Epactoides giganteus (scarab beetle)

First extinct, then discovered: the scarab beetle Epactoides giganteus

In 1817, the French botanist Nicolas Bréon found himself on Réunion, part of the Mascarene island group, which he would not leave again until 1833 due to health problems. During his time there he was director of the botanical garden Jardin du Roy (today Jardin de l’État), and he undertook numerous expeditions to southeastern Madagascar and to Sainte Marie, an island off Madagascar’s east coast.

We know that Bréon occasionally collected insects on Réunion and sent many of them to the Muséum national d’Histoire naturelle (MNHN) in Paris. It was precisely there that zoologist Michele Rossini in 2021 found a single female beetle from the scarab beetle subfamily Scarabaeinae that was not only larger than other species in the genus Epactoides, but also displayed a whole range of external features that distinguish it from the roughly 40 species belonging to the genus. Rossini recognized that it had to be a new beetle species and gave it the name Epactoides giganteus.

The label attached to the female holotype in the museum collection names Bréon as the collector and Réunion as the place where it was found. That would make it the first beetle of the genus Epactoides not endemic to Madagascar, but to Réunion. With the discovery of E. giganteus, the range of the genus Epactoides would thus suddenly expand—or would it?

In the original description published in the zoological journal ZooKeys in 2021, Rossini and his team discuss the origin of the new scarab beetle species, because specimens collected by Bréon had apparently been mislabeled on more than one occasion. The French entomologist Justin P. M. Macquart was the first to note this circumstance in 1843 in Diptères Exotiques Nouveaux ou peu Connus. He found that some of the flies Bréon had supposedly collected on Réunion and then sent to the MNHN were completely mislabeled. They were in fact species found in Europe. To be fair to Bréon, however, it should be noted that the beetle Gymnogaster buphthalma, which he collected and labeled as originating from Réunion, really was recorded on the island. Not all of the insects collected by Bréon were therefore mislabeled.

Epactoides giganteus – fact sheet

scientific name	Epactoides giganteus
original range	Réunion (Indian Ocean)
time of extinction	unclear, possibly in the 19th century
causes of extinction	unclear

Type locality: Réunion or Madagascar?

Because of what is known about the distribution of beetles in the genus Epactoides and about Bréon’s occasional mislabeling, Rossini concludes that it cannot be ruled out that E. giganteus was collected in Madagascar and then mislabeled. However, Malagasy Scarabaeinae beetles have been intensively studied in the field by several scientists over the past 20 years, and no additional specimens of E. giganteus have turned up. Of course, that could also be because the species has long been extinct. That would not be particularly surprising if one considers that Madagascar has lost around 44% of its native rainforests since 1950. It is therefore not far-fetched that E. giganteus, as a presumed forest-dweller, may have gone extinct in Madagascar long before the scarab beetle studies of recent years.

Scarab beetles (Scarabaeidae) are a family within the order of beetles (Coleoptera) to which many and very different animals belong. The scarab beetle family is in turn made up of several subfamilies; one of these is Scarabaeinae. It includes various rollers and tunnellers, dung beetles, rhinoceros beetles and Epactoides giganteus.

Since Rossini lacks direct evidence of a false label on the holotype of E. giganteus, he decides to regard Réunion as the place of origin of the new beetle species. One reason supporting this, he says, is that this island of the Mascarene archipelago has preserved the most forest habitats to this day. About a third of the island is covered by native vegetation. Rossini considers it possible that Réunion did and does harbor Scarabaeinae scarab beetles. After all, the other Mascarene islands also have or had beetles from the subfamily Scarabaeinae: on Mauritius, five extant species of the genera Nesovinsonia and Nesosisyphus have been described. From Rodrigues we know of no extant species, but of around twelve subfossil Scarabaeinae species discovered there. That proves that the local ecosystem of Rodrigues must once have possessed a rather rich Scarabaeinae fauna.

