Sam Wouthuyzen

Short-Distance Spawning Migration of Tropical Freshwater Eels

The freshwater eels have fascinated biologists because of their spectacular long-distance migrations between their freshwater habitats and their spawning areas far out in the ocean (1, 2, 3). Recent progress on the molecular phylogeny of freshwater eels suggests that they originated in the tropics (4), and information on the reproductive ecology and recruitment of tropical species will provide new insight into the evolution of the spawning migration of the freshwater eels (5, 6). However, the larvae (leptocephali) of the many sympatric tropical species are morphologically similar (7), so they are impossible to identify, and their spawning areas are thus virtually unknown. Recently, however, we have collected small leptocephali from around Sulawesi, Indonesia, and have used species-specific genetic markers to identify them as larvae of Anguilla celebesensis and A. borne-ensis, which provides the first definitive information about the general spawning areas of these tropical eels. Moreover, the discovery of a spawning area of A. celebesensis in Tomini Bay and the presence of small specimens of two species in the Celebes Sea indicate that, in contrast to the long migrations made by temperate eels, tropical eels make much shorter migrations to spawn in areas near their freshwater habitats. This difference in migratory behavior may reflect an evolutionary cline among freshwater eels that extends from tropical to temperate regions.

Seasonality of spawning by tropical anguillid eels around Sulawesi Island, Indonesia

Remarkably little is known about the life histories of the many tropical anguillid eels distributed across the Indo-Pacific region, and since the Danish expedition to study eels in the region in 1928 and 1929, research on these eels has only begun again in recent years. Sampling for anguillid leptocephali in the Indonesian Seas has been carried out recently to learn about the spawning ecology and larval distributions of tropical eels there. The leptocephali of Anguilla marmorata, Anguilla bicolor pacifica, Anguilla borneensis, Anguilla interioris, and Anguilla celebesensis were collected around Sulawesi Island both in May 2001 and October of 2002. The development of genetic identification techniques has enabled these leptocephali to be identified to species, and their distributions and sizes during different seasons indicated that there are differing life history patterns among sympatric species in the region. A. celebesensis was found to have been spawning in Tomini Bay of northeastern Sulawesi Island in March and April 2001, but apparently, no spawning had occurred in the late summer and fall of 2002. Studies on anguillid glass eels have suggested that tropical anguillids may spawn throughout much of year, but our research on leptocephali in Tomini Bay and data on the downstream migration of tropical anguillids in the major tributary to Tomini Bay indicate that A. celebesensis may have a distinct seasonal pattern of spawning possibly related to the regional monsoon cycles. This is the first evidence of seasonality of spawning in tropical anguillid eels whose life histories are only just beginning to be revealed.

Ariosoma-type Leptocephali (Congridae: Bathymyrinae) in the Mentawai Islands region off western Sumatra, Indonesia

BackgroundAriosoma-type leptocephali of the congrid subfamily Bathymyrinae are typically present in tropical and subtropical regions where they reach large sizes and are often abundant compared to other taxa. Different morphological species types of these larvae have been documented in the Indo-Pacific region, but few have been matched with their adult species, and their life histories are not known.ResultsA sampling survey for leptocephali off western Sumatra, Indonesia, collected 297 Ariosoma-type leptocephali of at least 12 different species of both the exterillium gut and non-exterillium gut types that could be distinguished using morphological characteristics. These leptocephali were collected at 23 of 24 stations over deep water but were not caught at two stations over the shelf between Sumatra and Java. They were most abundant in the 15- to 50-mm size range, but six species were also collected with sizes of > 140 mm, some of which were undergoing metamorphosis at sizes of 143 to 324 mm. Larvae of Ariosoma scheelei were most abundant, with small larvae of ≤ 25 mm present near Sumatra and the Mentawai Islands and larger larvae also present offshore. The other abundant larval types, sp. 1 and Ariosoma sp. 7 showed a different tendency with their small larvae being present more offshore, which suggests different spawning locations possibly linked to differing adult habitats or spawning behaviors.ConclusionsThe presence of at least moderately small leptocephali of most of these species suggests that their adults are present in this region, which indicates that there is a high biodiversity of these small benthic eels on the shelf and slope of the Mentawai Islands region off western Sumatra in the eastern Indian Ocean.

Leptocephalus larvae of two moray eels (Anguilliformes; Muraenidae), Gymnothorax sagmacephalus and Gymnothorax albimarginatus, identified from morphometric and genetic evidence

Two forms of muraenid leptocephali, collected from the western Pacific Ocean, were identified as Gymnothorax sagmacephalus Böhlke 1997 and Gymnothorax albimarginatus (Temminck and Schlegel 1846) on the basis of morphometric and genetic analyses. The leptocephali of each species were characterized, respectively, by counts of 172–175 and 186–191 myomeres, 43–44 and 47 predorsal myomeres, 109–113 and 127–134 preanal myomeres, and 100–104 and 118–119 last vertical blood vessel myomeres. Gymnothorax sagmacephalus leptocephali had minute melanophores over much of the head and body, closely resembling the condition in Gymnothorax minor (Temminck and Schlegel 1846), whereas those of G. albimarginatus not only had minute melanophores over much of the head and body, but also a pair of melanophore groups on the posteroventral and posterodorsal aspects of the head. Such groups are here considered to represent highly specific characters. Although a previous opinion postulated that G. sagmacephalus is a juvenile of G. albimarginatus, and the adult morphologies of the two species have a lot in common, they clearly differ in both leptocephalus morphology and genetic sequence. Therefore, G. sagmacephalus was concluded as being a valid species.

A comparison between the 2010 and 2016 El-Ninō induced coral bleaching in the Indonesian waters

Severe coral bleaching events are always associated with El-Ninō phenomenon which caused a rise in ocean temperature between 1-2°C and that they potentially kill the corals worldwide. There were at least four severe coral bleaching events occurred in the Indonesian waters. This study aims to compare the coral bleaching events of the 2010 and 2016 and their impact on corals in Indonesian waters. Long-term (2002-2017) remotely sensed night time sea surface temperature (SST) data acquired from Aqua MODIS Satellite were used in the analysis. Here, we calculated the mean monthly maximum (MMM)of SST as SST in normal condition in which coral can adapt to temperature; the differences between high SST in each pixel during coral bleaching events of the 2010/2016 and MMM SST, called hot spot (HS); and how long has HS occupied a certain water body, called degree of heating weeks (DHW, °C-week) and then mapped it. Results show that the MMM SST for the Indonesian waters is 29.1°C. Both bleaching events of 2010 and 2016 started and finished in the same periods of Mar-Jun and they nearly have the same pattern, but bleaching magnitude of the 2016 was stronger than 2010 with the mean SST about 0.4°C higher in May-June. The percentage of impacted areas of strong thermal stress on corals of Alert-1 plus Alert-2 status was higher in 2016 (39.4%) compared to 2010 (31.3%). Coral bleaching events in the 2010 and 2016 spread in almost all Indonesian waters and relatively occurred in the same places but with small variation in the bleaching sites that was caused by the strength/weakness of El-Ninō and upwelling phenomenon as well as the role of Indonesian through flow (ITF).