Relationship between dimorphism and drift in the Portuguese man-of-war

Highlights

• Dimorphism (left- and right-handed forms) occurs in the Portuguese man-of-war.

• More than 3,500 individuals were collected along the Basque coast in August 2010.

• Our model results suggest that most of these individuals were right-handed.

Abstract

One interesting feature of the Portuguese man-of-war (Physalia physalis) is its dimorphism, with the existence of two distinct forms (left- and right-handed individuals). The worldwide distribution of this dimorphic wind-propelled siphonophore is a much discussed but poorly understood phenomenon. Here we investigate the most likely region of origin and routes of more than 3,500 individuals of this species that were collected along the Basque coast (southeastern Bay of Biscay) in August 2010. We also test the hypothesis that most of these individuals were right-handed. To do this, we used the Sediment, Oil spill and Fish Tracking model (SOFT). From our model results, we can conclude that the region of origin of these individuals was probably located in the northern part of the North Atlantic Subtropical Gyre. The findings also suggest that our hypothesis is very likely to be correct.

Introduction

The Portuguese man-of-war (Physalia physalis) is a colonial organism that lives in warm tropical and subtropical waters (Totton and Mackie, 1960; Bardi and Marques, 2007; Mapstone, 2014; Ferrer et al., 2015; Munro et al., 2019). One interesting feature of the Portuguese man-of-war is the existence of two distinct forms (i.e., dimorphism). These two forms are mirror images of one another but otherwise identical. The dimorphism in the Portuguese man-of-war has been discussed by several researchers (e.g., Totton and Mackie, 1956, 1960; Woodcock, 1956, 1971, 1997; Bieri, 1959; Shannon and Chapman, 1983). Their observations reveal that under the influence of the wind one form (left-handed) moves to the right of the downwind direction, while the other (right-handed) to the left (see Fig. 1). Several years later, Iosilevskii and Weihs (2009) observed that the Portuguese man-of-war sailed with its float aligned with the wind direction in strong winds.

In 2010, the Atlantic and Mediterranean coasts of the Iberian Peninsula and the Canary Islands experienced their largest recorded number of Portuguese man-of-war. In August 2010, beach cleaning services collected more than 3,500 individuals along the Basque coast (southeastern Bay of Biscay). Unfortunately, there is no information on whether these individuals were left- or right-handed. Prieto et al. (2015) concluded that the meteorological and oceanographic conditions were very favourable for the occurrence of the 2010 swarm event along the Mediterranean coast. This conclusion agrees with the results obtained by Ferrer and Pastor (2017) for the Basque coast. Ferrer et al. (2015) suggested the use of a simple model based only on wind information to estimate the drift of the Portuguese man-of-war. This model was applied by Ferrer and Pastor (2017) to investigate the most likely region of origin as well as the routes of the Portuguese man-of-war that were collected along the Basque coast in August 2010 (hereafter referred to as the 2010 PMW, for simplicity). These authors concluded that the region of origin was located in the North Atlantic Subtropical Gyre (NASG, see Fig. 2(a)). In their numerical simulations, they did not consider the dimorphism in the Portuguese man-of-war.

Although many studies have been done on the Portuguese man-of-war, little information is available on issues such as its life cycle (including growth and reproduction rates, and lifespan), behaviour, ecology and dimorphism. Here we repeat the numerical simulations carried out by Ferrer and Pastor (2017) but considering the dimorphism in the Portuguese man-of-war (i.e., left- and right-handed individuals). Using a Lagrangian particle tracking model, we test the hypothesis that most of the 2010 PMW were right-handed. The results obtained from this study will improve our understanding of the distribution of this colonial organism in the North Atlantic Ocean.