Species-specific

preference?

Total area where adult

horseshoe crabs are

distributed can be very large.

It is easier to evaluate,

accurately, the abundance

of juveniles as they live on

their nursery beach for

more than nine years.

Juvenile horseshoe crabs

are relatively immobile, so

they are easy to count.

Horseshoe crabs are

arthropods, so they

undergo moulting to

increase in size.

In field studies, their growth is estimated indirectly from

the analysis of the size-frequency distributions

In field studies, their growth is estimated indirectly from

the analysis of the size-frequency distributions

Objectives

The present study aimed

to investigate the

abundance, growth and

population structure of

Chinese and mangrove

juvenile horseshoe crabs

at different nursery

beaches along Beibu

Gulf, southern China.

2. Materials and Methods

Beibu Gulf

Important nursery ground in China

2 m2 area

Study location – Beihai (Guangxi, China)

2 m2 area

Based on this information and a

brief overview on the survey

data from the monitoring study

by Guangxi Institute of Fisheries,

three nursery beaches with

different sediment characteristics:

a sandy beach at Xi Bei Ling

(XBL), a sand–muddy beach at

Jin Hai Wan ( JHW) and a

muddy beach of Xi Chang (XC)

were selected for the surveys.

Study location – Beihai (Guangxi, China)

Study sites description

Survey strategies

• Sampling time: 2009

• Six horizontal transects (width: 12 m) were set equally from

0.6 m to 1.6 m above chart datum per beach.

• The number of juveniles of

each whole transect was

counted.

Data analysisIndividual cohorts (size classes) were separated using the

modal progression analysis of Fish Stock Assessment Tool

II (FiSAT II)

The relationship between post-moult and pre-moult size

was used to compare the differences between the two

horseshoe crab species, according to the Hiatt growth

model.

Mortality (%) = (AInstar X – Ainstar X+1)/ AInstar X

3. Results and Discussion

3.1 Abundance

Table 1 Abundance of juvenile horseshoe crabs recorded at the study beaches.

Total

individuals

Total sampling

area (100 m2)

Density (mean ± SE)

(ind∙100 m-2)

T. tridentatus

XBL 1,742 972 1.79 ± 0.16

JHW 2,753 864 3.19 ± 0.28

XC 441 504 0.88 ± 0.16

C. rotundicauda

JHW 121 864 0.14 ± 0.02

XC 872 504 1.73 ± 0.14

3.2 Population structure

Figure 1 Juvenile T. tridentatus cohorts based on size (prosomal

width) frequency data at the three study beaches.

Figure 2 Juvenile C. rotundicauda cohorts based on size

(prosomal width) frequency data at XC nursery beach.

3.3 Growth

Table 2 Relationship between pre-moult and post-

moult prosomal widths of juvenile horseshoe crabs

(Hiatt growth model).

Equation R2

T. tridentatus

Sekiguchi (1988) Lt+1 = 1.12 Lt + 2.30 0.99

XBL Lt+1 = 1.33 Lt − 1.86 0.95

JHW Lt+1 = 1.24 Lt + 0.31 0.97

XC Lt+1 = 1.27 Lt − 0.86 0.98

C. rotundicauda

Sekiguchi (1988) Lt+1 = 0.92 Lt + 3.05 0.99

XC Lt+1 = 1.07 Lt + 3.74 0.97

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8 9 10

Mea

n p

roso

mal

wid

th (m

m)

Years

Sekiguchi et al. (1988)

XBL

JHW

XC

Figure 3 Growth rates of juvenile T. tridentatus as an

increase in mean prosomal width over time.

3.4 Mortality

Instar

stage

T. tridentatus C. rotundicauda

XBL JHW XC XC

Per

instar

Cumula-

tive

Per

instar

Cumula-

tive

Per

instar

Cumula-

tive

Per

instar

Cumula-

tive

7 7.0 - - - - - 59.5 -

8 59.3 62.2 42.1 - 23.9 - 29.5 -

9 - - 38.5 64.4 48.4 60.7 - -

10 58.9 - 52.6 70.9 56.5 77.6 39.9 35.2

11 - - 38.7 71.0 64.9 84.8 13.0 47.7

Table 3 Mortality (%) of juveniles at the study beaches.

Mortality was calculated at each successive instar and

accumulated across instars.

4. Conclusions

Juvenile T. tridentatus at 7th instar stage was in the majority in

the study area of XBL, while juvenile T. tridentatus at 8th instar

stage was dominant in the study area of both JHW and XC.

Juvenile C.rotundicauda at 7th instar stage was abundant in the

study area of XC.

Juvenile T. tridentatus at 7th instar stage was in the majority in

the study area of XBL, while juvenile T. tridentatus at 8th instar

stage was dominant in the study area of both JHW and XC.

Juvenile C.rotundicauda at 7th instar stage was abundant in the

study area of XC.

The relationship between pre-moult and post-moult sizes of

juvenile horseshoe crabs was generally fitted with the Hiatt linear

model, which showed juvenile T. tridentatus grows faster than C.

rotundicauda.

Juvenile T. tridentatus at 7th instar stage was in the majority in

the study area of XBL, while juvenile T. tridentatus at 8th instar

stage was dominant in the study area of both JHW and XC.

Juvenile C.rotundicauda at 7th instar stage was abundant in the

study area of XC.

The relationship between pre-moult and post-moult sizes of

juvenile horseshoe crabs was generally fitted with the Hiatt linear

model, which showed juvenile T. tridentatus grows faster than C.

rotundicauda.

Slow growth and high cumulative mortality rate (circa 50%) of

the two species were also found in this study.

Juvenile T. tridentatus at 7th instar stage was in the majority in the

study area of XBL, while juvenile T. tridentatus at 8th instar stage

was dominant in the study area of both JHW and XC. Juvenile

C.rotundicauda at 7th instar stage was abundant in the study area

of XC.

The relationship between pre-moult and post-moult sizes of

juvenile horseshoe crabs was generally fitted with the Hiatt linear

model, which showed juvenile T. tridentatus grows faster than C.

rotundicauda.

Slow growth and high cumulative mortality rate (circa 50%) of the

two species were also found in this study.

The present findings implied that once a horseshoe

crab population is locally extirpated, it takes a long

period of time for them to re-establish even after

restoration of their degraded habitats.

This is an extensive investigation to evaluate the

distribution, abundance and population structure of

juvenile horseshoe crabs at the three nursery beaches.

Baseline information:

further population dynamics studies

Acknowledgements

• My supervisor: Dr. Paul K.S. Shin

• Co-supervisor: Dr. S.G. Cheung

• Ms Qiongzhen Li

• All my labmates of CityU

Questions or Comments?

Thank you !