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Observations of natural spawning events are rare but critically important for the field of fertilization ecology. For broadcast spawners, knowing the natural behavior of spawning, including proximity of animals and the timing of spawning, is essential for informing spawning experiments and directing future study. Here we describe a natural spawning event that took place on the island of Maui in the winter of 2010. Groups of Tripnuestes gratilla were observed to spawn in the late afternoon shortly after a local high tide that was a lower high of a mixed, semidiurnal tide cycle. Urchins appeared to increase the probability of fertilization by ascending to the local high points of the reefs and spawning within groups of 2–5. Although the majority of the urchins that spawned were T. gratilla, two individuals of Echinothrix diadema were observed to spawn at the same time, raising questions about potential hybridization in the wild.

Gonad indices for two species of Diadema and two species of Echinothrix, including two color morphs of Echinothrix calamaris, are described for a 12-month period on Sosoikula Reef and Nukubuco Reef, Viti Levu, Fiji. Seasonal fluctuations in salinity (36.11±0.88 ppt) and water temperature (26.35±0.91°C) occurred. Gonad index data showed monthly reproductive rhythms, closely attuned to the lunar cycle. Diadema savignyi and Echinothrix diadema spawned on the full moon, and Diadema setosum and E. calamaris (white and brown color morphs) spawned on the new moon. Breeding periodicities coincided with the spring tides, thus maximizing chances of fertilization and dispersal. Such breeding cycles indicate how closely related species can co-exist with minimum risk of hybridization. Unusual sex ratios were found for all species, with an exceptionally low incidence of males. Reasons for this deviation from the typically reported 1:1 sex ratio may relate to the exceptionally high levels of tributyltin (TBT) recorded in Suva Harbor.[PUBLICATION ABSTRACT]

The 'Eastern Pacific Barrier' (EPB), 5400 km of uninterrupted deep water between the central and eastern Pacific, constitutes the greatest marine obstacle to the dispersal of shallow-water organisms. However, some species are found on both sides of the EPB. These 'transpacific' species are considered by 'dispersal' biogeographers as evidence of invasions through the barrier. 'Vicariance' biogeographers, on the other hand, think that transpacific species are morphologically conservative remnants of previously continuous distributions. We compared nucleotide sequences in a 642 bp region of mitochondrial DNA, and electrophoretically detected alleles in 17 enzymatic loci of central and eastern Pacific populations of Echinothrix diadema, an Indo-Pacific sea urchin recently reported from the eastern Pacific. Both types of molecules produced clear evidence of massive, recent gene flow across the EPB. Thus, rather than being isolated relicts of Tethyan distributions, conspecific populations from the eastern and central Pacific are genetically connected. Though the EPB is biogeographically important as a cause of speciation in many groups, it allows genetic connections in others, possibly through larval transport during El Niño events.

Echinoid populations can be major grazers on hard reef substrata and are consequently of importance in the bioerosion of coral reefs. There are no previous data on rates or patterns of bioerosion by the echinoid community in Indo-Pacific reefs. We surveyed densities of echinoid populations in 2 reef lagoons, the Tiahura section, northwest Moorea (Society Islands) and the central lagoon of Takapoto atoll (Tuamotu Islands), and determined echinoid erosion rate of the reef structure. Echinometra mathaei was the most numerous urchin in Tiahura (7.4 m⁻²) followed by Diadema savignyi (4.8 m⁻²), Echinothrix diadema (0.6 m⁻²) and Echinostrephus molaris (0.2 m⁻²). E. mathaei and E. molaris are small urchins processing small amounts of carbonate rock and the impact of their populations as bioeroders is limited at such densities, even for the numerous E. mathaei (1.0 g m⁻² d⁻¹). D. savignyi and E. diadema are large (mode test diameter 55 and 60 mm respectively) and their populations consumed 9.2 and 2.2 g m⁻² d⁻¹ of hard substratum respectively. In Takapoto lagoon, the same species were present (except E. molaris), but densities of urchins were extremely low and echinoid bioerosion was only of importance close to lagoonal inlets (6.2 g m⁻² d⁻¹). In situ growth of the hard reef substratum in Moorea is predominantly through growth of massive Porites colonies and estimated at 6 g m⁻² d⁻¹. The conclusion is that, with an echinoid bioerosion rate of 12.5 g m⁻² d⁻¹ the reef structure is being destroyed. The observed urchin densities are high for oceanic islands of the Indo-Pacific region and the possibility of a relation between human influence on the reef and urchin densities should be considered.

