Collecting Marine Gnathiid Isopod Fish Parasites with Light Traps

Summary

We present a method to collect marine gnathiid isopod fish parasites using light traps placed at field sites via breath-hold diving or scuba diving.

Abstract

A method to collect marine gnathiid isopod fish parasites with the use of light traps is presented. Gnathiid isopods are a major group of marine fish parasites that feed on blood and fluid from host fishes, mostly at night. Like ticks and mosquitos on land, they associate only temporarily with their host and spend most of their life free-living in the benthos. Given their high mobility and transient and predominantly nocturnal association with hosts, they cannot easily be collected by capturing free-living hosts. However, they are readily attracted to underwater light sources, creating the opportunity to collect them in light traps. Here the design and individual steps involved in the deployment and processing of specially adapted light traps for collecting free-living stages of gnathiid isopods are outlined. Sample results and possible modifications of the basic protocol for a variety of different sampling needs are presented and discussed.

Introduction

Parasitic crustaceans are important in the ecology and life histories of reef fishes. The biomass and energy they remove from their hosts are considerable and influence behavior, physiology, and survivorship¹. Gnathiid isopod crustaceans represent the most prominent group of fish parasites in tropical and subtropical reef systems, where they are both abundant and diverse^2,3 and are the primary food item of cleaner fishes^4,5. Gnathiids are generally 1-3 mm in size. They have unusual life histories in which o....

Protocol

Collection of samples was permitted by the Department of Agriculture-Bureau of Fisheries and Aquatic Resources (0154-18 DA-BFAR) in accordance with Philippine laws and regulations (RA 9147; FAO 233) and approved by the Silliman University (SU) animal ethics committee.

1. Light Traps

1. Construction
   1. Construct light traps from commercial polyvinyl chloride (PVC) tubes originally designed for plumbing. Use 10-15 cm diameter PVC cut to 30-40 cm length (

Discussion

Traditional light traps, such as those used for collecting larval fishes, are large and are suspended in water column³⁴. In contrast, the light traps described here are small and deployed on the sea bottom. These traps can be easily transported and rapidly deployed. They can be placed by breath-hold (free)-diving in shallow sites (as in this study) or on scuba in deeper sites, and attract both fed and unfed
juvenile stages.

Variations of the benthic light traps de.......

Materials

Name	Company	Catalog Number	Comments
Buckets, small sample containers		hardware store
Funnels	Supplier No. 2209-03	Funnels: AMERICAN SCIENTIFIC LLC SE - 75 mm (3”)	https://us.vwr.com/store/product/8884369/plastic-funnels
Main body of light traps (made from commercially available PVC sanitarty pipes)	(SKU 145640)	Alasco Sanitary uPVC Pipes Series 1000 107mm/4'	https://alascopvcpipes.com/product/alasco-standard-sanitary-upvc-pipe-series-1000/.  This brand can be found in the Philippines. Other simular brands can also be used
Modeling clay		Can be found in art suppliy and childreans toy stores	To seal the funnel after retreival
Plankton mesh (50-100 µm)		any reputable brand and source	https://www.adkinstruments.in/products/plankton-nets-in-various-mesh-size-1633936883
Screw on lids for the light trap		Alasco  Sanitary  Clean-Out  4"	https://alascopvcpipes.com/product/alasco-standard-sanitary-upvc-clean-out/. This brand can be found in the Philippines. Other simular brands can also be used
Scuba/snorkel equipment		any reputable brand and source
Stereo-microscopes		Scientific suppliers
Underwater touches		Princeton Tec Ecoflare or Fantasea Nanospotter 6023