Background

Richness or the number of species is, perhaps, the main attribute of biological diversity. There have been many efforts for the estimation of species richness, both for terrestrial and aquatic organisms and for plants and animals (Appeltans et al. 2012; Bouchet 2006; Foggo et al. 2003; Miloslavich et al. 2011; Mora et al. 2011). There are some groups of animals that are believed to have reached or are reaching their estimated number of species; however, most are far from having complete inventories (Bouchet 2006). New species to science and/or new geographic records of known species are reported often (e.g. Valencia-Giraldo et al., 2015). Echinoderms in general, and sea-urchins in particular, are included within the groups that are “not-well-known” (Bouchet 2006), and in regions where there is a paucity in studies and scarcity of experts, research efforts can easily lead to new records (e.g. Muñoz and Londoño-Cruz 2016) or the discovery of new species.

There are 79 sea-urchin species reported so far in Colombia (Benavides-Serrato et al. 2013; Muñoz and Londoño-Cruz 2016), of which 63.3% are found in the Caribbean and 36.7% in the Pacific. The family Diadematidae, a large and important family of sea-urchins, has only three representatives in the Pacific coast of Colombia [Astropyga pulvinata (Lamarck, 1816), Centrostephanus coronatus (Verrill, 1867) and Diadema mexicanum A. Agassiz, 1863]. Of these, A. pulvinata is rather rare, while C. coronatus and D. mexicanum are fairly common in coral reefs and less so in rocky reefs. The specimens of C. coronatus typically have white and black banded spines, a characteristic shared by other Diadematidae species (e.g. Echinothrix calamaris Pallas, 1774). The latter species, geographically restricted to the western Indian Ocean and Red Sea, has been reported only in one location in the entire Eastern Pacific: Cocos Island (Lessios et al. 1998), but recently we found some specimens of this species at Gorgona Island. Therefore, this paper adds a second locality for the Eastern Pacific and constitutes the first record of this species in Colombia.

Materials and methods

Three Echinothrix calamaris (EC) specimens were collected at Gorgona Island (3°00′55"N-78°14′30"W) on fringing coral reefs. One specimen (EC1) was found at La Azufrada reef (July 29, 2015) and two (EC2 and EC3) at Playa Blanca reef (April 19, 2016) at 2 and 3 m deep, respectively. The specimens were found unintentionally while conducting censuses of sea urchins. Once collected, the specimens were fixed in formalin (5%), and deposited in the Echinoderm Collection of Universidad del Valle (CRBMeq-UV), Cali, Colombia. These specimens are the first to be collected in the Colombian Pacific, and were identified using Coppard and Campbell (2006).

Results and discussion

Taxonomy

Order Diadematoida Duncan, 1889

Family Diadematidae Gray, 1855

Genus Echinothrix Peters, 1853

Species Echinothrix calamaris Pallas, 1774

We followed the identification key in Coppard and Campbell (2006) for the correct identification of the specimens, which showed the two distinctive arrangements and types of spines: Ambulacral (long, slender and hollow) and interambulacral (long, slender, and significantly larger and thicker than ambulacral spines) (Fig. 1). According to the ambulacral elevation and the form of the genital plates, our specimens match the brown color morph. In addition, an expert, Dr. Gordon Hendler, curator of echinoderms at the Natural History Museum of Los Angeles County, confirmed our observations with photographs of the collected specimens taken when still alive.

Fig. 1
figure 1

Side (a) and top (b) views of Echinothrix calamaris at La Azufrada (Gorgona Island, TEP)

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Measurements and museum vouchers (within parenthesis) of collected specimen are as follows: EC1 – test diameter (t.d.) 40.41 mm, test height (t.h.) 19.58 mm (CRBMeq-UV: 2015–001); EC2 – t.d. = 31.88 mm, t.h. = 12.77 mm (CRBMeq-UV: 2016–001); and EC3 – t.d. = 25.64 mm, t.h. = 11.07 mm (CRBMeq-UV: 2016–001). These measurements are very small in comparison to the ones reported by Coppard and Campbell (2006). There are possibly two explanations for this difference: 1) our specimens were still juveniles, while the data reported by the previous authors might belong to adult specimens; or 2) we have observed that echinoids from Gorgona Island tend to have a consistent smaller size than reports elsewhere, e.g. Diadema mexicanum has a mean size of 19.62 mm, with a maximum size of 41.50 mm, while the mean size reported by Coppard and Campbell (2006) for this species is 75 mm with a maximum of 92 mm; or both.

In the World Register of Marine Species (WoRMS), Kroh (2013) reports the documented distribution of E. calamaris as encompassing the Maldives (Laamu Atoll, Malé), Red Sea (Sewul) and South Andaman Islands; other localities are also included (Fig. 2), but they are not confirmed. These localities are all located in the Indian Ocean and Red Sea regions. Only once has this species been reported from a locality outside the Indo-Pacific: Cocos Island, in the Tropical Eastern Pacific (TEP; Lessios et al. 1996, Alvarado and Cortés, 2009). These authors argue that the warm phase of El Niño Southern Oscillation could favor the dispersal and settlement of this species, and considering this recent discovery, we believe this could be a plausible explanation for its occurrence at Gorgona Island. However, considering the extent of the Pacific Ocean and the vast expanse of open ocean devoid of islands that might serve as stepping stones (Gilpin 1980) between the Central Pacific islands and the TEP (known as the East Pacific Barrier or Filter; Briggs 1961), it may be necessary to consider other means of dispersal (e.g. ballast water, rafting) besides larval transport by currents as the mechanisms that could explain the presence of E. calamaris in the TEP. Given that this is the first record of this species in the Colombian Pacific and that it is the second confirmed record in the TEP, it is possible that E. calamaris is successfully extending its distribution to the entire tropical Pacific Basin, although the specific mechanism of colonization is not yet fully understood. It will also be necessary to monitor this species at Gorgona Island and other localities to unambiguously confirm the establishment of viable populations in the Colombian Pacific.

Fig. 2
figure 2

World distribution of E. calamaris according to the information in Kroh (2013). Details of the distribution in the Pacific (a) and Indian (b) oceans are presented. The new record of this species at Gorgona Island coral reefs (TEP) is highlighted (c)

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Conclusions

We report for the first time the presence of E. calamaris on coral reefs of Gorgona Island. On one hand, this increases the known species richness of Echinoids in the Pacific coast of Colombia (and South America), and on the other hand, it confirms the range extension of this species from Indian Ocean to the TEP.