INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 4, No 1, 2013 Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0 Research article ISSN 0976 4402 Observations on some aspects of biology of webfoot octopus, Octopus membranaceus Quoy and Gaimard, 1832 off Visakhapatnam, east coast of India Department of Marine Living Resources Andhra University, Visakhapatnam-530003, A.P. India raopykmlrau@gmail.com doi: 10.6088/ijes.2013040100002 ABSTRACT The biology of Octopus membranaceus off Visakhapatnam (Lat.17 41' N Long. 83 17'E) was studied. A Scale of four stages of maturity of gonads was identified. The mean length at first maturity was found to be 225 mm total length. Sex ratio indicated that there is a significant difference between males and females in their occurrence. Spawning takes place throughout the year with peak in September. Fecundity varied from 20,432 to 62,324. O. membranaceus was a benthic carnivore, feeding on fishes, shrimps, crabs and squilla in the order of importance. A common regression equation for both the sexes in O. membranaceus was W=9E-5 L2.372 Keywords: Octopus membranaceus, spawning biology, food and feeding habits, length-weight relationship, Visakhapatnam 1. Introduction Octopuses are of the order Octopoda, they inhabits many diverse regions of the oceans and seas, especially coral reefs. Octopuses are characterized by the eight arms, usually bearing suction cups. Unlike most of the cephalopods, the majority of octopuses have almost entirely soft bodies with no internal skeleton. Thirty eight species of octopuses belonging to the family Octopodidae, Tremactopodidae, Argonautidae abound the Indian seas including Andaman and Lakshadweep Seas (Silas, 1985). Octopuses caught mainly as by-catch in the bottom trawl. At present there is no demand for octopuses within the country except in the bait fishery. Due to the growing demand for octopuses in the international market, octopus fishery is gaining importance in the northeast region of India especially along Maharashtra coast (Sujith and Sarang, 2004). There are four species belonging to two genera namely, Octopus (O. aegina, O. dollfusi O. membranaceus) and Cistopus (C. indicus) of family Octopodidae found in the trawl catches at Visakhapatnam fishing harbor (Lat.17 41' N Long. 83 17'E). Among the four species, O. aegina and O. membranaceus were dominant and available throughout the year. These species were identified based on FAO species catalogue (Roper et. al., 1984). In the later revision on octopods (Norman and Hochberg, 2005) the genus Octopus was synonymised by genus Amphioctopus, even though the genus name retained as Octopus in the present study, due to unresolved taxonomy of Octopus membranaceus. The present study deals with some aspects of biology (spawning biology, food and feeding habits and length-weight relationship) of O. membranaceus represented in the trawl catches at Visakhapatnam, east coast of India. Received on July 2013 Published on July 2013 6
2. Materials and methods The present study was based on 107 specimens of Octopus membranaceus (145-475 mm TL and 20-250 g weight) collected from trawl catches at Visakhapatnam fishing harbor (Lat.17 41' N Long. 83 17'E) at regular intervals (twice in a month) from March, 2008 to February, 2009. The collected samples were immediately brought to the laboratory for further analysis. The color and general appearance were noted. The males and females were identified by the right third arm in males hectocotylized with well developed ligula. After measuring total length from tip of the longest arm to posterior most end of mantle (nearest 1mm) and weights were recorded (nearest 1g) of each specimen. The animals were dissected to note color and general appearance of the gonads. The gonads were then carefully removed and preserved in a 5% formalin solution. Estimation of fecundity gravimetrically (Simpson, 1959) was done on intact ovaries of stage III collected during the study period. Small piece of the ovary (egg strings) was taken as sample and weighed up to the milligram level in a Sartorius electronic balance (0.001mg accuracy). The egg strings are placed on a micro-slide and ova were counted under a binocular microscope. The number of ova present in the ovary was calculated using the formula: Weight of the ovary Fecundity = -------------------------------- X No. of eggs present in a sample (1) Weight of the sample Determination of spawning season was based on majority occurrence of mature female octopuses of the population in different parts of the year (West, 1990). The mean length at first maturity (LM) was determined for females by fitting logistic curve (King, 1995). The average length at which 50% of the population attains first maturity was considered as length at first maturity. For the study of food and feeding habits, the stomachs were separated, each stomach was considered as a unit and the stomach contents were first identified qualitatively to nearest taxon possible and their quantity was determine volumetrically. The points gained by each food item in all the stomachs examined were used to calculate the percentage of the different food items (Hynes, 1951). The Length-Weight Relationship (LWR) was derived using exponential hypothetical formula W= al b given by Le Cren (1951). Statistical analysis carried out by Micro Soft Excel. 