Invertebrates

Quantifying mobile invertebrate communities

Introduction

Mobile invertebrates are key components of marine ecosystems, serving as both ecological indicators and fulfilling critical functional roles in reef maintenance, herbivory, and nutrient cycling. The Pristine Seas invertebrate survey methodology provides standardized protocols for quantifying the abundance, diversity, and size distribution of conspicuous mobile invertebrates across different habitats and depths. These surveys complement our benthic composition data (LPI) by documenting the mobile fauna that drive essential reef processes and often serve as valuable fishery resources.

Method Overview

The invertebrate survey method employs a belt transect approach to systematically document conspicuous mobile invertebrates within a defined search area. This standardized approach enables quantitative comparison of invertebrate communities across sites, regions, and time periods.

Key Features

  • Defined search area: 50 m × 1 m belt transect (50 m²)
  • Focus taxa: Echinoderms, mollusks, crustaceans, and other conspicuous invertebrates
  • Size measurements: Targeted measurements of commercially important or ecologically significant species
  • Standardized effort: Consistent search time and methodology across all stations
  • Integration: Uses the same transect line deployed for LPI and coral recruit surveys

Field Implementation

Team Structure

The invertebrate survey is conducted by a specialized diver as part of the three-person benthic survey team. This integrated approach maximizes efficiency while ensuring consistent sampling across all benthic components:

  • One diver focuses on coral recruits and invertebrates
  • Another conducts the Line Point Intercept (LPI) survey for benthic cover
  • A third identifies corals at LPI points to species level

The recruit/invertebrate specialist completes the coral recruit survey first, followed by the invertebrate survey along the same transect, optimizing dive time while maintaining thorough coverage.

Invertebrate Survey Specifications
  • Transect dimensions: 50 m × 1 m (50 m²)
  • Search width: 1 m along one side of the transect line
  • Direction: From start (0 m) to end (50 m) of the transect
  • Depth strata: Typically 10 m and 20 m (matching other benthic methods)
  • Timing: Conducted after completing coral recruit quadrats

Collection Procedure

  1. Initial preparation:
    • After completing the coral recruit quadrats, return to the start of the transect
    • Prepare slate with data sheet and measuring guides
  2. Survey execution:
    • Begin at the 0 m mark and work toward the 50 m mark
    • Systematically search 1 m along one side of the transect line
    • Scan substrate, crevices, and undersides of structures
    • Maintain a consistent search speed
  3. Data recording:
    • Identify all motile invertebrates to species level and count them
    • For culturally and fishery important species, measure sizes in centimeters using slate
    • Exclude species permanently attached to the seabed (except for scallops, pearl oysters, and giant clams)
    • Note unusual behavior or interesting ecological interactions
  4. Photo documentation:
    • Photograph all invertebrates not identified in-situ to species
    • Capture additional images of unusual specimens or notable aggregations
    • Document evidence of harvesting or ecological interactions
  5. Quality control notes:
    • Note any deviations from standard protocol
    • Document search time and any interruptions
    • Record habitat and substrate characteristics that might affect invertebrate distributions

Measurement Protocols

For priority species, standardized measurements are taken using the centimeter-marked edge of the dive slate. Each taxonomic group has specific measurement standards to ensure data consistency:

Giant Clams
Maximum shell length for all Tridacnidae
Position recorded as embedded, loose, or partial

Pearl Oysters
Shell height including Penguin’s wing oysters
Measured from hinge to furthest shell margin

Gastropods
Shell width/length for Trochus, Triton, Conch varieties
Following SPC guide-specific measurement points

Sea Cucumbers
Body length when relaxed for all commercial species
Measured without disturbing the animal

Sea Urchins
Test diameter without spines for non-boring species
Especially important for Tripneustes and Diadema

Lobsters
Carapace length for all species
Measured from eye orbit to posterior carapace edge

Additional species may be included based on regional priorities, following the measurement protocols outlined in the SPC invertebrate measurement guide.

Priority Species Monitoring

Certain key species warrant additional documentation due to their ecological significance or commercial importance:

Crown-of-Thorns Starfish (COTS)

For each Acanthaster individual encountered, we document:

  • Size: Diameter from arm tip to opposite arm tip (cm)
  • Behavior: Activity coded as feeding (F), moving (M), or resting (R)
  • Habitat: Substrate type and coral species association
  • Aggregation: Number of other individuals within 5m radius
  • Condition: Any signs of disease, predation, or stress

This detailed monitoring helps track potential outbreaks that can significantly impact coral reef health and provides essential data for management interventions.

