Morphology and abundance of Crabs ( Decapoda) in Opi Lake, Enugu State, Nigeria

The morphology, distribution and abundance of fresh water crabs in all the continents including, Africa, Nigeria throughout the tropical regions is very paramount and forms the basis of stock assessment. The morphology and abundance of Crabs in Opi-Aji Lake, Enugu State, Nigeria were studied. Total number of 14 crabs (males) was collected from three randomly selected stations from July- September, 2018, during the peak of rainy season and flooding. All the crabs obtained were mainly Callinectes amnicola (Portunidae). Some of the morphological characteristics observed were; carapace length (CL) which ranged from 1.2-2 cm, dactylus length (2.5-3.7 cm), DL, polex length (1.9-2.7 cm), coxa length, CXL (0.5-1.2 cm), the chelipeds are strong and unequal, frontal margin length, FML (1.2-1.9 cm), orbit length, OL (0.7-1.1 cm), carpus length (1.1-1.8cm). At each station some physico-chemical determinations (surface water temperature, pH, dissolved oxygen, wind directions, turbidity, alkalinity and Total hardness) were carried out. From the three stations (A-C), selected for the crab sampling, station B (Lat. N 6^° 51^¹ 51’’, Long. E7° 23’ 14’’) has the highest percentage of crab abundance (52.19%) followed by station A (Lat. 6° 42^’ 8.298”N, Long. 7° 32^’ 52.536^’’) with 45.65% abundance, and station C (Lat. 6° 42^’ 9.9^’’ N, Long. 7° 32^’ 53.076^’’ E) with zero occurrence/or abundance of Callinectes amnicola. The DO values had a progressive increase in all the stations every month (July- September) while the observed changes are significantly different (P<0.05). The maximum pH values in the month of August (6.35±0.12) did not differ significantly (P>0.05) from the pH values between July and September. The optimum temperature 24.40±0.64°C in Opi-Aji Lake was observed in the month of July. However, the free CO₂ was significant in the month of July and August, while in the month of September, no significant change (P>0.05) was observed in the alkalinity level. Regression analysis revealed the relationship between dissolved oxygen and crab abundance and explained 39.79 % of the variation in crab abundance. Dissolved oxygen showed significant negative correlations with crab across the study months (r = -0.787, p < 0.05) while the Callinectes amnicola which are most abundant in the Opi Aji Lake in the month of July appeared to diminish thereafter. This is an indication that Opi Aji Lake is an important ecological zone for benthos macroinvertebrates conservation.