3. Results 3.1 Spawning biology 3.1.1Maturation Female maturity was categorized into four stages: Immature (Stage I) : The ovary is small and white. Maturing (Stage II) : The ovary is large and the oviducal glands are off-white in color. Mature (Stage III) : Loose oocytes are present in the ovary. Spent (Stage IV) 3.2 Length at first maturity (LM) : The ovary is flaccid with few number of loose oocytes present. 7
The mean length at which 50% of the individuals attained maturity at 225 mm in females (Figure 1). Minimum length of the octopus with ripe stage was observed at 200 mm in the octopus populations off Visakhapatnam. Figure 1: Length at first maturity in females of O. membranaceus. 3.3 Sex ratio The monthly sex ratio of males to females indicated that the dominance of females over males during most part of the study period except June, August and November, 08. The sex ratio for males to females was 1:1.54 (Table 1). Chi-square analysis of the data for the study period indicates that there is a significant difference (p<0.05) between the two sexes in their occurrence. Table 1: Monthly sex ratio for male and female of O. membranaceus at Visakhapatnam Month Total Number Male Female Sex Ratio Male: Female March 08 6 3 3 1 :1 0.2000 0.6547 April 08 20 9 11 1 : 1.22 0.9999 0.3173 May 08 - - - - - - June 08 9 6 3 1 : 0.5 0.2000 0.6547 July 08 5 2 3 1 : 1.5 0.2000 0.1573 August 08 2 2-1 : 0 0.2000 0.6547 September 08 20 9 11 1 : 1.22 0.1428 0.7054 October 08 - - - - - - November 08 7 4 3 1 : 0.75 1.4647 0.2905 December 08 - - - - - - January 09 17 4 13 1 : 3.25 10.7142 0.0010* February 09 21 3 18 1 : 6 4.9439 0.0261* Pooled 107 42 65 1 : 1.54 19.2218 0.0137* *Significant 3.4 Spawning season ᵡ2 p 8
Monthly percentage occurrence of females in different stages of maturity during March 2008 to February 2009 is given in Table 2. The ovaries of maturing and mature (stages II and III) observed throughout the year. The availability of more number of maturing and mature ovaries (stage II and III) in September showed that peak spawning in O. membranaceus. Table 2: Monthly percentage frequency distribution in females of O. membranaceus with different stages of maturity at Visakhapatnam Month Stage of Maturity I II III IV March 08 33.33-66.67 - April 08 36.37 18.18 27.27 18.18 May 08 - - - - June 08-33.33 66.67 - July 08 33.33-66.67 - August 08-50.00 50.00 - September 08-18.18 81.82 - October 08 - - - - November 08-33.33 66.67 - December 08 - - - January 09 69.23 30.77 - - February 09 61.11 38.89 - - 3.5 Fecundity The fecundity of O. membranaceus was estimated and it ranged from 20, 432 to 62,324 with mean 32,569 + 2,693.801 for the specimens collected from coastal waters off Visakhapatnam. The fecundity go linearly with the increase of octopus length, weight and ovary weight when fecundity values were plotted against the respective total length, weight and ovary weight of octopus (Figure 2, 3 & 4). Figure 2: Relationship between fecundity and octopus length in O. membranaceus. 9
Figure 3: Relationship between fecundity and octopus weight in O. membranaceus. Figure 4: Relationship between fecundity and ovary weight in O. membranaceus. 3.6 Food and Feeding habits Regular food items in the order of importance (overall % composition) from March, 2008 to February, 2009 (Figure 5) were fish (15.66%). The fishes encountered in the stomach of O.membranaceus were Leiognathus spp. and Bregmoceros sp. Shrimp (14.97%), consisting of Metapenaeus sp., Solenocera sp. and Acetes sp. Crabs (8.92%) were mainly Charybdis sp. Squilla (1.14%) were mainly Harpiosquilla sp. It indicated that O. membranaceus was a benthic carnivore. Monthly trends in food contents indicated that the fish and shrimp were the dominant food items in almost all months. Variations in the amount of feeding in different length groups showed that fish and shrimp were represented in all length groups. The highest 10