All giant clam species receive special attention with documentation of:

  • Species: Complete species-level identification
  • Size: Maximum shell length measured along longest axis
  • Position: Recorded as embedded (E), loose (L), or partially embedded (P)
  • Health: Assessment of bleaching status and physical damage
  • Habitat: Substrate type and depth within site

As iconic and vulnerable species, giant clams serve as important indicators of fishing pressure, environmental conditions, and reef health.

This comprehensive data collection provides critical insights into the status of invertebrate resources and informs sustainable management strategies.

Taxonomic Framework

Target Taxa

The invertebrate survey counts all motile invertebrates, with special attention to ecologically important and commercially valuable species. While we aim for complete enumeration, the following key groups receive priority focus:

Echinoderms
  • Sea stars (Acanthaster, Linckia)
  • Sea urchins (Diadema, Tripneustes)
  • Sea cucumbers (all commercial species)
  • Feather stars (crinoids)
Mollusks
  • Giant clams (Tridacnidae)
  • Pearl oysters (including Penguin’s wing)
  • Gastropods (Trochus, Triton, Conch)
  • Nudibranchs & Cephalopods
Crustaceans
  • Lobsters (Panulirus, Scyllarides)
  • Large crabs (Carpilius, Etisus)
  • Other large crustaceans

Identification Process

Our taxonomic approach adapts to the challenges of identifying diverse invertebrate taxa in the field. All motile invertebrates are identified to species level whenever possible, with photographs taken for specimens that cannot be immediately identified.

When immediate species-level identification is not possible, we employ a hierarchical approach to taxonomic resolution:

  1. Field names: Descriptive terms based on obvious morphological characteristics
    • Examples: “black sea cucumber with red papillae,” “blue-spotted sea star”
  2. Morphotaxa: Consolidation of field names into taxonomic units with distinguishing features
    • Examples: “Linckia sp. blue,” “Holothuria sp. black with red”
  3. Lowest defensible taxonomic rank: Formal taxonomic assignment based on available evidence
    • Examples: “Linckia laevigata,” “Holothuria atra”
  4. Functional group: Classification into ecological functional roles
    • Examples: “Grazing echinoid,” “Deposit-feeding holothuroid”
Identification Resources

Field teams utilize multiple resources to support accurate species identification:

  • Pre-expedition training: Familiarization with expected regional species
  • Field guides: Region-specific printed references available onboard
  • Digital libraries: Offline electronic identification guides
  • Photographic documentation: Systematic imaging for later verification
  • Expert consultation: Review of challenging identifications by taxonomic specialists

Ecological Classifications

Invertebrates are classified according to several functional frameworks to enable ecological interpretation beyond simple taxonomic groupings.

Feeding Modes

We categorize invertebrates into functional feeding groups that reflect their ecological roles:

Herbivores / Detritivores
Consume algae, detritus, or plant material
Examples: Many sea urchins, some sea cucumbers

Active Suspension Feeders
Filter feeders that actively generate a feeding current
Examples: Sponges, bivalves, bryozoans, tunicates

Passive Suspension Feeders
Rely on ambient water flow to capture particles
Examples: Crinoids, gorgonians, dendrochirotid sea cucumbers

Deposit Feeders
Process sediment for organic material
Examples: Many sea cucumbers, some sea stars

Predators
Consume other animals
Examples: Crown-of-thorns starfish, many gastropods, octopus

Resource Categories

Many invertebrates represent important fishery resources. We classify these according to their value and exploitation status:

High-Value Commercial
Primary target species in export fisheries
Examples: Sea cucumbers, lobsters, giant clams

Local Subsistence Resource
Species harvested primarily for local consumption
Examples: Trochus, octopus, many crabs

These classifications provide a framework for interpreting invertebrate patterns in the context of ecosystem function and human use.