Among extant crinoids, the feather stars are the most diverse and occupy the greatest bathymetric range, being especially common in reef environments. Feather stars possess a variety of morphological, behavioral and physiological traits that have been hypothesized to be critical to their success, especially in their ability to cope with predation. However, knowledge of their predators is exceptionally scant, consisting primarily of circumstantial evidence of attacks by fishes. In this study the question whether regular echinoids, recently shown to consume stalked crinoids, also consume feather stars is explored. Aquarium observations indicate that regular echinoids find feather stars palatable, including feather stars known to be distasteful to fish, and that regular echinoids can capture and eat live feather stars, including those known to swim. Gut-content analyses of the echinoid Araeosoma fenestratum (Thomson, 1872), which is commonly observed with large populations of the feather star Koehlermetra porrecta (Carpenter, 1888) in video transects from marine canyons off the coast of France, revealed elements of feather stars in the guts of 6 of 13 individuals. The high proportion of crinoid material (up to 90%), and the presence of articulated crinoid skeletal elements in the gut of A. fenestratum, suggest that these echinoids consumed at least some live crinoids, although they may have also ingested some postmortem remains found in the sediment. Additionally, photographic evidence from the northeast Atlantic suggests that another regular echinoid, Cidaris cidaris (Linnaeus, 1758), preys on feather stars. Thus in spite of the broad suite of antipredatory adaptations, feather stars are today subject to predation by regular echinoids and may have been since the Mesozoic, when this group of crinoids first appeared.

The sea urchin cardinalfish, Siphamia tubifer (Perciformes: Apogonidae), is unusual among coral reef fishes for its use of bioluminescence, produced by symbiotic bacteria, while foraging at night. As a foundation for understanding the relationship between the symbiosis and the ecology of the fish, this study examined the diel behavior, host urchin preference, site fidelity, and homing of S. tubifer in June and July of 2012 and 2013 at reefs near Sesoko Island, Okinawa, Japan (26°38′N, 127°52′E). After foraging, S. tubifer aggregated in groups among the spines of the longspine sea urchin, Diadema setosum, and the banded sea urchin, Echinothrix calamaris. A preference for D. setosum was evident (P < 0.001), especially by larger individuals (>25 mm standard length, P < 0.01), and choice experiments demonstrated the ability of S. tubifer to recognize and orient to a host urchin and to conspecifics. Tagging studies revealed that S. tubifer exhibits daily fidelity to a host urchin; 43–50 and 26–37 % of tagged individuals were associated with the same urchin after 3 and 7 days. Tagged fish also returned to their site of origin after displacement; by day two, 23–43 and 27–33 % of tagged individuals returned from displacement distances of 1 and 2 km. These results suggest that S. tubifer uses various environmental cues for homing and site fidelity; similar behaviors and cues might be used by larvae for recruitment to settlement sites and for the acquisition of luminous symbiotic bacteria.

Predators are important for regulating adult sea urchin densities. Here, we employ remote underwater video cameras to record diurnal predation on tethered sea urchins at Lizard Island on the Great Barrier Reef (GBR). We identified four fish predators of adult sea urchins ( Balistoides viridescens , Balistapus undulatus , Lethrinus atkinsoni and Choerodon schoenleinii ). Predator activity appeared to be site-specific. Balistoides viridescens and B. undulatus (f: Balistidae) were the two most important predators of Echinometra mathaei with the former handling E. mathaei significantly faster (mean 0.7 min) than B. undulatus (5.2 min). Balistoides viridescens also successfully preyed on 70 % of detections, while C. schoenleinii , B. undulatus and L. atkinsoni preyed on just 33, 17 and <1 %, respectively. Additionally, B. viridescens were behaviourally dominant among predator species and were observed as aggressors in 30 encounters with B. undulatus and 8 encounters with L. atkinsoni . In only one encounter was B. viridescens the recipient of any aggression (from B. undulatus ). In terms of relative vulnerability, of the three sea urchin species examined, E. mathaei were more vulnerable to predation than Diadema setosum or Echinothrix calamaris , with mean handling times of 1.2, 4.8 and 10.3 min, respectively. Balistoides viridescens and B. undulatus both appear to be able to play an important role as predators of sea urchins on the relatively intact coral reefs of Lizard Island. However, B. viridescens emerge as the most efficient predator in terms of handling speed and the proportion of detections preyed upon. They were also the behaviourally dominant predator. This preliminary study of the predators of sea urchins on the GBR highlights the potential significance of relatively scarce but functionally important species.