Vagrancy in paradise: documentation of the chevron butterflyfish Chaetodon trifascialis in Kaneohe Bay, Oahu, Hawaiian islands

Abstract

The chevron butterflyfish Chaetodon trifascialis is an obligate corallivore found on Indo-Pacific coral reefs. The typical northern extent of the distributional range of the chevron butterflyfish overlaps with the occurrence of one of its preferred corals Acropora cytherea on reefs of French Frigate Shoals in the Northwestern Hawaiian Islands. Here I document the occurrence of a single C. trifascialis in Kaneohe Bay, Oahu outside its typical range and discuss the role of biogeographic vagrants as potential colonizers that provide direct evidence of inter-island larval connectivity in the Central Pacific Ocean.

Background

The chevron butterflyfish Chaetodon trifascialis Quoy & Gaimard, 1825 is an obligate corallivore found on Indo-Pacific coral reefs. The species exhibits a strong ecological relationship with corals of the genus Acropora, in particular A. hyacinthus, and A. cytherea (Berumen & Pratchett, 2008; Berumen et al., 2012; Lawton et al., 2012; Pratchett et al., 2013; Pratchett et al., 2006). Typically, harems of C. trifascialis occupy a home range and feed primarily on Acropora corals (Yabuta & Berumen, 2013). Due to the specialist diet on corals, C. trifascialis has been suggested as an indicator species for the condition of coral reefs (Reese, 1981; Ohman et al., 1998). Globally, C. trifascialis is in “near threatened” status by the IUCN due to the susceptibility of corals to bleaching events (Carpenter & Pratchett, 2010). For example, the C. trifascialis was not observed from reefs that experienced massive bleaching events in follow-up surveys at sites where they had been previously documented (Pratchett et al., 2006). While C. trifascialis is distributed throughout the Indo-Pacific, it only occurs commonly in one location of the Hawaiian archipelago, French Frigate Shoals, where Acropora cytherea is well established (Asher et al., 2012; Randall, 2007; Grigg, 1981).

This study documents the occurrence of a single C. trifascialis in Kaneohe Bay, Oahu, Hawaiian Islands and discusses the role of biogeographic vagrants in this region as potential colonizers that demonstrate inter-island larval connectivity.

Methods

One specimen of C. trifascialis was observed and photographed during a dive on September 4, 2013 in a coral patch reef at 21°27′5.04″N and 157°47′25.537″W in Kaneohe Bay, Oahu, Hawaiian Islands (Fig. 1). The specimen was not collected because the author did not possess a Hawaii state collection permit for that particular species at the time of observation. The site was characterized by a stand of Pocillopora meandrina corals in the back reef of Kaneohe Bay near Sampan Channel.

Fig. 1

Map of the island of Oahu in the Hawaiian Islands. Red circle indicates the position of the observed specimen of Chaetodon trifascialis in Kaneohe Bay

Results

One specimen of C. trifascialis was observed and photographed within the branches of a Pocillopora meandrina coral colony at 4 m depth (Fig. 2). The fish was visually estimated as 5 cm TL and not collected. The coral colony was located in back reef habitat with isolated coral patches among surrounding extents of sand-bottom in Kaneohe Bay. No additional specimens of C. trifascialis were encountered during four dives (total time 2 h) in the general vicinity of the initial observation.

Fig. 2

Photograph of a specimen of Chaetodon trifascialis observed within the branches of Pocillopora meandrina in Kaneohe Bay, Oahu, HI

Discussion

The northern range of Chaetodon trifascialis has been observed on reefs of French Frigate Shoals in the Northwestern Hawaiian Islands, yet documented evidence of specimens has not been recorded elsewhere in the Hawaiian archipelago. For example, a regional-scale reef fish monitoring survey program of the Hawaiian Islands by NOAA from 2000 to 2016 did not observe a single specimen of C. trifascialis in the MHI (I. Williams pers. comm.). Prior anecdotes of C. trifascialis on Oahu reefs were described by Reese (Reese, 1981) but not supported with primary evidence, so this record represents the first documented observation of the species in the main Hawaiian Islands. Other observations of C. trifascialis on Oahu reefs were made in Kaneohe Bay in 1983 (R. Kosaki pers. comm.) and Hanauma Bay in the late 1980’s or early 1990’s (B. Mundy pers. comm.). Given known larval connectivity pathways, the specimen of C. trifascialis was most likely a vagrant species from Johnston Atoll (Kobayashi, 2006; Wren et al., 2016).

