Laboratory Experiment of Sexual Dimorphism in Crayfish

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Dimorphism is a sexual selection concept observed in most animals together with man and crayfish. Male crayfish are known to have long and large chela than their female counterparts. This experimented aimed at justification of the aforementioned hypothesis. Crayfish were sexually separated and the chela and physique measurements recorded and analyzed. The results showed that male crayfish has longer chelas than the lady crayfish. The outcomes of the findings justified the hypothesis that male crayfish have longer chela in contrast to the female hence sexually dimorphic.

Introduction

Sexual dimorphism is observable distinction in the appearance in color, size, shape and structure between females and males of the identical species. Sexual dimorphism is brought about by the genetic inheritance of sexual patterns in a genetic material. However, the crayfish apart from their sexual differences, other features are common and do not distinct differences (Moriyama). During the initial investigation, it is difficult to distinguish the female from male crayfish. In this experiment, the sexual dimorphism of the crayfish was investigated through the measurement of the chela and cephalothorax size. The determinants for sexual selection in this experiment involved the length of the claws/ chela and the length of the crayfish’s body. Generally, male crayfish have larger claws than female as they use them for mating. On the other hand female crayfish have larger tail size than male crayfish (Kuwai).

Method

During the experiment, ten pairs of crayfish were split from the bucket. The isolated ten pairs of crayfish were then separated into five males and five females respectively. The separation of the crayfish was based on the physical characteristics of the individual crayfish. We observed that the male crayfish had larger claws compared to their female counterparts and the female crayfish had long tails compared to their male counterparts.
The cephalothorax and the chela of each crayfish were then measured after the isolation, separation and identification of each crayfish sex. The measurements were done in centimeters and the results recorded into an excel sheet.
A tub with an inch of water was used to contain all the separated crayfish. For effective measurement to be obtained, two people were needed to handle each crayfish during the measurement. The data for morphological analysis was recorded in excel format to enable easy calculations. Using an excel data analysis software, t-value, p-value and the critical t-values of both the chela and body size for both the female and male crayfish were analyzed. The mean, n-value and the standard deviation for the chela and body length in centimeters for both the male and female crayfish was calculated and determined. The female to male ratios for chela and body length was calculated and a summarized data from the morphological analysis tabulated in the results section.

Results

Table 1. Summarized data from morphological analysis of the chela and body size.

Table 1. Summarized Data from Morphological Analysis

 

Chela Length (cm)

Body Length (cm)*

Ratio†

 

Male

Female

Male

Female

Male

Female

Mean

4.8

3.7

5.5

5.5

0.87

0.67

S.D.

0.734

0.646

0.538

0.593

0.100

0.121

n

40

40

40

40

40

40

Calculated t

     

Critical t

1.661585

1.669402

1.665996

p (one-tail)‡

1.92E-06

0.201969

9.17E-06

* Measured as the length of the thorax from the tip of the rostrum to the abdominal margin of the thoracic carapace

†The ratio of chela length to cephalothorax length
‡ α < 0.05

The results showed that the male crayfish had long chela length of 4.8 cm on average compared to the female chela of 3.7 cm on average. Both the female and male crayfish however showed almost equal length in their body sizes. The female crayfish however demonstrated long tail length compared to their male counterparts. The chelae to body mean ratio for the male and female crayfish was found to be 0.87 to 0.67.

Discussion

The findings of the experiment clearly indicated that the male crayfish have longer chela length than their female counterparts. The chela length was found to be 4.8 cm and 3.7 cm for male and female crayfish respectively. The mean body length for both the male and female crayfish was found to be 5.5 cm and 5.5 cm respectively. This indicated that both female and male crayfish has almost equal body length and this makes it difficult to distinguish the crayfish sexes based on the size of their bodies.
The findings of this experiment clearly supported the hypothesis that male crayfish have longer and larger claws or chela compared to their female partners who have shorter chela or claws. This is an indication that crayfish are sexually dimorphic species. The longer and larger claws in male crayfish are essential as they help the male to firmly hold on the female crayfish during sexual intercourse and reproduction. The longer chelas also cats a source of male dominance and competition. Morphologically, the female crayfish demonstrated longer tails than the male. The longer tails helps the female crayfish to create larger surface area for the attachment of her eggs to enhance prolonged survival of the eggs and the offspring and also to help her protect her sexual organs (Mullen 10).
Other species also demonstrate the concept of sexual selection. The peacock for example have beautiful tail feathers, the deer has beautiful antlers. Human beings also show the concept of sexual selection such as by falling in love by a beautiful lady or a handsome man. Morphologically, men who are tall, dark and handsome with deep voice win most beautiful ladies. So, the result of this experiment is justifiable in the sexual dimorphism of the crayfish (Stein 234).

Works cited

Kawai, Tadashi, and Neil Cumberlidge. A Global Overview of the Conservation of Freshwater Decapod Crustaceans. Cham: Springer, 2016. Internet resource.
Mullen, Dennis M, Michael L. Rutledge, and Sarah H. Swain. "Modeling the Process of Science: Investigating Sexual Dimorphism in Crayfish." Bioscene. 29.1 (2003): 7-13. Print.
Moriyama, Hiroshi. Sexual Dimorphism. , 2013. Internet resource.
Stein, Roy A. "Sexual Dimorphism in Crayfish Chelae: Functional Significance Linked to Reproductive Activities." Canadian Journal of Zoology. 54.2 (1976): 220-227. Print.

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