Data Workflow

Data Entry

Invertebrate observations are recorded on underwater slates during dives and transferred to standardized Excel fieldbooks on the same day. This immediate transcription preserves data accuracy while memory is fresh. The ISO3_YEAR_inverts_fieldbook.xlsx serves as the primary data repository for each expedition with three key components:

Fieldbook Structure
  • Readme: Expedition details, data entry protocols, and guidelines
  • Stations: Site information including depth, habitat type, and protocol adherence
  • Observations: Individual invertebrate records with taxonomic identification, counts, and measurements

The observations sheet includes validation rules that flag potential errors such as:

  • Missing taxonomic information
  • Unlikely size measurements
  • Invalid station numbers

Processing Pipeline

Our data processing transforms raw field observations into analysis-ready datasets through four sequential phases:

1. File Consolidation

First, we compile data from all divers and expedition legs into a unified dataset:

  • Merge all invertebrate records from multiple dives and sites
  • Standardize taxonomic names using reference taxonomy lists
  • Add metadata from the UVS Sites Fieldbook (coordinates, habitat type, etc.)
  • Create a complete dataset that preserves all original observations

2. Quality Control

Next, we verify data quality through a systematic review process:

  • Validate species identifications for consistency and accuracy
  • Flag outliers in size measurements for review
  • Cross-check invertebrate records with habitat types for ecological coherence
  • Ensure complete sampling coverage across all sites

Potential issues are flagged for expert review rather than automatically removed, preserving data integrity while ensuring scientific accuracy.

3. Density Calculation

We then calculate standardized density metrics for cross-site comparisons:

  • Compute abundance per unit area (individuals/m²) for each taxon
  • Generate biomass estimates for species with established length-weight relationships
  • Calculate summary statistics by taxonomic group, site, and region

A critical step in this process is the addition of zero values for taxa that were not observed on a particular transect but were found elsewhere in the region. This ensures that absence data is properly incorporated when calculating average densities and prevents biased estimates. Zero values are applied:

  • After taxonomic standardization but before aggregation
  • Based on the regional species pool established for each survey
  • Only for species that could reasonably occur in the sampled habitat type
  • Prior to calculating site-level and region-level metrics

4. Database Integration

Finally, we format the processed data for ingestion into our central database, creating four core output files:

  1. inverts_observations: Individual records with standardized taxonomy, counts, and measurements
  2. inverts_density_by_taxa: Density calculations by taxa and station
  3. inverts_stations: Station-level metadata and summary statistics (richness, density)
  4. inverts_taxa_summary: Regional occurrence patterns and average densities by taxa

These standardized outputs allow consistent analysis across expeditions and regions while maintaining connections to the original field observations.

Exploratory Data Analysis

Our analytical approach examines invertebrate communities through multiple complementary perspectives:

Community Composition

We analyze biodiversity patterns and community structure:

  • Species richness and diversity indices across sites and protection levels
  • Multivariate analyses (NMDS, cluster analysis) to identify assemblage patterns
  • Functional group proportions to assess ecosystem processes
  • Indicator species analysis to identify habitat associations

These analyses provide insights into the factors structuring invertebrate communities and their responses to environmental gradients.

Population Patterns

We examine population characteristics of key species:

  • Density comparisons across sites, habitats, and protection levels
  • Size frequency distributions to assess population structure
  • Biomass estimates for commercially important species
  • Recruitment patterns and juvenile abundance

These metrics help evaluate population health and the impacts of environmental conditions or harvesting pressure.

Spatial Patterns

By analyzing invertebrate distributions across space, we identify:

  • Hotspots of diversity and abundance
  • Habitat associations for key species
  • Depth-related community changes
  • Geographic patterns in commercially important taxa

Spatial analyses provide critical information for conservation planning and resource management.

Cross-Method Integration

We combine invertebrate data with other survey components to reveal ecosystem relationships:

  • Correlations between benthic cover and invertebrate communities
  • Predator-prey relationships between fish and invertebrates
  • Potential impacts of invertebrate grazers on coral recruitment
  • Comparisons between visual surveys and eDNA detection of invertebrate diversity

These integrated analyses provide a holistic understanding of reef ecosystem function and reveal the ecological processes that drive reef health and resilience.

Through these complementary analytical approaches, we transform field observations into ecological insights that inform conservation planning and contribute to our understanding of marine ecosystems across multiple scales.

Limitations

While belt transects are an efficient method for surveying many mobile invertebrates, several limitations should be considered:

  • Habitat complexity: Detection probability varies with substrate complexity
  • Cryptic behavior: Many invertebrates remain hidden within complex reef structures
  • Nocturnal activity: Daytime surveys miss species active primarily at night
  • Diver avoidance: Some mobile species actively avoid observers
  • Spatial clumping: Patchy distribution may result in high variance between transects
  • Taxonomic challenges: Some groups require specialized expertise for accurate identification

We address these limitations through standardized protocols, thorough diver training, adequate replication, and integration with complementary methods like eDNA and night surveys where possible.

References