Vagrant butterflyfish species are not uncommon. A review of geographical distribution shifts in 52 fish families found that butterflyfish (family: Chaetodontidae) had a significantly higher proportion of vagrants than expected by species richness given each family’s total number of species (Feary & Pratchett, 2014). For example, in the Azores and Madeira Islands off the western African coast, only vagrant butterflyfish species have been observed and there are no known permanent populations (Kulbicki et al., 2013). The vagrant species observed in the Azores, Chaetodon sedentarius, is commonly distributed in the western Atlantic and Caribbean region (Allen et al., 2010). Two butterflyfishes, C. lunula (Pyle et al., 2010) and C. unimaculatus (Myers & Pratchett, 2010a), are Indo-Pacfic species that are biogeographic vagrants in the Galapagos Islands. Furthermore, a single individual of the black butterflyfish C. flavirostris has been observed as a vagrant species at Easter Island (Myers & Pratchett, 2010b). These examples are not comprehensive but provide a sample to demonstrate the relative commonness of butterflyfish vagrant species.

In the Hawaiian archipelago, larval connectivity from Johnston Atoll represents the most likely source for periodic colonization of reef fauna, such as butterflyfish (family: Chaetodontidae) and stony corals of family Acroporidae (Kobayashi, 2006; Wren et al., 2016). The overlap in butterflyfish species composition between the Hawaiian archipelago and Johnston Atoll is nearly complete, with the exception of C. fremblii which has not been observed at Johnston Atoll (Randall, 2007; Kosaki et al., 1991; Randall et al., 1985; Wagner et al., 2014). The lined butterflyfish C. lineolatus has been rarely observed at Johnston Atoll (Randall et al., 1985), so could be described as a biogeographic vagrant there. The Acropora corals that are the primary prey for C. trifascialis are commonly found at French Frigate Shoals in the Northwestern Hawaiian Islands but have been very rarely sighted in the main Hawaiian Islands (Asher et al., 2012; Grigg et al., 1981). Isolated Acropora cytherea colonies have been recorded off the islands of Kauai (Grigg et al., 1981; Maragos, 1977; Kenyon et al., 2007) and Oahu (Kosaki et al., 2013) but attempted resightings for these colonies have failed. While most singleton species arrivals to the Hawaiian Islands probably perish, several large colonies of Acropora gemmifera were observed in April 2013 on the Kona coast of Hawaii Island (Walsh et al., 2013). The size of the A. gemmifera corals suggested that they had been present for decades in the area. Grigg (Grigg, 1981) hypothesized that Acropora were in the process of post-Pleistocene recolonization from larval dispersal outside of the archipelago. Given the obligate feeding relationship of C. trifascialis on Acropora corals, a successful colonization of C. trifascialis would require the pre-establishment of Acropora to settlement sites in the main Hawaiian Islands. The probability of this occurrence is low but not impossible especially if range expansions of these Acropora species to the MHI are facilitated by the predicted increases of sea surface temperatures under changing climates (Baird et al., 2012; Yamano et al., 2011). Under future warming conditions with enough larval supply and survival, the successful colonization and establishment of both Acropora species and C. trifascialis from Johnston Atoll to sites in the Main Hawaiian Islands may be possible as a continued recolonization of the Hawaiian archipelgo (Grigg, 1981; Kobayashi, 2006; Maragos & Jokiel, 1986).