Laboratory Growth, Reproduction and Life Span of the Pacific Pygmy Octopus, Octopus digueti'

Octopus digueti Perrier and Rochebrune, 1894 was reared through its life cycle at 25°C in a closed seawater system using artificial sea water. Two field-collected females produced 231 hatchlings: 193 hatchlings were groupcultured while 24 were isolated at hatching and grown individually to allow precise analyses of growth in length and weight over the life cycle. All octopuses were fed primarily live shrimps. Maturing adults fed at a rate of 4.7% of body weight per day and had a gross growth efficiency of 48% . Growth in weight was exponential for the first 72 days and described best by the equation: WW(g) = .0405e·0646t. The mean growth rate over this period was 6.4% increase in body weight per day (% jd) , with no significant difference between male and female growth. From 72 to 143 days , growth was logarithmic and described best by the equation: WW(g) = (6.78 x 1O6) t3 . 13 . Females grew slightly faster than males over this growth phase. During the exponential growth phase, mantle length increased at a mean rate of 2.1% per day, declining to 1.1% per day over the logarithmic phase. No attempt was made to describe mathematically the period of declining growth rate beyond day 143. The primary causes of early mortality in group culture were escapes and cannibalism. Survival was good despite high culture density: 73% survival to date of first egg laying (day Il l). Survival was better among the isolated growth-study octopuses: 88% to the date of first egg laying (day 130). Mean life span was 199 days in group-reared octopuses and 221 days in the growth-study octopuses. There was no significant difference between male and female life span. Progeny of the group culture were reared at similar stocking densities and fed predominantly fresh dead shrimp and crab meat. This diet resulted in cannibalism, with only 6% survival to first egg laying on day 128. Fecundity in this group was lower. Octopus digueti is a good candidate for laboratory culture and biological experimentation because of its small size, rapid growth, short life span, and good survival in group culture. O CTOPUS DIGUETI PERRIER AND R OCHEBRUN E, 1894 is a small, benthic octopod inhabiting tidal flats in the northern Gulf of California (lat. 31°N, long. 114°W). Octopus digueti (Figure I) shares several morphological and ecological characteristics with the Atlantic pygmy octopus, Octopusjoubini Robson, 1929, and the Pacific blue-ringed octopus, Hapalochlaena maculosa Robson, 1929: similar adult size (20 to 60 g), large egg size (8 to 9 mm, Figure 1), short life cycle (5 to 8 months at 25°C), and the habit of living within empty mollusc shells (Tranter and Augustine, 1973; Hanlon, 1983a; Hanlon and Forsythe, 1985). To date there is no published information on the biology and life history of Octopus digueti . We present here basic life cycle information that will contribute to ecological studies and to the evaluation of this species for use in laboratory culture and biological studies. MATERIALS AND METHODS The closed culture systems and methodology are described in detail in Forsythe and 104 t Manuscript accepted February 1987. 2The Marine Biomedical Institute, The University of Texas Medical Branch, 200 University Boulevard, Galveston, Texas 77550-2772. The Pacific Pygmy Octopus-DERuSHA, FORSYTHE, AND HANLON 105 FIGURE I. A. Embryos of O. digueti at three stages of development (25C): freshly laid, 3 to 4 weeks and ventral view at 5 to 6 weeks (immediately prior to the second reversal) . B. An adult female O. digueti (ML approximately 5ern). Hanlon (1980) and Hanlon and Forsythe (1985). Briefly, the culture system consisted of a 2 m diameter circular fiberglass waterconditioning tank, 75 ern deep, of approximately 2000 I capacity and was the site of all water-conditioning processes. Two fiberglass culture troughs, 2.4 m x 68 em x 30 cm, were supported on the upper rim of the waterconditioning tank and received freshly filtered artificial seawater (Instant Ocean®) continuously. All octopuses were cultured within these troughs. Two female octopuses inhabiting mollusc shells (Pecten sp . and Muricanthus sp.) were collected in the northern Gulf of California near Puerto Penasco, Sonora, Mexico. They were transported to the University of Arizona (Tucson), where they laid eggs in aquaria prior to shipment to Galveston. The brooding females were packed in separate plastic bags containing approximately 31 of sea water and an equal volume of pure oxygen, then shipped by air to Galveston. Upon arrival they were placed directly into a culture system with the same temperature and salinity. A culture of O. joubini was in progress in the system at that time . The two brooding females and their eggs were kept in separate plexiglass chambers (19 em x 14 em x 10 em) with nylon screen sides (800 j1m), which allowed water circulation but prevented escape of hatchling octopuses. With the onset of hatching, hatchlings were removed from the chambers daily, counted and placed into shallow (water depth, 3-4 em) hatchling trays (50 em x 40 cm) within the main group culture troughs. One hundred ninety-three octopuses were placed into the group culture over a 23-day period. This group-reared population was provided with increased horizontal space as animal size increased, the first occurring on day 21 when half of the population was placed into a second hatchling tray. Twenty-four hatchlings less than 15 hours old (nine from brood I, 15 from brood 2) were isolated in individual growth chambers to allow precise measurements for a growth study through the full life cycle. The sides of the tightly lidded chambers (7 em x 5 em) were replaced with fine nylon screen. The chambers were supported in the culture trough and supplied with strong water circulation. The octopuses were given larger dens and growth chambers as they grew. Hatchlings in both the group culture and growth study were fed ad libitum on live mysidacean shrimps. As the octopuses grew, they were provided larger live foods , progressing from mysidacean shrimp (3-10 mm) to palaemonid grass shrimp (7-25 mm) to penaeid shrimp (2-6 ern). Crabs, fishes, and gastropods of appropriate size supplemented the shrimp diet. Beginning at hatching, each animal in the isolated growth study was measured every 14