Report on the spawning of Sargassum fish, Histrio histrio
Article by Binu Varghese, Ph.D., Trivandrum, Kerala, India. Copyright 2006.
Photos by the author
The fishes were collected from the floating/drifting sargassum weeds off Vizhinjam coast (South west coast of India, Near Kovalam Beach) to the marine aquaria and Research Centre of Central Marine Fisheries Research Institute, Vizhinjam. Fishes (6-8) were kept in 5 ton circular FRP quarantine tanks for their forward transition to the exhibit tanks. Two of the fishes were observed to have highly inflated abdomens. The second evening between 1600 -1630 hrs there was a splashing sound in the quarantine tank and upon inspection a transparent gelatinous object was found. It was found floating on the water surface (see photographs below). The egg mass/raft appeared to be double layered with slightly larger middle region and blunt ends at both sides. The length of the egg mass/raft was found to be about 30-35 cm. The egg mass/raft was transferred to a 500 L tank in a flow though system after an hour. Moderate aeration was given initially as the air bubbles started attaching to the gelatinous membranes. The airstone was removed and only slight bubbling near the surface was allowed to give water movement. By the second evening, the gelatinous matrix began to disintegrate. Microscopic analysis of the eggs found them to be undivided/unfertilized and decaying.
Folded egg mass/raft in beaker:
The next week, another spawning was observed in exactly the same manner. This time the egg mass/raft was taken for photographs after giving sufficient time for fertilization (if any). Again, unfortunately, no embryo development was observed.
Size female (mm): 100-120
Food given: Live mysids, small fishes, boiled mussel meat
Tank: Fishes were kept in 5000 liter FRP quarantine tanks
Unfortunately, I couldn’t observe any courtship behaviour or pre-spawning activity. The shear size of the egg mass/raft indicates the need of external pressure to release the massive egg mass. The females were observed in a mature condition along with a group attached to drifting sargassum weeds. The females had round abdomens, apparently in a bursting condition. The female made a splashing sound while releasing the egg mass. In both cases, the egg mass/rafts were found attached to the air tubes just where it touched the water surface. It’s definitely a promising species for reproductive biology studies.
Spawning occurred in the open water column near the surface. The splashing sound heard indicates the release occurred very close to the surface. In both cases the females released the eggs near the surface and the egg masses were found laid over the airline floating on the water surface. This may be a spawning behaviour in this species as in nature the fishes were found among the floating sargassum weeds. This behaviour might be an adaptation for ensuring proper fertilization in this species as their entire habitat is highly mobile.
There is every possibility that fertilization is external. As in both the cases the eggs were found unfertilized and no cell division was noticed. It’s also very unlikely to have internal fertilization in this species as the ova are well protected inside the gelatinous matrix.
There are two possible points where fertilization could occur:
(1) when the ova are still in the matrix and the sperm find their way into it. This is like that of their family members like lion/scorpion (Pterois) where the egg balls are formed, the male fertilizes them and his sperm enters into this matrix. In this case, the lack of egg development may be due to the absence of males/mature males in the group.
Microphotograph of Eggs/Ova:
(2) Another probability is that fertilization occurs after the dissolution of the gelatinous structure. If the fertilization occurs at this time, then timing has to be very essential. In the open ocean, the chances of fertilization are limited if the egg mass is moving as sargassum fishes tend to be slow movers and rarely find it comfortable to move away from the weeds. This also explains the behaviour of surface spawning and the necessity to attach/hang the egg masses on floating objects. The morphology and mobility of the sperm of the species will be of great interest.
Egg mass/raft description
Large (30-35 cm), gelatinous, transparent, apparently double layered structure with numerous eggs egg/ova embedded in the matrix spaces (see photos). The middle portion of the egg mass/structure was distinctly wider than both ends, though the ends were not tapering (some thing like elliptical with blunt ends). I was unable to locate any exterior opening of the egg mass, perhaps because I took too much care not to spoil the eggs.
No development was noticed; the matrix disappeared on the second day. This provided great space for the microbes to develop and the eggs were decayed.