Techniques for collecting interstitial opisthobranchs

by Michael Schrödl

Searching
- prefer coarse, subtidal sand (or gravel, shell grid, small coral rubble) from places with continuous currents, and/or from the lower intertidal with just modest wave action
- take just the surface layer (some centimeters)

Before Extraction
- put the sample in a bucket/box, with sea water just covering the sand surface (!)
- let the sand sample stand (in the shade), at least for 1 day; specimens will migrate to the oxygenated surface layer and accumulate there.

Extraction
- take some sand (a few spoons) from the surface layer
- put the sand in a 1 l beaker (or in a half plastic bottle)
- add about triple volume of an isotonic Magnesium chloride solution
- mix it, but shake very carefully
- wait at least 10 min
- shake the beaker gently to suspend organic particles/opisthobranchs
- quickly put the liquid through a 80µm sieve (avoid sand to get in)
- turn around the sieve and wash (seawater; squeeze bottle) the particles into a dish
- put the Magnesium chloride solution back into the beaker
- repeat the suspending procedure more and more fiercefully, until abundance of extracted organic substance/specimens gets notably less

Analysis
- wait 5 min to get the opisthobranchs reactivated in the dish
- look for opisthobranchs under a binocular microscope
- pay attention to all slowly (!) moving creatures that 1) do not swim (if touched), 2) do not have chaetae, 3) do not invert or evert large parts of the body
- most gastropod species either have a head with tentacles, a foot, and/or a shell; mesopsammic specimens with shell are either Caecum (prosobranchs), basal heterobranchs or opisthobranchs; remember that benthic species from the surface layer may be included as well.
- shell-less mesopsammic opisthobranchs usually show at least one of the following features: a clearly defined foot, one or two pairs of cephalic tentacles, a pair of eyes, a visceral sac separated from the head-foot complex, calcareous spicules.
- taking photographs of living animals is essential for identifying and later description
- all specimens, except for those to be fixed and used for molecular analysis, must be relaxed before fixation by slow, drop-wise (!) addition of Magnesium chloride solution until their bodies no more react to water movement, i.e. slightly to abruptly shaking the dish. Any premature addition of fixatives will cause heavy contraction of specimens.

Authorship details
Schroedl, M., 2006 (February 9) Techniques for collecting interstitial opisthobranchs. [In] Sea Slug Forum. Australian Museum, Sydney. Available from http://www.seaslugforum.net/factsheet/inteextr

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