Blue lobster

Panulirus gracilis is a spiny lobster of the family Palinuridae, described by Streets in 1871 ^[1]. In Peru it is called «langosta verde», and it is fished commercially throughout its range, which runs from Baja California in Mexico to Paita in Peru, plus the Galapagos Islands ^[2]. The IUCN Red List classifies it as Data Deficient ^[3].

Unlike many other lobsters in its genus, it has smooth abdominal segments with no transverse groove, and its third maxilliped has no exopod ^[2]. The plate that carries the antennules bears 4 strong spines set in a quadrangle, and abdominal segments 1 to 6 each show, next to the rear edge, an unbroken whitish stripe that stands out because it is edged with dark colour in front and behind ^[2]. The carapace is brownish or bluish green, nearly plain or faintly and irregularly mottled, and the antennae are greenish at the base and bluish green along the flagella ^[2].

Males reach at most 32 cm in total length and females 30 cm; the carapace measures from 1 to 13 cm in males and from 1 to 12 cm in females ^[2].

Males and females differ in proportions: in Manabí, Ecuador, the relation between abdomen length and carapace length showed that, at 80 mm of carapace, a female has an abdomen of 129.3 mm and a male one of 118.7 mm ^[4].

Similar species

• Panulirus penicillatus: Its abdominal segments have a transverse groove, the 4 spines of the antennular plate are joined at the base and the legs carry yellowish lengthwise streaks; in the eastern Pacific it lives mostly on islands off the coast, such as the Galapagos ^[2].
• Panulirus inflatus: Only abdominal segments 1 to 3 have a thin whitish line, and segments 4 to 6 carry a row of large whitish spots instead; it is known from the west coast of Mexico, from Baja California to Oaxaca ^[2].

It ranges through the eastern Pacific from Baja California in Mexico to Paita in Peru, and the Galapagos Islands, in shallow coastal waters from 0 to 18 m, among rocks, cracks and crevices ^[2]. A comparative review adds that it is not confined to clear-water rocky bottoms, since it also occupies gravel and sand bottoms near river mouths, where the water is often very turbid, whereas Panulirus inflatus lives only on rock; this suggests it is more of a generalist, although where the postlarvae of either species settle is still unknown ^[5].

A study by the Peruvian Marine Research Institute (IMARPE) in Tumbes, drawing on earlier work, places it in the rocky infralittoral and on mixed sand and gravel bottoms, and puts its southern limit in Piura ^[6]. In the Galapagos, between 2002 and 2008, its fishing yield was highest in the western region of the archipelago, and the fishers who target it most work mainly off the south and west of Isabela Island ^[7].

Its larvae pass through 11 phyllosoma stages, almost identical to those of Panulirus inflatus, and the length of that larval phase is still unknown ^[5]. The postlarva, called the puerulus, swims without feeding, measures 7.0 to 8.9 mm in carapace length and has spatulate antennae 2.7 times as long as its body ^[5].

At Zihuatanejo, Mexico, mark and recapture showed that males grow about 0.96 mm of carapace per week, almost twice as fast as Panulirus inflatus ^[5]. At that site the smallest egg-bearing female on record measured 47.5 mm in carapace length, and the size at which half of the females carry eggs is 80.0 mm ^[5]. It breeds over much of the year: one female can produce up to 4 broods per season, perhaps more, embryos take about 3 to 4 weeks to hatch, and at the same size it lays more eggs, and smaller ones, than Panulirus inflatus ^[5].

In Manabí, Ecuador, 703 lobsters caught between June and September 2010 were examined: females outnumbered males, with 0.72 males per female in the net catch and 0.63 in the diving catch, and the share of egg-bearing females rose in August and September ^[4]. Animals taken by divers at 7 to 12 m depth were larger, averaging 84.1 mm in carapace length, than those caught with nets in shallower water, at 3 to 5 m, at 73.4 mm ^[4].

It eats many kinds of invertebrates and shows a clear preference for molluscs, as Panulirus inflatus does ^[5]. At a rocky site in Zihuatanejo shared by both species, studied in 1979 and 1980, the rise of Panulirus inflatus in October coincided with a fall in Panulirus gracilis, several of which were recaptured from October to December on a gravel and sand bottom 3 km away, which suggests the former pushes it out of the richest feeding patches ^[5].

In IMARPE tanks, what was observed suggests the lobsters see little and are guided mainly by chemical cues picked up by moving their antennules, and several fought over the same piece of bait, which the authors read as territorial behaviour ^[6]. Fishers in Tumbes, like some authors, regard the octopus as its main predator, and in that region lobster beds overlap with those of Octopus mimus ^[6].

In the Galapagos Marine Reserve, between 2002 and 2008, its catch per unit effort was higher on daytime trips than on night trips and fell as sea surface temperature rose ^[7].

On the IUCN Red List its global category is Data Deficient ^[3].

According to a 2018 study, Peru has no minimum landing size and no management plan for the species: it is fished all year with bottom gillnets, which take egg-bearing females and small animals, cause bycatch and ghost fishing, and individuals of only 34 mm carapace length have been taken ^[6]. As an alternative, traps with a 15 by 15 cm entrance were trialled; in tanks they caught lobsters and sorted them by size, but at sea they caught none ^[6].

In Manabí, Ecuador, 98 % of the lobsters caught in 2010 were below the legal size of 103 mm carapace length, and the authors proposed a monitoring programme during the fishing season and a closure ^[4].

A climate change vulnerability assessment of the fishery resources of the tropical ecosystem off northern Peru, between 3° S and 6° S, gave it medium vulnerability ^[8].

Not medical advice. For a sting, a poisoning or an allergy, seek medical care.

In the Galapagos Islands, mercury was measured in leg muscle from 14 specimens of Panulirus gracilis: on a dry-weight basis, values ranged from 0.12 to 1.26 mg/kg, with a mean of 0.35 mg/kg ^[9]. Once converted to wet weight, nearly all concentrations fell below United States and European Union regulatory limits, and only 1 of those specimens exceeded them; the authors recommend also analysing tail muscle, which might accumulate more mercury ^[9].

It is fished commercially throughout its range and sold in local markets; it is taken with trammel nets, by hand or with traps ^[2]. In mainland Ecuador it has been fished since 1960, with bottom nets and, from the 1980s, also by hookah diving; that country exports on average 145 t a year of this species and Panulirus penicillatus to Italy, the United States, Belgium and Spain ^[4].

In Tumbes the fishery is entirely small-scale: it is worked from wooden boats or rafts close to the shore and supports coastal communities that depend on the resource ^[6].

In the Galapagos Marine Reserve some 1146 fishers hold permits for the archipelago's two lobsters, and those from the port of Villamil, on Isabela, focus mainly on Panulirus gracilis ^[7].

Peruvian literature on the species centres on Tumbes: according to a study published in 2018, the main fishing area and natural beds lie between Acapulco and Bonanza, and the fishing trials were made along the coast at an average depth of 9 m, between 6 and 12 m ^[6]. The 1991 FAO catalogue puts its southern limit at Paita ^[2].