Arbacia spatuligera

Arbacia spatuligera is a sea urchin of the order Arbacioida and the family Arbaciidae ^[1]. It lives in the southeastern Pacific, from northern Peru to southern Chile, and is mostly known from less than 30 m depth, although a recent study recorded it between 36 and 63 m ^[2].

A genetic study of 140 individuals, collected between 11°59′ S (Peru) and 36°45′ S (Chile), found no structure among localities over about 3200 km and a low genetic diversity; the authors relate this pattern to the dispersal potential of the planktonic larva ^[3].

It is a regular urchin with a circular to subpentagonal test reaching 70 to 80 mm in diameter. Alive it is dark brown to dark green, with paler, usually white, spines. The spines on the upper side are short and club-shaped, and those at the edge of the test are long and slender, about as long as the test diameter, up to 50 mm ^[2].

Diagnostic characters include a narrow bare interambulacral area, reduced to the midline in large specimens, poorly developed upper ambulacral tubercles that do not reach the ocular plates, and ambulacral pores larger than those of other species in the genus ^[2].

A population at 53 m depth off Pichidangui showed a variant with long brown spines reaching the apex and a dark green test, although its COI gene sequence was identical to that of the typical morphotype. The authors treat it as a possible response to the deep, calm environment ^[2].

Similar species

• Arbacia dufresnii: Mortensen (1935), cited in recent literature, separated it from Arbacia dufresnii by colour, with no green or red patches in the interambulacrum, and by the club-shaped upper spines ^[2].

The species occupies the Warm Temperate Southeastern Pacific province, between about 6° S (Chiclayo, in northern Peru) and 42° S (Puerto Montt, in south-central Chile). A historical record from Guayaquil, near 2° S, is considered doubtful because no museum specimen comes from Ecuador ^[2].

The range lies within the Humboldt Current System, which carries nutrient-rich water northward; the authors of a genetic study consider that this current system may influence the dispersal of the species ^[3].

The species lives in the subtidal zone, traditionally at less than 30 m ^[3]. In 2019 and 2022, 128 individuals were found between 36 and 63 m on four mesophotic reefs in central Chile, on rock, two shipwrecks and a sandy slope with debris, which extends the known depth ^[2].

The life cycle includes a planktonic larva of about 30 days, followed by settlement and metamorphosis into the benthic adult ^[3]. This larval phase is shorter than that of other coastal invertebrates, such as the urchin Tetrapygus niger, at about 3 months, and the authors propose it as one possible cause of the low genetic diversity observed ^[3].

Among 140 individuals sequenced for a 630 base-pair fragment of the COI gene, there were 25 haplotypes, and a single one of them grouped 114 individuals, spread across the whole sampled range. The star-shaped pattern and the demographic analyses indicate a recent expansion of the population, dated to 33000 to 47000 years ago ^[3].

In the laboratory, gametes are obtained by injecting a potassium chloride solution into the perivisceral fluid of adults; those eggs and sperm are then used to measure fertilization rates ^[4].

In the mesophotic populations of central Chile, the highest abundance was recorded on the Pichidangui rocky reef, between 40 and 58 m, where there was anything from a single individual to aggregations of 11 urchins on the rock. The deepest shipwreck, at 63 m, had the lowest abundance, with 10 individuals ^[2].

At those sites it shared the habitat with the striped anemone Anthothoe chilensis, and on the sandy slope three individuals were found next to the anemone Antholoba achates; there, abandoned fishing gear served the urchins as substrate ^[2].

The species has no commercial value and is little known; most studies are fertilization and survival bioassays ^[3].

Those bioassays use it as an indicator of water quality. In the Biobío region of Chile, the fertilization of its eggs was 14 % and 22 % in two sectors with organic discharges from fishmeal plants, against a 70 % mean in the remaining sectors. It was correlated with dissolved oxygen, pH, turbidity and oil and grease ^[4].

In Peru, the material for a genetic study was collected by scuba diving, between 10 and 30 m depth, at a locality at 11°59′ S on the central coast, during 2016 and 2018; the source does not let us name the exact site here ^[3].

The deepest records, 36 to 63 m, come from Chile and required technical diving with a rebreather, so they are not accessible to recreational diving ^[2].