University of Tasmania

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Gravid spot: a surrogate for predicting progress of embryonic development and reproductive output in a live bearing fish (Gambusia holbrooki)

conference contribution
posted on 2023-05-24, 15:39 authored by Lokman Norazmi, Gary PurserGary Purser, Jawahar PATILJawahar PATIL
Brooding females of livebearing fish can be identified by the presence of the gravid spot; a dark pigmented spot located at a lateral and cranial position to the anal/genital pore. However, its use to predict embryonic development and clutch size remains unexplored. This study quantified visual attributes (intensity and size) of the gravid spot in relation to key features of internal embryonic development and clutch size in Gambusia holbrooki, a pest fish of concern to Australia. Observations show that the colour of the gravid spot arises from progressive melanisation on the surface of the ovarian sac. As predicted, the intensity and size of the gravid spot were closely linked with both developmental stage and clutch size, suggesting their reliable use as external surrogates. This reliability was harnessed to design and document downstream experiments on gestation, parturition and hormonal sex reversal. The gestation period was very sensitive to a small change in temperature—significantly longer (F=364.58; df=1,48; P<0.05) when reared at 23°C (897±43.93 degree days) compared to 25 °C (715±48.5 degree days). However, temperature did not have significant impact (P>0.05) on clutch size or diel timing of parturition which occurs predominantly in the morning (0900-1100h under 16:8h light: dark photoperiod, time on 0600h). The first-ever description on the birth posture of G. holbrooki fry (progenies) during parturition where the tail of the fry emerged first with a few exceptions of head-first, twin and premature births, was also documented. The reproducibility and utility of the relationships imply that they are also relevant to management of wild populations such as for stock assessment, monitoring environmental impacts, and control of pest populations among others. The observed embryonic superfetation and its relevance to reproductive adaptation and management will also be discussed.


Publication title

Australian Society for Fish Biology - Oceania Chrondrichthyan Society Conference abstract book


Institute for Marine and Antarctic Studies

Event title

Australian Society for Fish Biology - Oceania Chrondrichthyan Society Conference

Event Venue

Hobart, Tasmania

Date of Event (Start Date)


Date of Event (End Date)


Repository Status

  • Restricted

Socio-economic Objectives

Control of pests, diseases and exotic species in coastal and estuarine environments

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