How E. giganteus got from Madagascar to Réunion

It is therefore possible that E. giganteus is the first endemic beetle from the subfamily Scarabaeinae to have been discovered on Réunion. But how did it reach Réunion, 550 kilometers away, if the genus Epactoides is of Malagasy origin? Rossini suspects that E. giganteus or its ancestors, like other species from Madagascar, reached Réunion through oceanic dispersal. In this process, terrestrial organisms (animals, plants, but also seeds) cross water from one landmass to another. This can happen, for example, by travelling on floating vegetation or tree trunks.

Because the Mascarene island chain, which lies on an underwater plateau, has never been connected to other landmasses since its formation, the origin of all animals and plants in the archipelago can only lie in dispersal across water. According to Rossini, some now-submerged continental fragments also formed in the northern part of the Mascarene Plateau over the last 65 million years. These may have facilitated dispersal from Madagascar to Mauritius, which formed eight to ten million years ago. From Mauritius, living beings then probably reached the geologically younger islands of Réunion and Rodrigues, which formed only two million years ago.

E. giganteus differs from its Malagasy relatives

Beetles of the genus Epactoides differ in appearance: some are uniformly dark, others also have large and symmetrical yellow spots. Some appear shiny, others dull. But all have one thing in common: with a body length of two to five millimeters, they are very small. Not so E. giganteus, which can easily be distinguished from its Malagasy relatives by its body length of nine millimeters.

Rossini, whose first description of the species is based exclusively on a well-preserved female specimen, argues that E. giganteus truly belongs to the genus Epactoides because of its external morphology, which in many respects matches that of other beetles in the genus, such as the oval body and the fact that it is rather flat from back to belly. Despite this, E. giganteus also shows a number of external traits unique within this genus. Rossini mentions, among other things, the clypeus with four teeth, a form that occurs in no other beetle of the genus. And while E. giganteus has large eyes, the other Epactoides beetles tend to have narrow ones.

The phenotypic traits that distinguish E. giganteus from its Malagasy relatives suggest that Bréon really did collect it on the island of Réunion. These may have been special adaptations to the habitat that developed after it reached Réunion from Madagascar. Perhaps its enormous size, which can be more than twice that of the Epactoides species from Madagascar, is also a form of adaptation to the conditions on the smaller island (island gigantism).

Why did Epactoides giganteus go extinct?

Although Réunion is considered the Mascarene island with the largest share of original vegetation today, around 70% of its endemic land vertebrates have gone extinct since the arrival of Europeans in 1665. These include, among others, Bory’s white bat, the small Mauritian flying fox, the lesser yellow bat (Scotophilus borbonicus), the Réunion pigeon, the Réunion ibis, the Réunion night heron (Nycticorax duboisi), the Réunion swamphen and the Réunion giant tortoise. Hardly anything is known about the impact that settlement of the island had on invertebrates and insects in particular. Tropidophora carinata, a land snail that occurred on Mauritius and Réunion, died out at the end of the 19th century—presumably due to habitat loss.

In Lost Land of the Dodo (2008), ornithologists Anthony Cheke and Julian P. Hume illustrate the massive transformation the Mascarene archipelago has undergone since human settlement. According to earlier reports and ecological studies, the three Mascarene islands were completely covered with dense forests at the time of their discovery. Uncontrolled logging, excessive hunting, the accidental and deliberate introduction of foreign animal species and vast numbers of invasive plant species had devastating effects on local flora and fauna. Today, it is above all sugar cane cultivation that is changing the ecosystems.

There are therefore many possible causes that may have led to the disappearance of E. giganteus. Rossini points out that future studies of the scarab beetle subfamily Scarabaeinae on Réunion would be needed to confirm or refute the possibility that E. giganteus is or was endemic there. Various beetle groups have been discovered on the island in recent decades, but no systematic and geographical monitoring aimed at detecting scarab beetles has yet been carried out. During manual collecting campaigns without standardized methods, however, some introduced Scarabaeinae scarab beetles of the genus Onthophagus have already been recorded on Réunion.

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