The Sizes, Growth and Reproduction of Arrow Worms (Chaetognatha) in Light of the Gill-Oxygen Limitation Theory (GOLT)

Daniel Pauly,Weiwei Xian,Nicolas Bailly,Elaine Chu,Cui Liang

doi:10.3390/jmse9121397

Abstract

The Chaetognatha are a marine invertebrate phylum including 132 extant, carnivorous species in nine families and two orders, but with unclear protostomian affinities in the animal kingdom. We document the gradual recognition of the distinctiveness of chaetognaths by early taxonomists, with some emphasis on the often-overlooked studies by Chinese marine biologists. The carnivorous arrow worms are understudied relative to their importance in the marine zooplankton, where they rank second in abundance after the herbivorous copepods. Although arrow worms lack gills or other dedicated respiratory organs, we show that the Gill-Oxygen Limitation Theory (GOLT) can be used to explain how temperature and respiration affect their growth and related life-history traits. Notably, we present a reappraisal of evidence for size–temperature relationships between and within chaetognath species, and for the relationship between their temperature-mediated oxygen demand and their growth patterns. Von Bertalanffy weight growth curves of Ferosagitta hispida (family: Sagittidae) based on earlier aquarium experiments by various authors are presented, which suggest (a) a good fit and (b) that the life span of chaetognaths is much lower than suggested by the authors of several published growth curves drawn onto length–frequency samples from the wild. In addition, we show that chaetognaths attain first maturity at a fraction of the maximum length they can attain that is similar to the corresponding fraction in fishes. Overall, we suggest that the manner in which the oxygen they require enters the body of small marine invertebrates, although often neglected, is a crucial aspect of their biology. In addition, based on our result that arrow worms conform to the GOLT, we suggest that this theory may provide the theoretical framework for the study of growth in the other water-breathing ectotherms lacking gills.

Highlights

The first published record and drawing of an arrow worm, from the North Sea, are in Slabber [37] (46–48; Pl. 6, S4), where it was described as a “sea-worm”
The Aphragmophora are mainly planktonic, and have been sampled during oceanographic campaigns or surveys, while the Phragmophora are mainly benthopelagic (Heterokrohniidae), and benthic (Krohnitellidae, Spadellidae), i.e., require the targeted and dedicated samplings that have been historically more frequent in the Northern than the Southern Hemisphere. This becomes more evident when the planktonic Eukrohniidae are added to the Aphragmophora, resulting in 68 species in the Northern and 41 species in the Southern
It is symptomatic that many of the minute spadellid species are known only from the type specimens, and that the most recent one (Spadella kappae) was described near the “Station Biologique de Roscoff” [104,105], a French marine station established in 1872 on the coast of the English

Summary

Introduction

The Chaetognatha (i.e., “bristle jaws”), or arrow worms (Figure 1), belong to a marine invertebrate phylum with unclear protostomian affinities in the animal kingdom, and are spread through all oceans from the surface down to 5000 m, with a 6000 m record to be confirmed. Of the 132 extant species currently recognized [1], 58% are pelagic, and 42% benthopelagic or benthic [2]. Their biomass is about 1/3 that of copepods globally, and over 1/5 of the total zooplankton in the North Atlantic [3]. Arrow worms are carnivorous, feeding preferentially on copepods, and on other small invertebrates and fish larvae [7,8,9].

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