percentage (32.08%) of fish was observed at 400-500 mm length groups, where as in shrimp the highest percentage (17.65%) was noticed in 101-120 mm length groups. Figure 5: Percentage composition of different food items in guts of O. membranaceus 4. Length - Weight Relationship (LWR) The regression equation for the length weight relationship of males, females and sex combined calculated as: Males: W= 0.000419 L 2.1331 Females: W= 1E-5 L 2.6836 Sex combined: W= 9E-5 L 2.3724 The growth coefficient (b) was 2.1331, 2.6836 and 2.3724, where the regression coefficient (r 2 ) was 0.84, 0.80 and 0.82 for males, females and sex combined respectively. The growth coefficient (b) indicated negative allometric growth in O. membranaceus. 5. Discussion Hatanaka (1979) followed a scale of four stages of maturity in females of Octopus vulgaris of the north western Africa. Mangold (1987) described a scale of four stages of maturity namely immature, maturing, mature and spent in the investigations on the biology of octopuses. Yedukondala Rao and Mohana Rao (2010) described a scale of four stages of maturity in Octopus aegina. The present study was also noticed four stages of maturity (immature, maturing, mature and spent) in O. membranaceus. The size at first maturity was determined for females of O. membranaceus at a length of 225 mm TL. According to Hatanaka (1979) the smallest size observed for a mature female of O. vulgaris was 379 mm TL and almost all species larger than 600mmTL were mature in north western coast of Africa. Yedukondala Rao and Mohana Rao (2010) reported the size at first maturity for O. aegina was 275mm TL in the trawl catches off Visakhapatnam. Hatanaka (1979) observed two spawning seasons in O. vulgaris one from May to June and the other in September. Paust (1988) stated that the peak period of egg laying in Alaska is April May in giant octopus Enteroctopus dolfleini. The spawning season in O. vulgaris runs from February to October with peak at April, May 11
and August (Silva et al., 2002). Oosthuizen and Smale (2003) reported that the spawning season of O. vulgaris on the temperate south east coast of South Africa was summer. Katsanevakis and George (2006) reported two spawning peaks in O. vulgaris in the Mediterranean, a main during late winter spring, second one during late summer- early autumn. Yedukondala Rao and Mohana Rao (2010) reported the spawning season of O. aegina during December February at Visakhapatnam. In the present study it appears that O. membranaceus spawns throughout the year. The availability of more number of mature ovaries in September probably shown peak spawning season at trawling grounds off Visakhapatnam. Females were found to dominate the intertidal area (sex ratio 2:1) while no differ found subtidally (sex ratio 1:1) in O. vulgaris on the temperate south east coast of South Africa (Oosthuizen and Smale, 2003). Yedukondala Rao and Mohana Rao (2010), reported that the males were dominated the catches, but chi-square analysis indicated that there is no significant difference (P>0.05) between the two sexes in O. aegina. The present study on O. membranaceus reveals that there is a significant difference (p<0.05) between the two sexes in the catches, sex ratio for male to female was 1:1.54. Paust (1988) estimated the fecundity of giant north Pacific octopus Enteroctopus dolfleini ranged from 20,000 to 1, 00,000 eggs. Mangold (1997) reported fecundity ranged from 1,00,000 eggs to 5,00,000 eggs in O. vulgris. Oosthuizen and Smale (2003) reported the individual fecundity ranged between 42,200 and 7, 90,000 eggs in O. vulgaris on the temperate south eastern coast of South Africa. Ignatius and Srinivasan (2006) estimated the fecundity ranged from 2,962 to 8,820 in individuals of O. aegina at Mandapam, Palk Bay region. Otero et.al (2007) estimated the potential fecundity in O. vulgaris was 2,22,244 ± 1,160.31 oocytes. Yedukondala Rao and Mohana Rao (2010) reported the fecundity ranged between 19,875 and 71,774 eggs in O. aegina. In the present study, fecundity estimated in O. membranaceus ranged from 20,432 to 62,324 eggs, which is more or less similar to O. aegina. Various components of the food spectrum indicated that the O. membranaceus was bottom feeder, feeds on fish and crustaceans. Food items such as fish, shrimp, crab and squilla are actively mobile and are hunted before they are ingested. Octopuses are known to feed on crabs, bivalves and gastropods (Fiorito and Gherardi, 1999). In the present study gastropods and bivalves are not identified in the guts of O. membranaceus, this is mainly due to the specific feeding behaviour of O. membranaceus on mollusks. Hanlon and Messenger (1996) observed polychaetes along with crustaceans, cephalopods and bony fishes, but in the present investigation, polychaetes and cephalopods are not encountered in guts of O. membranaceus. Oosthuizen and Smale (2003) reported octopuses apart from crustaceans and teleosts in O. vulgaris, but in the present study octopuses are not encountered in guts of O. membranaceus. Yedukondala Rao and Mohana Rao (2010) reported fish, shrimp, crab and squilla in guts of O. aegina. Similar food items also reported in the present study. Yedukondala Rao and Mohana Rao (2010) calculated the common regression equation for both the sexes of O. aegina was Log W = 4.07472 + 2.4600 Log L. A common regression equation for both the sexes in O. membranaceus was log W = 9E-5 log L 2.3724 (r 2 = 0.82) in the present study. Acknowledgements The authors are thankful to the Head, Department of Marine Living Resources, Andhra University, Visakhpatnam for providing laboratory facilities. 12
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