For the first time, we report the occurrence of a CM species and its competitive success over corals at Lakshadweep atolls. No previous studies had observed the presence and competitive invasion of CMs in any of these atolls even though some of these atolls were previously subjected to thorough surveys. Although the occurrence of other Rhodactis sp. was reported from various reefs in the western Indian Ocean, there are no previous records of R. bryoides from this region. R. bryoides has previously been recorded from the central Indi-Pacific Ocean. First described from the Torres Strait (Haddon & Shackleton 1893), this species remains less known till date, having a few records from the western Pacific Ocean regions such as Australia, Mariana Islands, Coral Sea and the South China Sea off Vietnam (Paulay et al. 2003; GBIF 2019). It is morphologically distinct from the four other morphologically similar congeners (Chen & Miller 1996). In the nearby atolls of Maldives, the presence of an ambiguous Rhodactis sp. (then doubtfully identified as Discosoma cf. rhodostoma) was previously reported and speculated to be causing a phase shift (Zahir et al. 2009).
Various species of corallimorpharia live solitary in deeper waters or as colonies in the shallow waters. Rhodactis bryoides is a colonial corallimorph, and they inhabit shallow waters. Corallimorpharians have a worldwide distribution, with many of the colonial corallimorpharia occurring in the tropics primarily aided by the asexual mode of reproduction (Den Hartog 1980). Longitudinal fission occurring in the larger polyps is the most common form of asexual reproduction (Chen et al. 1995b; Chadwick‐Furman & Spiegel 2000; Chadwick‐Furman et al. 2000). Corallimorpharians also show inverse budding which is achieved when a nodule of the pedal disk rises up and pinch off from the maternal polyp (Chen et al. 1995b). Different modes of asexual reproduction help corallimorpharians to increase their population size. Doubling time of Rhodactis rhodostoma is significantly higher than Scleractinians (Chadwick‐Furman & Spiegel 2000). Corallimorpharians are known to exhibit a higher biomass turnover than Sccleracctinians in less than five weeks (Chadwick 1987). Consequently, they are expected to be more efficient and successful competitors for space on tropical reefs.
We strongly believe that the present occurrence of R. bryoides is a case of invasion in this region. For a non-indigenous species to become invasive, there must be a source, it should survive and thrive in the new place. Whether a species has successfully or unsuccessfully invaded elsewhere can substantiate if it is invasive or not (Mc Neelay 2000; Norström et al. 2009). Evidence from our study indicates that they are actively reproducing within a short time span, successfully establishing a population by effectively outcompeting the dominant coral species. This is apparent from the permanent quadrat data, where an increase of ~25 % of coverage was noticed in a short duration of six months.
However, from the currently available data, it is not possible to elaborate on the triggering factors behind the invasion and the seemingly successful initial level of proliferation of this corallimorph at Kavaratti. We recorded R. bryoides towards the shore of Kavaratti, where the depth of the lagoon is minimal (<1m). The coral colonies at that location were found to be healthy except at the area of the corallimorph invasion. There are studies describing the capacity of CMs to thrive in hostile conditions which give them an advantage over Scleractinians (Muhando et al. 2002). They prefer nutrient-rich areas with low visibility, which helps them outcompete hard corals as the latter requires clear and low-nutrient water (Kuguru et al. 2004). Usually, most Scleractinians have limited success in shallow waters as they are exposed to air and sunlight and wave damage. However, this part of the reef was found to be devoid of any such stressors. The reef at which R. bryoides was located is beyond the level of a neap tide exposure. Further, hardly any anthropogenic disturbances were noticed at the site. Being tropical in location, the reefs of Lakshadweep are prone to bleaching, associated with ENSO and multiple bleaching events had been recorded from 1998 till date (Arthur 2008; Arora et al. 2019). Nevertheless, R. bryoides was observed to be overgrowing live corals rather than growing on the dead ones. Hence, the findings point to a successful invasion of R. bryoides in a healthy reef patch without any apparent facilitation by extrinsic factors, but rather by the inherent biological ability of that species to outcompete the corals.
From the initial observations itself, it was evident that the dominant P. cylindrica is losing the fight for space. Since competition for space is a never-ending process in coral reefs (Muko et al. 2001), the CMs use several life-history traits that allow them to overgrow the shallow reef substrate. They are competitively superior to some coral species and have well-suited anatomy which gives them an edge in the battle for space. Their elongated marginal tentacles help them to kill Scleractinian corals. (Langmead and Chadwick-Furman 1999). Additionally, clonal replication ability of CMs allows them to rapidly overgrow spaces on the reefs (Chadwick-Furman and Spiegel 2000). CMs successfully use their mesenterial filaments and acrospheres as weapons against their adjacent organisms in the competition for space (Chadwick 1987; Langmead & Chadwick-Furman 1999a). Naturally, mesenterial filaments were used in digestion, but when they come in contact with a Scleratnian coral, CMs gather the mesenterial filaments in mouth or body walls and extrude onto the opponent’s body (Chadwick 1987). The mesenterial filaments come in contact with the enemy causing tissue necrosis; prolonged contact may lead to death (Chadwick 1987; Kuguru et al. 2004).
An established invasive corallimorpharia will be very difficult to be expunged, especially from a highly connected ecosystem such as coral reefs. Nevertheless, the extent of damage to the coral reefs it can cause may justify efforts to control further spread and manage established populations. If this situation continues, given this rapid pace of expansion, we can expect that the CMs might be able to destroy the entire reef by overthrowing the coral within a few years. This can further lead to a phase shift in the reef, as noted in other coral reefs (Kuguru et al. 2004; Work et al. 2018; Zahir et al. 2009). Understanding the biology of this species, the interaction with other benthic groups, and the environmental descriptors promoting the invasion is vital in prioritising the management actions. The Lakshadweep atolls' strategic position and their connectivity to many nearby atolls and fringing reefs (Athira 2018) call for the immediate adoption of coordinated and integrated invasive species management strategies.